BASIC PRINCIPLES OF PLANT SCIENCE
A Reference Unit for Teaching Basic Principles
Vocational Agriculture Courses
Floyd L. Northrop, Area II Supervisor
Departmniit of Agricultural Education
University of Florida
Division of Vocational, Technical, and Adult Education
Walter R, Williams, Jr,, Director
Agricultural Education Section
Harry E. Wooed, upervisor
O, C. Norman, Program Specialist
R. W, Scull, Assitant
In view of the broadened program in agricultural education to include
"any occupation involving knowledge and skills in agriculture subjects",
it is evident that basic principles must be taught during the first two or
more years in the high school agriculture curriculum.
References that are concerned primarily with production practices are pre-
sently available to vocational agriculture teachers. This reference titled
"Basic Principles of Plant Science" represents a different approach, and is de-
signed to fulfill the need for organized subject matter dealing with basic prin-
ciples. Much of the organized body of plant management knowledge is deeply rooted
in the basic principles of plant science. By bringing the two together -- the
WHY and the HOW -- learning will be expedited and interest in performance will
be increased. A comprehensive understanding of the basic principles underlying
a procedure or practice, in connection with a problem, Increases the efficiency
of the individual, and equips him to make decisions on other problems involving
the same or similar principles. Indeed, a practice taught without the under-
standings of the underlying basic principles will certainly not equip the students
S to adjust to a rapidly changing agriculture.
The best use of the basic principles seems to be this: Select and incorporate
into the course of study those principles that are truly basic to agriculture then
"round out" the instruction by including productive enterprise jobs, improvement
projects, supplementary practices, and other activities which can be worked into
a supervised experience program to develop understandings and skills involved.
As already implied, it is not intended that basic principles be used to re-
, place or de-emphasize the student's supervised experience program. Indeed, it is
Highly doubtful that basic principles can be effectively taught unless they are
put into actual practice. Numerous experiences with the applications of these
basic principles should result in the permanent and productive vocational educa-
tion of our students.
Teachers are urged to supplement this reference with additional basic refer-
V' ences, as long as they are reliable, and to use pictorials, specimens, films, film-
strips, and other visuals to make the information more meaningful.
We are greatly indebted to the Agricultural Education Department of Missis-
sippi State University and the Division of Vocational and Technical Education of
the Mississippi Department of Education for permission to reproduce their mater-
ials in this publication.
Floyd L. Northrop,
Subject Matter Specialist
Table of Contents
I. Classification of Agricultural Plants . . . . . . ... . . 1
II. Parts of Plants -- Functions of Each ... ........... .. ... 5
III. Reproduction Systems and Reproduction . . . . . . . . .13
IV. Plant Growth -- How It Takes Place . . . . . . ... . 35
V. Plant Nutrition .... .......... ............. ..39
VI. Plant Diseases .................... .... ...... ...53
VII. Plant Insects (Common to Farm Crops) . . . .. . . . . .61
VIll. Weed Science .................... ........ ...71
Classification of Agricultural Plants
1. How are agricultural plants classified?
Plants are classified in many ways. In this reference unit, however, con-
sideration will be given primarily to their classification according to (a) life-cycles
and (b) botanical classes.
The life-cycle of a plant refers to the duration of the plant's life.
Life spans vary from one season to many centuries. From the "life-
cycle" standpoint plants are classified into:
(1) ANNUALS. These are plants that complete their life-cycle within
one year's time. Annuals grow from a single seed, mature, reproduce,
or produce seeds, and die in a single season.
Annuals are of two kinds: summer annuals, and winter annuals.
Summer annuals, such as beans, peas, an corn, are planted in the
spring; they form seed and die before late fall. Winter annuals, such
as wild winter peas and vetch, are planted in the fall; the seedlings pass
through the winter in a rather inactive state and resume growth in the
spring. The plant dies after seed is produced in the summer.
(2) BIENNIALS. Plants classified as biennials live two seasons. During
the first year they produce vegetative parts only. The first season is a
storage season. Depending upon the species, some parts of the plants --
stems, leaves, or roots -- have a supply of stored food. The plant, using
the stored food, grows during the second season and bears the flowers.
After seeds are produced the plant dies. Cabbage, beets, carrots, and
sweet clover are examples of this category.
(3) PERENNIALS. Perennials are plants that live more than two seasons.
Most perennials form their vegetative organs -- stems and leaves -- the
first year, but do not produce flowers until the following season, or later.
Trees, shrubs, Sericea lespedeza, white clovers, and bermudagrasses are
a few of the perennials. Cotton will grow as a perennial if climatic
conditions permit. (It grows, however, as an annual in most cotton
growing sections of the United States because low temperatures kill it.)
In fact, there are places in the United States where a second crop of
cotton is produced from a single planting. The stalk is cut off above the
ground after the first crop has matured. If warm weather continues it
produces new vegetation from the stump.
Perennials are further classified as deciduous or evergreen. Deci-
duous plants shed all their leaves each year during the dormant or resting
period -- winter. Evergreens retain green leaves during the entire year,
even though they shed leaves. Pecans, most oak trees, and spireas are
examples of deciduous plants; pines, the Live oak, hollies, and cedars
are some of the many plants classified as evergreens.
B. Botanical Class.
What is the importance of classifying plants by botanical -- scienti-
fic -- names?
Man has always given common names to plants and animals. In
many cases, various regions have their common names for the same plant
-- or animal. The use of common names is all right as long as those using
them remain in the same community. But common names often lead to
confusion. In some cases, a single common name refers to two or more
entirely different plants. For example, what is a "buttercup"? Several
different plants are referred to as "buttercups". So, the duplication of
names is one problem resulting from the use of common names -- and
there are others. In addition to preventing this duplication, botanical
-- are usually descriptive
-- show a systematic relationship to other organisms, and
-- are used by people of all countries.
A system of classification has been worked out whereby plants can
be identified properly. Classification is a means of placing plants that
are similar into the same group. Each group has certain characteristics
that all its members possess. Other groups have their distinguishing
Botanical names are based on Latin descriptions and must be ap-
proved by a Congress, or committee, of plant scientists. By agreement,
the world's plant scientists have given only one name to each species,
and have agreed that Latin, which no country is now using, will be the
The following example shows how one plant, commonly known as
corn, is botanically classified:
*Phylum: Spermatophyte (seed plants)
Family: Gramineae (grass family)
Species: Zea mays
(Plants are further classified into varieties, such as Dixie 82 corn.)
In classifying plants -- or animals -- the kingdoms are, as shown
previously, divided into phyla, classes, and so on.
*Phyla in the plant kingdom are divided into four divisions:
(1) Thallus plants -- plants not having stems, roots and leaves.
a. Algae -- simplest green plants.
b. Fungi -molds, mildews, yeasts, mushrooms, smut, and
ringworm fungi. Simplest non-green plants. Do not
manufacture their own food.
c. Bacteria -- microscopic, one-celled plants. Disease
and non-disease forming types.
(2) Mosses, and Liverworts.
(3) Ferns, and Club mosses.
(4) Spermatophytes (seed plants)
In this reference unit we shall be primarily concerned with the spermato-
phytes. See-a-producing plants are known as spermatophytes, and include conebear-
ing and flowering trees, shrubs, flowering herbs, and so on, which have leaves,
stems, and roots.
Botanists classify seed-producing plants according to their flower-parts.
(For example, all grasses have similar flowers, while flowers of the various legumes
After being divided into phyla, plants are further divided into classes.
Spermatophytes are classed as gymnosperms and angiosperms. The gymnosperms
are the more primitive seed plants. "ymnosper means "naked" seed, or seeds
not enclosed in an ovary -- pines, firs, and others. They are all woody, perennial
forms and, with few exceptions, are evergreen plants. The angiosperms include all
the flowering plants. Their flowers produce the fruit, or ovary, in which the seeds
are enclosed -- an enclosed structure. Pea pods, acorns, and apples are examples
of such seed-enclosing structures.
A further division is that of classes into orders, and then orders into
families. In the case of angiosperms, families are iied into monocotyTe- onous
and dicotyledonous families. (The word "mono" means one; "di" means two; "coty-
ledon" means seed-leaf.) The word monocotyledonn" refers to a seed plant having
only one seed leaf, as is true with grasses. "Dicotyledonous" plants are those
having two seed leaves -- cotton, beans, etc.
The following is a list of the most important seed-producing plant families
(with examples) classified under the mono- and dicotyledon divisions:
(1) MONOCOTYLEDON -- one cotyledon (seed-leaf) per seed.
(a) Grass family. Corn, other cereal grains, fescue, grasses.
(b) Liyamily. Lilies, onions, tulips, wild onions, and wild
(c) Sedge family. Nutsedge erronously called "nutgrass".
(2) DICOTYLEDON -- two cotyledons (seed-leaves) per seed.
(a) Legume (pulse) family. Beans, peas, peanuts, vetch, clovers,
lespedezas, alfalfa, and kudzu.
(b) Nightshade family. Potato, tomato, pepper, tobacco, and
(c) Mustard family. Turnip, mustard, radish, wild mustard, etc.
(d) Morning-g ory~famly. Morning-glory, sweet potato, and
(e) Mallow fa ily. Cotton, okra, hollyhock.
(f) Rose family. Rose, peach, pear, apple, plum, and straw-
Families of plants are then classified into genera, and then into species.
Botanical, or scientific, names are made up from the genus and the species. An
example of this is corn, which is botanically named Zeamays. This is similar to
calling a man Jones john. There may be several of the "Jones boys" but only one
Jones john (Avena sativa -- oats). There may also be a Smith john (Oryza sativa --
rice), but he will be an entirely different individual.
Each plant species may have several different varieties. For example,
there are many varieties of Zea mays (corn): Dixie 55, Dixie 82, etc.
I/. Parts of Plants Functions of Each
1. What are the structures (parts) of seed-producing plants?
As mentioned in Unit I, the information in this publication is primarily
concerned with spermatophytes. Spermatophytes, the highest form of plants,
are known as seed-bearing plants or as flowering plants. (These are plants in
the fourth phylum of the plant kingdom, and are the ones of economic impor-
tance to agriculture.)
The main parts of spermatophyte plants are the:
(a) Leaves )
(b) Stems ) vegetative organs
(c) Roots )
(d) Flowers, and )
) reproductive organs
(e) Seeds, or fruit )
The vegetative structures will be discussed in this unit -- Unit II. The
reproductive structures will be covered in Unit IV, "Reproductive Systems and
2. What are the functions of the vegetative organs -- leaves, stems, and roots?
The greatest factory on earth is the green plant! It manufactures its own
food. The change from a seed to a mature, food or fiber-producing plant,
which bears new seed for the following crop is one of nature's greatest wonders.
Leaves, stems, and roots have functions of their own, but work harmon-
iously together. Refer to Figure 1 for a summary of these functions. Other
information in this unit, and Units II through IV supply additional information.
3. What influences root growth?
Root systems vary considerably in their extent of growth. Even an
abundant root system may not come in contact with more than two percent of
the total soil particle surface. Root growth is primarily influenced by:
(a) Soil moisture content. Roots grow better in soil which has adequate
moisture, but is not completely water soaked. Roots sometimes appear to have
grown in "search" of water, but this appearance merely results from the fact
that other roots on the same plant could not grow into areas of dry soil as
rapidly as the roots which were closer to the water supply. This is the reason
why the soil should be deeply soaked when plants are watered. This enables
the plant roots to grow faster toward deeper horizons where the soil remains
moist for a longer period of time.
FEMALE FLOWER FUNCTIONS:--
(b) Store food --
(a) Conducts water and raw
minerals from soil to leaves
(b) Conducts manufactured food
from leaves to other plant
(c) Produces leaves and displays
them to light
(d) Supports leaves, flowers and
(e) Stores food reserves in some
plants -- Irish potato, Johnsongrass,
cabbage hearts, etc.
--MALE FLOWER FUNCTION:
(c) Food storage in
*Refer to Unit IV
(a) Absorb water and raw
(b) Anchor plant
(c) Store food reserves --
in some crops
FIGURE 1. Functions of Leaves, Stems, Roots, and Flowers
(b) Soil fertility, and acidity. Adequate fertilization and liming can cause a
plant to extend its root system. This is probably a secondary effect. A well-
fertilized plant produces more food in the leaves. This food is then available
for the roots to use as energy to grow farther into the soil.
(c) Soil aeration and structure. Tight, compacted soils, and hardpans often
limit root growth.
4. What are the kinds of:
Leaves are simple and compound in type. A leaf is said to be
simple when the blade is all in one piece, as in the case of the leaves of
corn, and apple, oak, and elm tree leaves. A compound leaf is one that
is divided into two or more leaflets, as is true with leaves of the locust
tree, clover plant, and roses.
Stems may be classified as:
(1) Soft stems -- corn, herbs, etc.
(2) Woody stems -- cotton, shrubs, etc.
(3) Single woody trunks -- trees
c. Roots ?
When seeds begin to grow, the young root is usually the first part
to emerge from the seed coat. The first, or primary root, lengthens
rapidly, and quickly penetrates downward into the soil. After a short
period of growth, branches of secondary roots begin to appear. This takes
place first near the top of the primary roo-fthen proceeds downward.
Root hairs form on these secondary -- lateral branch -- roots.
There are two kinds of root systems:
(1) The taproot, or primary root system, and (2) the fibrous system.
In plants having taproots, the primary root continues to grow down-
ward. It remains the largest root of the root system and is know as the
taproot. The taproot, with its branching secondary roots, is the taproot
system. Some taproots are fleshy, as in the case of turnips, carrots, and
beets. Others, like the taproot of the pecan tree, are woody.
Corn, grasses, and many other plants have a different type of root
system, known as the fibrous system. In these plants the primary roots
live only a short time. TFIepermanent root system grows from the nodes --
joints -- at the base of the stem, or stalk, as in the case of the corn plant,
and are known as brace roots. These are adventitious roots since they
arise from the stem tissue rather than from a primary root as described
above. Fibrous roots usually have more small branches than taproots. The
roots of this type system are ideal for absorption of water and minerals,
and help hold the soil together. Many fibrous-rooted plants are also deep
FIGURE 2. Kinds of Roots.
Root hairs develop near the tip of all roots. These are the tiny,
white, fuzzy hair-like growth that develop in the root hair zone near the
end of the lateral root branches. Their function is that of absorbing most
of the water and minerals used by the plant. As the root tips move down-
ward and outward into the soil new root hairs form near the tip. Those
formed earlier wither away. Root hairs are thought to give off acids or
other substances which aid in dissolving minerals and breaking up soil
particles. Sometimes, root-secreted acids etch away sides of rocks leaving
Adventitious roots are of several kinds -- those that develop above
the ground from the stem, or even from leaves. The roots which grow from
the joint of the corn plant, just above ground level -- prop or brace roots
-- are often classified as a part of the secondary roots, but are actually
a kind of adventitious root. These roots actually grow into the ground,
develop many small branches, and become the major part of the total root
system of the plant. (If soil is piled around the stem additional brace roots
develop from the next joint above the soil line.) Another example of
adventitious roots are the aboveground roots of Bermudagrass, English Ivy,
Boston Ivy, Poison Ivy, and Winter-Creeping Euonymus. These, and many
other climbers or runners, produce climbing roots or clusters of roots at
joints along the stem. The roots of many of these plants cling to a wall or
to some other type of support, attaching the stem securely. Stolon roots,
an adventitious type, as on the strawberry, first appear aboveground on the
ends of stems or runners. Later they take root in the ground and develop
like normal roots.
Some plants have a special kind of root, different from that of most
plants of agricultural importance. Aquatic, or water, roots are found on
such plants as the water hyacinth. The roots of the bald cypress tree,
native to the Southern United States, usually grow in swamps and are
rooted in water-covered soil. These roots, known as "cypress-knees",
extend above the water to help brace the tree and supply air to the sub-
merged roots. In general, plants living in submerged habitats, such as
rice, have modified conducting tissues.
Some tropical plants produce aerial roots. This is true with tropical
orchids that live on trees. Their aerial roots absorb water from the atmos-
phere. The small amount of debris collecting around their roots is
sufficient to supply their mineral requirements.
5. How do roots absorb water, and other needs from the soil ?
Plants absorb water through their root system, primarily the root hairs.
This takes place through a diffusion process called osmosis. Osmosis, is de-
fined as the diffusion or passage of a solvent (eg., water) through a differen-
tially permeable membrane. This direction of movement is from a region of high
concentration (of the solvent) to one of lower concentration. Another way of
stating it is, that the water moves from a place where it is more active to one
where it is less active.
The water, after being absorbed through the outer layer of root cells,
passes from cell to cell until it reaches the xylem. Upon reaching the xylem
tubes in the roots -- we might think of these tubes as water pipes in a plumb-
ing system -- it is transported upward through the stem; from there the route of
travel is through the leaf veins into the leaf where the water is combined with
carbon dioxide to make simple sugar. This process of sugar formation is called
Mineral elements in chemical solution in the soil also enter the plant
through its root system. These chemical elements are carried upward in the
water and move with it to all parts of the plant. Using the simple sugar formed
by photosynthesis as the "building blocks" and the chemical solution as "mortar"
the plant builds all other substances needed for its growth -- wood, fiber, vita-
mins, proteins, protoplasm, cell walls, etc. This will be discussed later in
Chapter IV. This upward flow of substances, or materials through the "pipes",
is called the sapstream.
The upward movement of water -- the sapstream -- is primarily due to the
"pull" caused by evaporation of water from the leaves. If the evaporation
through the leaves -- transpiration -- results in the plant losing more water
than the root system can supply the plant will wilt.
In a plant which is not wilted the cells are full and firm just as a paper
sack or a balloon is firm when filled with air or water. This firmness, or
turgor pressure, in cells exists because the minerals, sugars and other sub-
stances in the cell have an affinity for -- attract -- water which enters through
the cell membrane by the process of osmosis. Osmosis is the diffusion of water
through a differentially permeable membrane from an area of high water concen-
tration -- low sugar -- to an area of low water concentration -- high sugar.
When the inside of a cell has a high concentration of dissolved substances,
water will diffuse into the cell until the pressure built up by the addition of
water is high enough to prevent more water from entering the cell.
The water lost by transpiration is replaced either from the water in the
cells or by water which the roots absorb from the soil. If the roots are unable
to supply water to the plant, then the cells which lose water become flaccid --
limp -- and begin to collapse. Wilting occurs when enough water is lost from
the plant cells to cause them to lose their firm and distended condition.
By understanding the way absorption takes place it can be seen why the
transplanting process can be so hazardous for a plant. Great care must be under-
taken to keep a large ball of earth around the roots or wilting will occur, and
the plant experiences so much "shock" that it will die. It is usually impossible
to remove a plant from the soil without removing a large part of the root system.
The main water-absorbing part of the system -- the root hair zone near the root
tips -- is likely to be left in the soil when the plant is removed. The type of
plant, deciduous or evergreen, the time of planting, and the quality of the
setting job also greatly influence the outcome of transplanting. Deciduous
plants, or those that shed all their leaves during the dormant or resting period
(winter), experience less shock than evergreens and may be transplanted with
a higher degree of safety. Plants without leaves have very little surface from
which water can evaporate. Many deciduous plants are transplanted bare-root
during the dormant season, usually between mid-November and the last of
February in Mississippi; however, even deciduous plants that are transplanted
with a ball of earth on the roots experience less shock than they do when plant-
During the dormant season the growth requirements of plants are greatly
reduced, particularly for deciduous plants. This means that during this period
plant growth stops except for root growth which decreases considerably. With
this great reduction in transpiration of water, transplanting can be done during
the dormant period with a greater degree of safety. But keep in mind, that
even during the dormant periods evergreens should be transplanted with a ball of
earth, and that even deciduous plants live and grow better when transplanted
with the soil left intact on their roots.
Some of the aboveground plant growth should be pruned at, or prior to,
the time the plants are dug. Usually about one-third should be removed, with
the amount varying due to the time of transplanting, size and condition of plant,
and the quality of the setting job. Since all of the roots are not removed from
the ground this pruning is necessary to "balance" the aboveground growth with
the remaining root system. In other words, the roots that are left on the plant
that is being transplanted cannot continue to supply the requirements of the
6. How do plants respond to their surroundings?
At the tip of the young root there is a root cap, that serves to protect
the delicate end. As this cap pushes its way through the soil by the growing
root behind it, its outer surface is worn away. New cells, added to the inner
surface, keep the cap repaired.
Immediately behind the root cap, at the tip of the root proper, cells are
constantly dividing. This region is the growing point of the root. Behind the
growing region newly formed cells are constantly elongating, causing the root
tip to penetrate further and further into the soil.
The roots as well as the entire plant, respond to various stimuli. The
response of a pat, or any of its parts, toward or away from a stimulus, 7W
known as Tro-ism. IFthe respnse is toward the stimu us, as in the case oT a
root growing into the soil, the response is positive. However, if the response
is away from it, the tropism is negative, as in the case of the stem of the corn
plant growing away from the earth. With the aid of these tropisms the plant
is able to adjust itself to its surroundings, enabling it to make the best possible
use of its environment.
There are several kinds of tropisms:
Geotropism refers to the response caused by gravity, which strongly in-
fluences root growth. Geotropism directs the growth of the root toward the soil
and its supply of water, gases, and minerals. Without this stimulus, roots would
not likely grow deep enough to reach these vital substances.
Regardless of the position in which seeds are planted the primary root be-
gins to grow downward. Therefore, there is no need to worry about seed plant-
ed upside down. The positive influence of gravity causes the radicle, which be-
comes the primary root, to grow downward while the hypocotyl and epicotyl --
or stem, which becomes the stalk, grows upward in response to negative tropism.
Phototropism refers to the plant's response to light; however, plants vary
in their requirements for light. Some prefer full sun, while others do better in
varying degrees of shade.
In general, light tends to slow down the elongation of plant cells. When
a plant is lighted from one side the cells exposed to the light cease to become
longer while the cells on the shaded side continue to elogate. This difference
in extension of cells causes the stem to bend in the direction of the light source.
If the plant turns enough toward the light so that the light intensity is equal on
all sides then the curving movement stops and the plant continues to grow
straight toward the light. Such turning movements can take place only in
young plants or young stems which have not developed woody tissue.
A further illustration of the advantage of certain tropisms to plants is
furnished by the recovery of the normal position of plants such as corn, oats,
etc., that have "lodged". Because of heavy rains or wind, or both, these plants
are at first prostrate, or semi-prostrate on the ground. If the plants are not too
old they are brought again to their normal upright positions through phototropic
(light), and geotropic (gravity) response.
The following is a summary of tropisms affecting plants:
Roots do not grow in search of water and minerals, as commonly believed,
but roots in moist areas grow faster than roots in the very dry areas.
///. Reproductive System and Reproduction
Up to this point we have discussed the plant parts, or organs, which are
primarily concerned with the vegetative functions -- those through which the in-
dividual plant provides for its own needs. The function of reproduction has noth-
ing to do with the welfare of the individual plant's vegetative function. Actually,
in many plants, reproduction is always a forerunner of the death of the individual
plant. This is true in annual and most biennial plants which die soon after the
formation of flowers and seeds; only the seeds remain alive. The reproductive
function is to "give birth" to new individual plants and thus to perpetuate and
multiply the species and to scatter them over a larger area.
In this unit we shall discuss flowers, fruits, and seeds, the plant struc-
tures which directly relate to reproduction. We shall discuss the various processes
by which reproduction is accomplished -- pollination, fertilization, seed develop-
ment, and germination. In addition, other things such as the various methods of
reproducing, and the classes of market seeds will be covered.
Let us begin the study of plant reproduction with seeds -- what they are,
how they are borne, formed, and so on.
1. What are seeds?
Seeds are many things. Above all else they are a way of survival of
their species. They protect and sustain life. Seeds are food for man, and animals,
and other living things; and they are raw materials for the fashioning of a multitude
of products by people. They can be of great value to mankind, or they can be a
source of trouble. Seeds are containers of embryonic plants, the embryos of a new
generation. Thus, seeds are many things, but everything about seeds -- their num-
bers, forms, and structures -- has a bearing on their main purpose, to insure con-
tinuing life. They contain, in addition to an embryo, seed coats, and stored food
which nourishes the young plant from the time it starts to grow until it can produce
its own food by photoshythesis (Unit IV -- Plant Growth). )
2. How are seeds borne?
Seeds are borne by two great and different classes of plants. These
classes are the gymnosperms, and the angiosperms. (Refer to Unit I, question 1,
"B", in this publication.) The gymnosperms or "naked seed" plants, have no ovaries,
no flowers, and no fruits, although they do have seeds. The gymnosperms include
the conifers which bear their seeds in pairs at the bases of the scales of the cones.
In plants of the more highly developed and much larger class, the angio-
sperms -- vessels for seeds -- the ovule and the seed develop within an ovary, the
seed vessel. The ovary is the part of the flower that contains the ovule with its
egg, or female sex cell. The ovary later becomes a fruit with the developed ovule
or ovules -- seeds -- inside.
Deep within the ovary of the mother flower -- or between the scales of
a seed cone -- lies the ovule, which contains an embryo sac and its tiny egg. The
egg must be fertilized by a sperm cell from a pollen tube before it can start to de-
velop into an embryo and so perpetuate the parent's life.
3. What is the structure (parts) of a seed?
Seeds vary in size, shape, and manner of developing. The bean seeds
are typical of dicotyledonous plants and the corn kernel is illustrative of the mono-
cotyledonous type. (Refer to Unit I, question 1, "B".) These seeds will be used
in discussing the parts of seeds, and their functions.
A. The Bean Seed -- Its Structure, Development, and Early Growth.
The mature bean seed is usually kidney-shaped. It starts out as a
mass of tissue, the nucellus which develops into the ovule. In one part
of the ovule is the embryo sac, containing the egg cell. The ovule is
covered by two coats, or integuments, and is attached to the ovary wall,
or placenta, of the fruit by a stalk, the funiculus, through which it re-
ceives nourishment. The integuments are slightly separated at one point;
this opening, through which the pollen tube usually enters is called the
SEED COAT OPENED
Cotyledon scar -~ -,- Plumule
FIGURE 3 Diagram of the Structures of a Bean Seed.
During the development of the ovule into a seed several changes
(1) The integuments become firm and hard, forming the seed
coat, the testa.
(2) The fertilized egg cell divides into many cells which dif-
ferentiate into the following parts of the embryo:
(a) Cotyledons, two seed leaves. These are a part of the
new plant, and are filled with food -- mainly protein
and oil -- which will be used during germination.
(b) Plumule, the stem tip -- often with fairly well-develop-
(c) Hypocotyl, which connect the root and cotyledons, and
(d) Radicle, the embryonic root.
(3) The stalk drops off, leaving on the seed a scar, the hilum.
Ripe bean seeds have no endosperm, a substance contained in seeds
of monocotyledonous plants. The endosperm is stored food which is used
for germination and early growth. As the bean seed grows in the pod,
the embryo absorbs the endosperm about as fast as it is produced. The
food from the endosperm is usually totally absorbed into the embryo -- the
two cotyledons -- by the time the bean is ripe.
The growth, or germination, of the plant takes place as shown in
the following steps:
Terminal bud .
Steps in the Germination of a Bean Seed.
(1) The first step is the absorption of water an oxygen into the
seed. This permits the use of stored food for the energy
necessary to carry out the remaining steps.
(2) Then the young root breaks through the testa, and soon be-
comes imbedded in the soil.
(3) The hypocotyl curves into a loop form, the top of which is
usually called the arch. The hypocotyl grows rapidly in
length, causing the arch of the loop to push its way to the
soil surface, often turning from side to side around soil par-
ticles. After the surface is penetrated, the cotyledons are
lifted up as the arch straightens.
(4) Finally, the cotyledons spread apart, exposing the plumule
to air and light. The cotyledons serve as leaves in that the
stored food in the cotyledons is transferred to the other
growing parts of the plant. This food is usually sufficient to
form the first leaves on the plumule. As new leaves form
they take over the food making functions for the plant. In
some species, such as cotton, soybeans, and radishes, the
cotyledons turn green and function in the food making pro-
cess. The cotyledons shrivel as their stored food is used and
are usually shed as soon as the young seedling plant has
absorbed its food. After this shedding, the stem and leaves
continue to grow.
After several weeks the bean plant is large enough to produce flowers,
which develop into pods containing seeds, thus completing the life-cycle. (The
structures of a bean seed are shown in Figure 3.
B. A Corn Seed -- Its Structures, Development, and Early Growth.
A seed of corn, commonly called a grain or kernel, is actually a com-
plete fruit; therefore, it really corresponds to the bean pod and its contents
rather than to the individual bean seed. Yet, there is only one embryo in
each grain and it completely fills the fruit. The outer coat of the kernel is
formed from the ovary wall of the flower.
The kernel of corn is similar to the individual bean seed in that it:
(1) Contains an embryo, the young seed plant
(2) Contains food for use of the young seed plant during its early
(3) Has marks upon it.
Located on one side of the kernel is a depression beneath which lies the
tiny embryo, or young plant. On the top of the grain there is a slight promi-
nence, the silk scar which marks the place where the silk -- style of the fe-
male flower organ -- was attached. This is more prominent in popcorn than it
is in dent varieties. At the base there is a stalk ')y which the kernel was at-
tached to the cob during its development.
In the bean the cotyledons are part of the embryo and the stored food of
the seed is already in the embryo itself. Corn differs from the bean in the re-
lative size and position of its embryo, which is at one side of the food supply.
This food supply, the endosperm, is composed primarily of starch. The endo-
sperm is formed in the ovule but has not been absorbed into the embryo as was
the case in the bean. Corn has a single cotyledon called the scutellum. This
cotyledon is the part of the embryo that digests and absorbs the food for its
use. The cotyledon is attached to the embryonic axis and is surrounded by the
starchy endosperm. It never appears above the ground -- as is true in the case
of beanseed. The corn embryo has its leaves rolled into a tight, pointed bud,
the plumule, an adaptation which enables it easily to pierce the earth above
it. Adventitious roots, called "prop roots", grow out from the stem to support
the plant. The true root, or radicle, grows from the lower part of a short hy-
(The structures of a bean seed are shown in Figure 3.
4. How are seeds formed?
A. Flowers and Seed Formation.
A flower exists to produce seed.
(1) REPRODUCTIVE ORGANS.
To do this the following organs
_ Staminal column
FIGURE 5 Diagram of a Cotton Flower.
(a) Stamens -- pollen producing parts.
Another -- pollen bearing part of stamen.
Filament -- stamen part supporting the anther.
(b) Pistil -- ovule-producing organ, within which egg cells
Stigma -- pollen-receiving structure.
Style -- part of the pistil that supports the stigma.
Ovary -- part of the pistil that produces the ovule or ovules.
Ovule -- structure containing the embryo sac in which the
egg is found.
(2) FLORAL ENVELOPE*.
The reproductive organs are the most essential -- they are
necessary if there are to be seed; however, organs of another type,
called accessory organs, protect the essential organs of the flower
from destruction, and attract insects. These organs are the:
(a) Sepals -- outermost leaflike parts covering the flower be-
fore it opens ; collectively known as the calyx.
(b) Petals* -- leaflike structures above and inside the calyx
that are usually brightly colored; collectively they are call-
ed the corolla. Many petals attract insects, humming birds,
and other creatures -- due to color, odor, and nectar secre-
*Both sepals and petals -- or floral envelope -- are missing in some
angiosperms; in some, only the petals are missing. In grass flowers,
they are much different in shape. (Refer to Types of Flowers be-
Types of Flowers
A flower that has all four organs -- both reproductive, and both floral --
is called a complete flower. One that does not is called an incomplete flower.
In addition to being complete and incomplete, flowers are classified as
being perfect and imperfect. Perfect flowers, as in apples, etc., are those with
both stamens and pistils. If either the stamens or pistil is missing the flower is im-
perfect -- although other parts may be present -- as in corn, pussywillow, and
Complete flowers, as mentioned previously, have all flower parts (repro-
ductive -- stamens and pistil, and floral -- sepals and petals) in the same flower and
may be self-pollinating, as in flowers of cotton, peas, beans, wheat, roses, and
If the flowers are imperfect, sometimes the staminate (male) and pistillate
(female) flowers are found on the same plant, and the plants on which they grow are
said to be monoecious. (Monoecious is derived from the Greek words meaning "one
house".) Flowers of monoecious plants are frequently cross-pollinating, but in some
cases self-pollination occurs when pollen from a flower reaches the stigma of a fe-
male flower on the same plant. Examples of monoecious plants are corn, water-
melons, cucumbers, and other members of the gourd family. (Corn is both self- and
cross-pollinating.) On the other hand, individual plants of some species -- such as
hollies, willows, cottonwoods, and many fruit trees -- have imperfect flowers with
only one sex on the plant. Plants of these species are said to be dioecious. This
means that flowers of one plant may have only stamens -- male, or pollen-producing.
parts, while flowers of another plant may have only pistils -- female or ovule-
producing parts. The imperfect flowers of dioecious plants must be cross-pollinated
Female plants of this type require the presence of pollinators -- bearing male
flowers -- within a reasonable distance of insect flight, or for wind pollination.
B. The Pollination and Fertilization Process.
Before the ovule, or seed, can begin to develop, the egg cell (female)
must be fertilized. This development begins when the male sperm from the pollen
unites with the egg cell in the ovule.
Pollen is distributed by various means: gravity, wind, and man, animals,
insects, humming birds, and other living things.
The following is an example of the pollinating process in a complete
flower. (The process is similar in perfect and imperfect flowers, except that the
former may be self-pollinated while the latter are cross-pollinated.)
Pollen from the anther is distributed by one or more means, as mentioned
previously, to the stigma. The stigma receives the pollen, which germinates and
develops a slender, threadlike tube that grows through the style, into the ovary and
finally reaches the ovule and the embryo sac. The two sperms in the pollen grain
move through this tube and enter the embryo sac. Union of one sperm and the egg
forms the zygote. The second sperm unites with two other nuclei in the embryo sac
and forms an endosperm cell.
Another example of how seed are formed is shown below, using the corn
plant to tell the story. (Keep in mind that these plants are monoecious -- their
reproductive organs are formed in flowers on separate parts of the plant.) Pollen
from the staminate flower -- male flowers on the tassel -- upon reaching the pistil-
late flower -- ovaries and silk on the cob -- develops a pollen tube which grows
through the style, or silk, to the ovules, or "blisters" on the cob. Each blister is
an ovary which contains only one ovule. One sperm cell unites with the egg in the
embryo sac and the other unites with two other cells in the embryo sac to form a
primary endosperm nucleus. As development continues, the endosperm and embryo
enlarge and the mature grain contains both endosperm and embryo. (This is differ-
ent from a bean development. In the bean the embryo absorbs the endosperm almost
as fast as it is formed and by the time the bean is mature all the endosperm has been
transferred into the cotyledons.) If the ovule is not fertilized, then no seed deve-
lops and a bare spot remains.
The flowers on one tassel may furnish as many as 15,000,000 pollen grains;
however, only one pollen grain is needed for the development of each kernel, or
grain of corn. The pistillate -- female flowers -- number approximately eight
hundred. It is estimated that not more than 5 percent of corn kernels are fertilized
by self-pollination. In other words, corn is both self- and cross- pollinating.
A summary of the development of reproductive plant structures leading to
seed formation may be given in five steps:
FIGURE 6 How Plants Reproduce (sexually).
(1) The formation of reproductive parts -- stamens and pistils in flower
(2) The opening of the flower -- anthesis, which signals the sexual
maturity of those organs
(3) Pollination, which consists in the transfer of the pollen from the
stamen to the pistil; germination of the pollen; the formation of
the pollen tube; and union of the sperm with the egg -- fertiliza-
(4) Growth of the fertilized egg and its differentiation into an embryo,
plus a surrounding coat -- the seed; and
(5) Maturing of the seed, usually with an accumulation of stored food,
either in storage organs of the embryo -- cotyledons -- or as
endosperm, or a combination of the two.
5. What is the relationship of fruits to seeds?
As mentioned previously, fertilization ends the work of the flower. As
the sepals, petals, and stamens wither, seed development hormones force the plant
to provide its full energies for the development of the ovary and its contents, the
ovules. Later the ovary and its ovules ripen. In many plants, fruits develop. A
fruit can be defined as a ripened ovary, with or without its associated parts. On
the other'hand, a seed is the matured ovu-Yte-tat s enclosed in the fruit. For ex-
ample, an apple is partial ly composed of the ovary, and inside This fleshy fruit are
the seeds from which new plants grow.
One important fact about seeds must be remembered: that the new plant
grows from a seed -- not from a fruit. Yet, the fruit is very important to the seed
in that it surrounds the seed and protects it from water loss, insect and disease at-
tack, and other dangers while it is developing; and later the fruit may serve as a
vehicle for distributing the seeds. Fruits may serve as food for man, animals, and
birds, or may serve as a wing or parachute for dispersal.
6. How are fruits classified?
True fruits develop directly from the ovary and include the berry, stone-
fruits, nut, grain or kernel, pod, and capsules.
Fruits are first classified into fleshy and dry structures. They are further
classified as to type, and then dry fruits are further classified as dehiscent and in-
dehiscent. Dehiscent refers to those fruits that split along definite seams, such as
beans, peas, etc. Indehiscent, on the other hand, refers to those that co not open
along definite seams when ripe.
The following is a summary of fruits according to their types:
Classification of Fruits (according to type)
Pome Apple, pear, quince
Stone, or drupes Peach, plum, cherry, apricots
Berry Tomato, grape
Modified berry Orange, tangerine, lemon, cucumber,
Multiple fruit* Mulberry, pineapple
Aggregate fruit* Blackberry
Accessory fruit* Strawberry
Dry Fruit (Dehiscent)
Pod Bean pod, pea pod, peanut(in the shell)
Capsule Cotton boll, iris
Dry Fruits (Indehiscent)
Nut Pecan, acorn, hickory nut
Grain Grasses, corn, oats, wheat
Winged, or key fruits Maple, ash, elm
Achene Sunflower, dandelion, buckwheat.
*A multiple fruit is one formed from a number of flowers.
An aggregate fruit is one formed from a single flower; but rather than a single pistil it has
a number of ovu-containing organs called carpels, which enlarge the Form of fruit.
An accessory fruit is one in which a nonfloral tissue has enlarged. All members of these
types are not necessarily fleshy.
7. How are fruits and seeds dispersed?
To fulfill their functions, seed must be scattered. Otherwise, (1) the
seedlings would choke one another if all were dropped together; (2) growth would
be retarded; and (3) extermination, in some instances, might occur -- some unfav-
orable condition might kill all of them.
Wind, water, man, and animals may serve as distributing agents to scatter
seed, as may natural mechanical devices such as exploding pods. To be distributed
by wind, a seed or a fruit must, or course, be light. Plumes, tufts, down, and
wings enable the wind to carry them away. In the case of the tumble weed, the
entire aboveground growth breaks off, and is blown by the wind, scattering seeds as
it rolls along the ground.
8. What are the principles of germination -- conditions necessary, and relation of
When a seed is fully matured it is ripe. Some seeds are able to grow or
germinate within a very short time. Others must remain in dormant state, or resting
period, during which life processes take place very slowly. This period may extend
from only a few days, as in the case of the willow, to several years. Some have
even remained dormant for almost a hundred years, and then germinated when con-
ditions became favorable. Many seeds remain dormant throughout the winter season
and germinate during the following summer. This is true in colder climates of the
seed of annuals, as well as some perennials. In many cases, as foresters have
learned, cold weather increases the percentage of seeds that germinate in the spring.
The ability of seeds to grow is called viability. Viability of seeds depends on:
(1) amount of food stored in the endosperm and cotyledon, or cotyledons; and (2)
dormancy conditions, such as heat, drowth, and cold, which may be enemies of the
embryo, even though it is enclosed in protective seed and fruit coats. Warmth and
moisture lower viability, while cool, dry places are ideal for storing seeds.
Purchasers should check the viability of seed. Tags on sacks of commer-
cial seed show the viability of the seed. Seeds in containers marked as being 90%
viable means that 90 seeds out of every 100 seeds planted should germinate. This
is not necessarily ture, however, since viability may vary because only relatively
few samplings are made.
Certain conditions are required for germination. Most seeds require at
least three: (1) moisture, (2) correct temperature, and (3) oxygen.
Some seeds require light to germinate properly, and if buried in the soil,
will not germinate. This is true of many of the small grass seeds. Considerable
variation in each of these requirements exists among different kinds of plants.
Prior to germinating, seeds usually absorb considerable water, causing
them to swell and their seed coats soften. Too much warm moisture during the period
of germination and later growth encourages the growth of fungi and bacteria which
may result in seed decay. Cold wet conditions will do the same thing. Fungi often
grow at cool temperatures.
Seeds of many water plants germinate under water where there is enough
moisture, a fairly even temperature, and oxygen which is dissolved in the water.
This, of course, is not true with most land plants. Even though they need moisture
they cannot germinate under water because they need more oxygen than will dis-
solve in water. Land plants require less moisture. Some seed, such as the seed of
plants that grow in arid places such as deserts, are able to germinate with the minute
amounts of moisture supplied by dew.
During the germination process, the cells of seedlings are rapidly divi-
ding, requiring a much higher rate of respiration than older plants. This is the rea-
son why the oxygen supply of seedlings is critical. This necessitates that the soil
in which seeds are planted be loose, and that the seeds be planted sufficiently near
the soil surface. The required depth for planting is dependent primarily on the size
of the seed, soil conditions -- temperature, water, and the type of seed -- mono-
cotyledon or dicotyledon, etc.
The food stored in seeds undergoes changes during the process of germi-
nation. The embryo produces an enzyme known as diastase which changes starch
to sugar, and the cells of the embryo absorb the sugar. This change accounts for
the sweet flavor of sprouting seeds. In many seeds the stored food is in the form of
oil. Oil-digesting enzymes convert the oil to simple compounds which the plant
9. How are plants reproduced -- other than by true seed?
Plants, other than being propagated or multiplied by true seeds or sexual
reproduction, may be produced by vegetative means, known as asexual reproduction.
Asexual methods include grafting, budding, layering, and other methods essential
in reproducing plants by buds.
Vegetative propagation is used for two basic reasons:
(1) Some plants do not normally produce seeds and, therefore,
must be reproduced vegetatively, as in the case of the seedless grape,
sugarcane, and certain sterile hybrid plants. Then other plants, such as
the common English Ivy, rarely produce flowers. Vegetative propagation
is the most economical way to produce them.
(2) Many seed-bearing plants do not breed true; in other words,
seedlings that grow from their seeds do not closely resemble the parents.
This happens with many species -- tree fruits, wood ornamentals, grasses,
sweet potatoes, bulbs, and others. They would generally resemble their
parents, but the identifying characteristics that made the originals famous
would be lost. For example, seedlings from pecan or peach seed would
produce fruits, but these would probably not possess the desired variety
characteristics -- the fruits would be smaller, and otherwise vary. This
is usually the case when the parent is a hybrid. Seeds produced on hy-
brids will grow into plants which show many different combinations of
the characteristics found in the parents of the hybrid. However, a bud
taken from a hybrid plant will develop into a stem, and finally into a
plant, which is in reality the original hybrid plant.
Vegetative reproduction has its limitations. Although it is a high-
ly desirable means of maintaining the "status quo" in a variety, its use-
fulness is restricted to this purpose. Consequently it cannot be used in
developing new varieties, except in extremely rare instances when "sports"
or mutations occur. "Sports" are plants, or parts of plants, that abruptly
show c noticeable different appearance from the rest of the plant. "Sports"
may be in the form of changes in colors of the fruit, leaves, double
flowers instead of single ones, and so on.
When plants are increased by vegetative reproduction, the aim is
to stimulate the growth of buds on "borrowed" roots and stems.
D. Types of Buds.
There are essentially two types of buds of importance in vegetative
propagation -- normal and adventitious buds. Normally buds form at the
base of a leaf, or terminally on the plant stem. Buds may develop into
leaves, flowers, or stems. Some plants may be reproduced from buds by
removing a leaf with attached normal bud and rooting it, as in the case
of rooting many house plants such as the Begonia.
Adventitious buds, develop from previously differentiated tissue,
In posit ions -other -iithose in which normal buds are found. To illus-
trate: (1) Cuttings made from blackberry roots and set out can form ad-
ventitious buds which grow into new shoots, thus perpetuating the plant.
(2) A pecan tree set during the planting season leafs out during the fol-
lowing season, but the live buds die after leafing out takes place. The
stem or trunk, remains alive and buds underlying the trunk's outer surface
may force out the following growing season. These buds may be adven-
titious, or they may be "true buds"; that is, axillary buds which have
remained dormant since the trunk was first formed. (3) Another example
is that of adventitious buds resulting in sprouts that grow from a stump after
a tree has been cut. Many of these, however, are also "true buds".
E. Methods of Vegetative Propagation.
When plants are propagated by methods other than from true seed --
vegetatively -- the structure of the plant itself determines the type of
vegetative method that can be used. For example, pecans may be pro-
pagated by budding, or by grafting scions from papershell varieties onto
stocks of seedling plants; while strawberry plants are usually propagated
by "layering" stems, or runners.
Vegetative propagation may be applied to both belowground and
(1) Belowround parts. Belowground parts include roots, tubers,
stolons, rhizomes, ti lers, Tulbs, corms, and mycelium. Vegetative pro-
pagation of these parts is accomplished in a variety of ways. For example,
plants often are increased directly from true roots by two primary methods:
(1) Cuttings of whole roots, as if the case of Irish potatoes, which are
propagated by planting cut off portions that contain an "eye" or true bud,
or by planting the entire tuber; and by using (2) root suckers, bulbs, and
corms of ornamental flowers.
There are two variations of the usual root-cutting method. They
are induced suckering, and undisturbed whole-root cutting. Induced
suckering is accomplished by partially girdling or cutting an exposed root
in such a way that the partly severed root receives some nutrients from
the parent plant. Undisturbed whole-root cuttings merely goes one step
further. The exposed root is severed completely and the cut-off portion
is left in the soil.
(2) Aboveground parts. The regenerative process of propagating
plants by means of aboveground parts is accomplished in several ways, as
discussed in the following paragraphs.
(a) In layering, roots are sprouted on stems -- while they
are still attached to the parent plant. Layering may be
done by covering part of the plant with soil, or by an
air-layering process. The latter process is accomplished
by cutting a deep gash into the stem, wrapping the wound
with moist sphagnum moss, and then covering the area
with polyethylene plastic to keep in the moisture. (The
air-layering method is successful for many plants that
are hard to root by other methods.)
(b) In cuttings, plants may be regenerated by taking a leaf
or stemcutting from a plant and inserting it into a root-
ing medium. Roots form from the nodes or joints of the
cutting. There are three general classes of stem cut-
tings: namely, (1) from growing softwood; (2) semi-
hardwood, and (3) from ripened or mature wood Plants
that may be propagated in this way include the pine-
apple, mother-in-law tongue, camellia, gardenia,
begonia, coleus, and many others.
(c) In budding, a single bud is inserted into an incision be-
neat"hhebark of a stock of the plant on which it is to
grow. Shield, or "T" budding is the most common and
perhaps the easiest budding method. Roses, many fruit
trees, and other plants are propagated this way.
(d) In grafting, a part of one plant -- a scion -- is grafted,
or attached to the "stock", or understockk", in such a
way that they unite and grow. (The cambium layers of
the two parts are placed in contact with one another.)
Scions may be grafted on homoplastic -- like -- or
heteroplastic -- unlike -- stocks. An example of the
former is the grafting of a scion from a papershell pecan
tree onto the stock of a wild seedling pecan. Heteroplas-
tic grafting, for example, is done when pears are grafted
on quince rootstocks to produce dwarf pears; when lilacs
are grafted on ligustrums; and when plums are grafted on
peach rootstocks. In actuality, there are many such
grafts that can be made.
Note: The following references contain information on the asexual method
of reproducing plants:
(1) Plant Propagation Florida Department of Agriculture, Bulletin #178
(2) Home Propagation of Ornamental Trees and Shrubs, U. S. D. A.
(3) SEEDS, Yearbook of Agriculture, 1961, U. S. D. A.eWashington,
D. C. pp. 134-144
(4) University of Florida Extension Circulars #127A Mist Propa-
gation; 141B Propagation of Ornamentals by Layering; and #148
Home Propagation Units
You may also find helpful information in Botany and Biology Textbooks.
10. How are improvements made in plants?
Great strides have been made in improving plants, especially during re-
cent years. But there is evidence that the history of plant improvement dates way
back--undoubtedly to prehistoric times after man abandoned his nomadic habits and
settled in more or less permanent quarters.
The Arabs were the first people to recognize sex in plants. They learned
that they had to plant a few male trees in their date gardens to get dates. On
through the centuries man, in his humble way, continued to select plants and seed
of superior quality. Then, during the 19th Century, a botanist named Johann Mendel
made a great discovery. His findings, known as Mendel's Law, explains the inher-
itance of many characters in plants and animals from their parents. At first, his
findings were not accepted by other botanists, but were confirmed later--around
1900. This heralded a new world of knowledge.
The accomplishments during the period of time since Mendel's Law was con-
firmed have been many, especially during recent years. Accomplishments in the future
will probably be far greater as newer and more improved methods are discovered.
Plant breeders now have many tools and techniques to work with to improve
plants--sexually with seed, and asexually with cuttings, etc. There are two basic
methods of obtaining Improved plants:
(1) Selection of chance seedlings. This method is of considerable value
in developing new varieties. Because plants are not always like their
parents, and since many new varieties are actually mixtures of many slight-
ly different types, it is possible to get improved strains through the
selection of seedlings. The plant improver carefully studies his plants,
and if he finds superior individuals or a strain he selects and increases
it. Selection is merely a matter of saving seed from the superior plant
and keeping it separate from other seeds of the same crop. In vegetatively
propagated crops, selection of the superior plant consists of marking
the plant and using it as a source of buds, scions or other vegetative
parts, depending upon the method of propagation needed for that crop. For
example, most of our current major apple and pear varieties were selected
as superior trees many years ago and have been propagated vegetatively
Closely allied to this method is the process of plant introduction;
that is, obtaining plants from a different geographical or ecological
region which may have certain qualities not found in varieties at hand.
An example of this is Turkey wheat, introduced from Russia, which has
been valuable as a parent for further improved varieties.
(2) Controlled breeding. This is a more sophisticated level of im-
provement and involves several techniques, some of which become quite
complicated. The process of introduction of parent material, frequently
referred to as "germ plasm", is invaluable in these procedures. After
making controlled crosses between introduced strains -- breeding with
known parents -- the breeder may evaluate tens of thousands of seedlings,
and further evaluate hundreds of selections, before finding one worthy
of increase and development into a new variety. In the case of plants
normally propagated vegetatively, such as most fruits, and some vege-
tables, selection of a hybrid plant can be made, but in the case of
plants normally propagated from seed, such as most agronomic and vege-
table crops, much further work is necessary to insure that the plants will
come true from seed. By inbreeding -- self pollination -- selected
plants for several generations ( 6 to 8 or more) true-breeding lines are
developed. Each generation following can then be expected to closely
resemble the previous generation.
Hybridization is a valuable procedure in plant breeding beyond
those given in "1" above. After a hybrid is formed the variation in
later generations may result in plants which have the desirable charac-
teristics of both of the parents of the cross and be free of the defects
which these parents had. Other plants may be just the opposite -- pos-
sess mainly desirable characteristics. The plant breeder can, therefore,
select the superior plants and save seed only from them. He discards the
low quality plants. By controlled cross-pollination among good plants
or self-pollination of good plants he is able to develop true breeding
strains, which will produce only desirable plants. This possibility for se-
lection in a population which possesses new combinations of characteris-
tics of the parents has led to many new varieties which are much superior
to either of the parental strains. In other words, the breeder in this case
is inbreeding the hybrid, which he obtained by outbreeding his inbreds.
One method of insuring trueness to type from seed is the use of
hybrid seed from inbred -- true-breeding -- parents, a technique that is
epitomized by the tremendously successful use of hybrid seed corn. It
might be worthy of note that this is one of the techniques most coveted
by some of the Iron Curtain countries in their current attempts to improve
Method of producing hybrid seed corn.
(1) First, it is necessary to produce pure lines. This is accomplished
by in-breeding (self-breeding) open-pollinated varieties which have been
selected for highly desirable characteristics such as high production,
disease and drought resistance, early maturity, and resistance to lodging.
Plants are self-pollinated or "selfed". This is done before pollen-shed
by placing paper bags over the developing ear (the "shoot"), and another
over the tassel.
When the shoot has developed far enough for its silks to be fully
exposed beyond the husks the shoot bag is removed long enough for the
pollen collected in the tassel bag to be shaken onto the silks. In this
way the shoot is self-pollinated with pollen produced by its own plant.
Seed produced in this way are then planted the following year.
The most promising of these plants are bagged and self-pollianted. This
process is continued for a number of years, usually six years at least and
sometimes as many as ten. Each year the plants produced from one ear
become more and more alike, until finally the plants from any one ear
are so similar that all seem exactly alike. These similar-appearing groups
are known as "inbreds". It is possible to obtain many inbreds from each
ear secured from the original selfing. These vary in many ways, including
the number of ears per plant, ear height on the plant, the number of
leaves, and the color of the leaves and stem.
(2) After inbred lines--strains--are secured, the next step is to
cross these lines. This is done by planting two inbreds on rows side by
side. All plants of one inbred are detasseled prior to pollen shed. This
allows natural-(cross-) pollination by the other inbred, provided it is
isolated from other corn, as required by state regulations. Single-crosses
are the result. Ears on the detasseled rows had to be pollianted by wind-
blown pollen from the tow which had the tassel. Only these ears carry
(3) "Double-crossing" of plants the following year is the final step to
be accomplished before the seed are available to the farmer. Double-
crossing is done by crossing single-crosses. One single-cross is detassel-
ed and the shoots on these plants are pollinated by the other single cross.
In producing double-crosses the ratio of pollen-bearing plants to
seedbearing plants is usually 3 to 1. This means that 6 rows of seed-bearing
plants are bordered on each side by a row of pollen-producing plants.
(The ratio is usually 2 to 1 when producing single-crosses.) A large field
may have many blocks of plants arranged in this manner. Such fields are
kept isolated from other lines of corn, as required by state regulations.
Even though, regulations may vary according to the size of the field, in
all instances a certain number of rows of the pollen parent must surround
the entire field. The plants provide a sufficient number of "buffer zone"
plants to reduce the proportion of pollen blown in from other sources, to
a very low level.
INBRED A INBRED B
(BxA) PRODUCED IN
AN ISOLATED FIELD
PLANT, (BXA) PLANT, (C D)
FIGURE 7 Steps in the Production of Hybrid Corn.
11. What are the purposes of seed certification? What are the classes of
certified seed? Who supervises the certification?
Each state has its own seed certification practices, regulations,
and agencies. In Florida the Certification Seed Law is found in Chapter 575
of the Florida Statutes. The State Department of Agriculture has made avail-
able reprints of this law along with Rules of the Department of Agriculture
relating to this legislation. This reprint entitles CERTIFICATION SEED LAW,
RULES AND REGULATIONS may be secured from the Commissioner of Agriculture.
Selected portions of this reprint are given below.
CHAPTER 575-CERTIFICATION SEED LAWS
(1) Certification of Seed-Any grower of agricultural or vegetable seed,
located in Florida, may make application to the commissioner for inspection
and certification of his crop for seed purposes, under rules and regulations
covered by this chapter. The commissioner, or his authorized agents, shall
make all necessary inspections, issue official seals and tags for marking
containers of "certified seed" and "registered seed" as are necessary to safe-
guard the privileges and service provided.
(2) Fees for Certification-The commissioner may fix, asses and collect,
or cause to be collected, fees for the certification inspection service, the
same to be paid in such manner as he may direct. Such fees shall be large
enough to meet the reasonable expenses incurred by the commissioner or his
agents in making such inspections as may be necessary for carrying out the
provisions of this chapter.
(3) Rules and Regulations-The commissioner may mal(e all necessary rules,
regulations and standards to carry out the provisions of this chapter, after
notice of hearing to all growers of certified seed.
(4) Employees-The commissioner may employ such assistants, inspectors,
specialists and others as may be necessary to carry out the provisions of this
chapter, to fix their salaries and to pay same from such funds as may be avail-
able for the purpose.
CHAPTER 7E-5-RULES OF THE DEPARTMENT OF AGRICULTURE, DIVISION OF INSPECTION
(1) Purpose of Certification-The purpose shall be to maintain and make
available to the public, through certification, high quality seed of superior
crop plant varieties so grown and distributed as to insure genetic identity
and genetic purity.
(2) Fees-A fifteen dollar ($15.00) eligibility fee for each kind of
seed shall be paid by applicant before requests for certification will be accepted.
(3) Eligibility Requirements for Certification of Crop Varieties-Only
those varieties that are approved by a Technical Advisory Committee representing
the State Agricultural Experiement Station, Agricultural Extension Service, and
College of Agriculture, University of Florida, and accepted by the Florida
Department of Agriculture shall be eligible for certification.
(4) Classes and Sources of Certified Seed and Color of Tags Used-Four
classes of seed shall be recognized in seed certification: namely, breeder,
foundation, registered and certified. These classes of seed shall meet the
standards of the Florida Department of Agriculture for the respective crops.
These classes are defined as follows:
(a) Breeder seed (white tag) handled by breeder. Breeder seed is
seed directly controlled by the originating, or in certain cases the sponsoring
plant breeder or institution, and which provides the source for the initial and
recurring increases of foundation seed.
(b) Foundation seed (white tag) handled by Florida Foundation Seed
Producers Association. Foundation seed shall be seed stocks that are so han-
dled as to most nearly maintain specific genetic identity and purity and that
may be designated or distributed by agencies represented on the Official
Technical Advisory Committee. Production must be carefully supervised or
approved by representatives of this committee. Foundation seed shall be the
source of all other certified seed classes, either directly or through regis-
(c) Registered seed (purple tag). Registered seed shall be the progeny
of foundation seed that is so handled as to maintain satisfactory genetic identity
and purity and that has been approved and certified by the Florida Department of
Agriculture. This class of seed should be of a quality suitable for the pro-
duction of certified seed.
(d) Certified seed (blue tag). Certified seed shall be the progeny of
foundation, registered or certified seed that is so handled as to maintain satis-
factory genetic identity and purity and that has been approved and certified by
the Florida Department of Agriculture.
(e) At the discretion of the Florida Department of Agriculture, a
grower may be permitted to continue production of foundation, registered or
certified seed from lots of seed which were fully inspected but rejected be-
cause of factors which did not involve genetic purity.
(5) Handling of Crop Prior to Inspection-Roguing of off-type plants,
objectionable crop plants, and weeds is required prior to field inspection.
(6) Seed Handling and Curing-Harvesting, curing, cleaning, and bagging
of certified seed is required to be done under supervision of the department's
inspector and shall be performed only on the grower's premises or in commercial
cleaning and curing establishments which have been inspected and approved by
the operator and inspected and approved by the department's inspector prior to
processing each lot of certified seed.
(7) Samples and Sampling of Seed-A representative sample of two pounds of
each lot of seed as it is offered for sale shall be obtained by the Florida
Department of Agriculture. Not more than two samples of any lot of seed will
be drawn after grower has finished cleaning and processing his seed; then the
average germination of the two samples will be the percentage used-not the
highest germination found.
(8) Tags and Seals-
(a) All stocks of certified seed shall have an official tag and seal
properly affixed to each container prior to sale. Sealing requirements will
depend upon the crop and methods of handling.
(b) All official certification tags and seals will be supplied by
the Inspection Division, Florida Department of Agriculture, at a reasonable
cost and must be affixed to seed containers under the supervision of its
official agent; the tags and seals complying with the certification standards.
All official tags will carry statement of certification and under what authority,
and the following information: kind, variety (or strain), stock number and
(a) Field inspections. Field inspection shall be made prior to
harvest when the crop is in bloom or at such time as the condition of the
crop can be best determined.
(b) Harvesting and curing inspections. Inspections will be made
during the period of harvesting and curing to insure proper cleaning and
identification of the seed in transit from the harvester to the curing or storage
plant. Responsibility for thoroughly cleaning the threshing machine or har-
vester rests with the grower.
(c) Processing inspections. The processing, warehousing, pack-
aging, sealing and tagging of seed containers with official certification
tags will be made under the supervision of the department's inspector, who
will have access at all times to the warehouses or storage places for per-
forming his official duties.
(10) Bags or Containers-All certified seed must be sold in bags or con-
tainers approved by the Florida Department of Agriculture.
In this connection it may be well to include information as to Florida's
laws regarding all seed provided for sale. A reprint of the Florida Seed Law,
Rules and Regub tions (Chapter 573, Florida Statutes) is also available from
the State Department of Agriculture. Some of the provisions of this law
are given below:
(1) Every person, except as provided in subsection (4), before selling,
distributing, offering for sale, exposing for sale,handling for sale, or soli-
citing orders for the purchase of any agricultural or vegetable seed or mixture
thereof, in the state, shall first register with the commissioner as a seed
dealer, giving the name and location of each place of business at which such
seed are sold, distributed, offered for sale, exposed for sale or handled for
sale and the name and address of each representative soliciting orders for
purchase of any agricultural or vegetable seed, and at the time of registra-
tion shall pay to the commissioner an annual registration fee for each such
place of business or each such representative based on the gross receipts
from the sale of such seed for the last preceding license year.
(2) A receipt or acknowledgment from the commissioner of such registra-
tion and payment of such fee or fees shall constitute a sufficient permit for
such dealer to engage in or continue in the business of selling, distributing,
offering or exposing for sale, handling for sale or soliciting orders for the
purchase of any agricultural or vegetable seeds within the state until the
first day of July next thereafter, subject to compliance with the other require-
ments of this compliance with the other requirements of this law.
(3) Every person selling, distributing, offering for sale, exposing
for sale, handling for sale, or soliciting orders for the purchase of any
agricultural or vegetable seed in the state, other than as provided in
Section 578.14 shall be subject to the requirements of this section. Pro-
vided, that Florida state agricultural experiment stations shall not be
subject to the requirements of this section.
(4) The provisions of this act shall not apply to farmers who sell un-
cleaned, unprocessed, unpackaged and unlabeled seed, but shall apply to far-
mers who sell cleaned, processed, packaged and labeled seed in amounts in
excess of five thousand dollars worth of cleaned, processed, packaged and
labeled seed of any farmer shall be exempted from the provisions of this
(5) Label requirements-Each container of agricultural or vegetable seed
sold, offered for sale, exposed for sale or distributed within this state for
sowing or planting purposes shall bear thereon or have attached thereto, in
a conspicuous place, a single label containing all information required un-
der this section,
(6) Commissioner to prescribe uniform analysis tag-The commissioner
shall have the authority to prescribe a uniform analysis tag required by
Labels on different types of certified and of non-certified seed may well
be used as references in connection with the study of seed laws.
An up-to-date listing of growers of Florida certified seed may usually
be secured from the Commissioner of Agriculture in Tallahassee.
IV. 'Plant Growth How It Takes Place
1. What are plant cells? What role do cells play in plant growth?
Cut a thin slice from a leaf, root, stem, or any other Iliving plant structure.
Examine the slice under a microscope. You will find that it is made up of many
small chambers, separated by thin, transparent walls. You will also notice that
within each of these tiny chambers there is a liquid, an almost transparent sub-
stance called protoplasm. This is the living material of the plant. It is made
up of a chemical mixture of water, proteins, starches, sugars, fats, and mineral
salts. The mass of protoplasm in a single such cavity is called a protoplast.
These protoplasts, with their surrounding walls, are called cells. They are the
units of structure and function in plants. Each cell may be-dTeined as a unit
mass of protoplasm, in which plant life takes place.
Cells vary considerably in size, shape, wall thickness, and to a certain
extent in the nature of the material of which the wall is made.
Most plants are made up of many cells -- hundreds, thousands, or even
billions of them. Thus, the organs in which the various activities of the plant
go on are, in plants of an agricultural nature, multicellular. The division or
multiplication and enlargement of these cells results in growth of the plant.
Cell division takes place in meristems. Meristems are areas where cells
are capable of division. Meristems are found at growing points of roots and
stems and in the cambium layer of plants. When a new cell is formed it is un-
specialized. Soon, however, it differentiates into a specialized cell and be-
comes part of specialized tissue (leaf, wood, storage, conductive tissue, etc.).
(*Refer to Units II and III for additional information related to this unit.)
2. How does plant growth take place ?
The tiny plant emerging from seed must live from stored food until it can
send roots down for minerals and force green leaves through the soil's surface
into air for sunlight. Once through this stage the young plant can make its own
But how is the plant able to make, utilize, and store this food? To accom-
plish its mission in life, several processes or functions must be performed --
processes such as absorption, photosynthesis, digestion, assimilation, respira-
tion, transpiration, response to environment, sexual reproduction, and cell di-
All these processes are inter-related; however, the more we learn of the
physiology and biochemistry of the cell and organism, the more we come to
appreciate that no hard and fast dividing line can be drawn between the
"anabolic" or building processes and the "catabolic" or degradative processes.
Certainly, photosynthesis is a building process, in which energy is stored in an
easily available chemical form, but many of the reactions and molecules involved
in respiration, which we generally think of as a "tearing down" process. In
many cases, if you were able to observe a molecule being changed, or "meta-
bolized" in the cell, you could not tell whether it was being built up or torn
down until you saw its final destination. For example, sugars are broken down,
we generally say, into 3-carbon molecules, but subsequently they may be either
degraded, in the sense of energy content, into carbon dioxide; or they may be
combined to form part of a fat molecule. Now, were the initial changes into
the 3-carbon compounds a process of breakdown, or building? Similarly, if
they are energetically degraded into carbon dioxide, the energy released might
be used in building cell walls, or other energy-requiring anabolic functions;
in other words, most building processes are dependent For their energy on the
break-down of other materials.
To illustrate these processes, or functions, let us consider how they take
place in the corn plant. Let's think of this plant as a factory, with all of its
auxiliary systems of transporting, processing, and distributing materials --
materials necessary to provide the needs of the plant itself, and to provide
man and animal with some of the necessities of life.
When the tiny plant is no longer dependent upon the food stored in the
seed it begins to take in raw materials -- minerals, water, gases -- from which
it can fashion its finished product -- food for man and animals. The water and
minerals are gathered or absorbed from the soil by the roots, carried up the
stem's xylem tubes, or "plumbing system" to the leaves. In the leaves these
materials are transformed, or manufactured, into a form of food that can be
utilized and stored by the plant. Photosynthesis assists the plant in doing this.
("Photo" means light, and "synthesis" means putting together.) So, we might
define photosynthesis as the process whereby certain living plant cells combine
water, carbon dioxide, and light energy in the presence of chlorophyll and
various enzymes, to form carbohydrates -- mostly glucose, a simple sugar, and
to release oxygen into the atmosphere.
Corn and other plants perform this remarkable activity every day. How-
ever, it takes place most rapidly in temperatures from 80 to 900F.; it decreases
with lower temperatures, stops when the temperature is near the freezing point
or when it exceeds 1000F. At night it stops, unless artificial light is provided.
We understand what happens, but scientists have never been able to
duplicate photosynthesis, although recent workers have been able to carry out
partial processes in the test tube. This is because it is so closely associated with
the process of life itself. In other words, photosynthesis is a function of living
Briefly, photosynthesis takes place in the following way. Carbon dioxide
from the atmosphere enters the leaves through small openings -- the stomata --
in the epidermis, or outer layer of the lear's cells. This carbon dioxide -- CO2
-- combines with water -- H20 -- to form a simple carbohydrate. This com-
bination will not take place by itself. As with all chemical reactions energy
is involved. In this case, the energy of light -- either direct or indirect --
working through a complex system of enzymes, is used to cause the combination
of C02 with H20. Thus, the living cells, containing chlorophyll -- a complex
green substance, carbon dioxide, and water, produce glucose, a simple sugar,
plus oxygen. The chemical proportions involved in this reaction are:
6 CO2 plus 6 H20 light energy C6H1206 plus 602
(6 molecules of (6 molecules (1 molecule of (6 molecules
carbon dioxide) of water) simple sugar) of oxygen
The released oxygen, often called a waste product of photosynthesis, is vital
in the life of other plants, as well as animals. The simple sugar contains no
mineral elements. It is merely food which the plant can use to combine with
other elements to form other materials in the plant. The light energy necessary
to make this reaction proceed is present in the sugar, having been converted to
chemical energy. This sugar is later broken down by other processes -- digestion
and respiration -- forming carbon dioxide and water, and releasing energy.
Over all, this breakdown is the reverse of photosynthesis, but many of the re-
actions involved are not identical.
Of the processes listed, in a sense, photosynthesis is by far the most im-
portant in the biosphere (the total of all organisms on earth). This is the initial
energy source occurring the in the plant. Other processes are either food
transfer, food-changing or food-using processes. Part of the food made by
photosynthesis must be "burned" for energy to make the other processes go.
Carbon, hydrogen, and oxygen make up nearly all of the total weight of the
plant. For example, a bale of cotton removed from the field will average, for
Lbs. of Lbs. of
Carbohydrate N P K
500 Ibs. of lint cotton 425 1 1/2 3
1000 Ibs. of cotton seed 263 37 6 10
And one ton (2,000 Ibs.) of Alfalfa hay will contain about 43 Ibs. of nitrogen,
4 Ibs. of phosphorus and 36 Ibs. of potassium. Nearly all of the remaining dry
weight of these products will be carbon, hydrogen, and oxygen. All of the carbon,
hydrogen, and oxygen is derived from the process of photosynthesis.
After photosynthesis has gone on in the leaves for some time, part of the
sugar produced is changed into the insoluble and somewhat more complex carbo-
hydrate, starch, which is formed within the chloroplasts. Part or all of this
stored starch, may be changed back into sugar during the night, or at other times
when photosynthesis is less active. This change of starch or other foods such as
fat, or certain proteins, into soluble forms is called digestion.
The conversion of these carbohydrates, fats, and proteins, and simpler
nitrogenous foods into the living material -- protoplasm -- of the cell must
then be accomplished. This conversion is called assimilation.
Plant cells must exchange gases while all this is taking place. Except
when photosynthesizing this involves giving off carbon dioxide by the plant to
the air and taking up oxygen. This respiration is the same in animals and plants
and is constantly occurring in all higher plant cells that are not actively
Transpiration is the loss of water vapor from the plant, primarily via the
leaves. Plants transpire considerable water during their lifetime. It is said that
the water given off, or transpired in one day from the average size tree is
sometimes as much as 80 gallons. Although it may under some circumstances
result in increased mineral uptake, transpiration might be considered as an
inevitable consequence of the requirement that an effective photosynthetic
leaf be able to take up carbon dioxide rapidly. The same modifications of the
leaf that allow it to do this result in water loss. This is a process of great
importance to the agriculturist, since one of the major problems of plant
husbandry is the provision of water to the crop. If there were little or no
transpiration, there would be little need for water. Think of the tremendous
problems of wilting and "burning that would result with insufficient trans-
pirationl And also think of the tremendous agribusiness that results from
transpiration! But for this phenomenon, all of the irrigation engineers, equip-
ment manufacturers, their salesmen, and all of the water conservation experts
would be out of a job. On the other hand if plants could not transpire water or
exchange gases, consequences would be even worse -- no life! (Transpiration
is largely controlled by the stomata in the leaf.)
V. Plant Nutrition
Farming is no longer a "plow boy" proposition. The modern farmer must
use his "pencil" -- brain -- as well as "brawn". He must, among other things,
have a knowledge of soils, and plants, and plant nutrition.
Under the American system of competition efficient production and suc-
cess go hand in hand. This is true, not only in industry, but on the farm as well.
This is where plant nutrition fits into the picture -- proper nutrition helps provide
1. What nutrients are needed for plant growth?
Farmers today recognize that plants are living things, and like man and
animal, must have adequate food for normal growth. They have learned that they
are indispensable friends. Furthermore, they know that mineral nutrients will
always be needed -- much of it will have to be supplied through commercial
sources. They are growing crops; the crops remove the minerals, or fertilizing
elements from the soil; these elements must be replaced, or crop growing will have
to stop. The processes may be compared to a bank account; some has to be put in
in order to draw it out.
Plants absorb minerals from the soil and use them to make new tissue
from the simple sugar produced by photosynthesis. They do this by taking in,
primarily through their roots, the elements -- nutrients -- which are utilized in
building body tissue and energy. Many of these elements are the same as those
required by man and animal.
Plants must have access to all the necessary food elements, in not only
adequate amounts, but also in forms they can use efficiently. In other words, it
isn't enough to have different elements present, in different forms, but they must
be in balance -- to form an adequate, well-balanced "diet".
Plants require 16 elements for proper growth and development. When-
ever one or more of these elements are not present in sufficient amounts, plants
either do not make satisfactory growth, or die.
Ten of the 16 plant food elements are required in fairly large amounts.
Try using the "Simple-Simon" saying for easier remembering.
Elements Chemical Symbol "Simple-Simon Saying"*
Carbon C See
Hydrogen H H
Oxygen 0 0
*Read down: C. Hopkins Cafe mighty good
Six additional elements -- boron, manganese, copper, zinc, molybdenum,
and chlorine -- are usually present in sufficient amounts on most soils.
Of the 10 that are required in fairly large amounts, three are usually
present in sufficient amounts in air and water -- carbon and oxygen in air, and
hydrogen and oxygen in water.
Plants have 3 sources from which they get their nutrients; the air and
water, as previously mentioned, and the soil. With 3 of the elements coming from
air and water, this leaves 13 that must come from the soil. Most soils contain these
13 elements in varying amounts, in addition to others not known to be needed by
plants. Some soils are deficient in one element, some deficient in another, and
still other soils may be deficient in several, or even all of them.
Mineral elements, of course, can be added to soils in the form of manures
and chemical compounds. Broadly speaking, any chemical compound used for supply-
ing one or more of the essential plant-food elements is a chemical fertilizer.
2. What are the functions of the:
A. Primary (major) Nutrients? -- the 3 most needed by plants.
Gives dark green color to plants
Induces rapid growth
Increases yields of leaf, fruit, or seed
Improves quality of leaf crops
Increases protein content of food and feed crops
Feeds soil micro-organisms during their decomposition
of low-nitrogen organic materials
Elements.. Chemical Sv m
Most soil nitrogen is present in the organic matter in the
topsoil, or plow layer. Thus, soils low in organic matter are
usually low in nitrogen. In other words, as the soils of the earth
vary in their organic matter content, so do they vary in nitrogen
Nitrogen is quickly exhausted from many soils by bacterical
breakdown of soil organic matter, erosion, leaching, and by grow-
ing crops, which require relatively large quantities of this nutrient.
Therefore, it must be replaced frequently to maintain soil produc-
SStimulates early root formation and growth
Gives rapid and vigorous start to plants
S Hastens maturity -- seed formation
SStimulates blooming and aids in seed formation
Gives winter hardiness to fall-seeded grains and hay crops
SIs extremely important to germinating seedlings
Is very important in the conversion of sugar to other
Except for nitrogen, unsatisfactory growth more often is due
to a shortage of this element than any other. Phosphorus is intimate-
ly associated with all life processes (See Unit IV, "Plant Growth --
How It Takes Place") and is a vital constituent of every living cell.
Thus, without phosphorus there could be no life.
S Imparts increased vigor and disease resistance to plants
SProduces strong, stiff stalks, thus reduces lodging
Increases plumpness of the grain and seed
S Is essential to the formation and transfer of starches,
sugars, and oils
S Imparts winter hardiness to legumes and other crops
Many of this country's soils are far richer in potash than in
nitrogen or phosphorus. Unfortunately, most of the soil's potash is
locked up in forms that plants cannot readily use. Therefore, avail-
ble potash is a more important consideration than is total potasFhin
the soil. Sandy soils are usually low in this element.
AVERAGE PLANT FOOD CONTENT OF FERTILIZER MATERIALS
Materials 100 Ibs.
Percent Plant Food Material
1" P205 K9 I a Ade Availability A dity Ilkali yl
1 Ammonium Nitrate
2 Amonium Sulfate
3 Ammo-Phoa A
4 Ammo-Phoe B
5 Ammoniated Superphosphate
6 Anhydrous Ammonia
7 Aqua Ammonia
8 Calcium Nitrate
9 Cal Nitro
10 Nitrate Soda
11 Nitrate Soda Potash
12 Potassium Nitrate
14 Calcium Cyanamide
15 Castor Pomace
16 Cocoa Shell Meal
17 Cottonseed Meal
18 Dried Blood
19 Fish Scrap
20 Quana, Peru
22 Sewage Sludge
23 Soybean Meal
24 Tankage, Animal
25 Tankage, Garbage
26 Tankage, Process
27 Tobacco Stems
28 Steamed Bone Meal
29 Raw Bone Meal
30 Basic Slag
31 Ground Rock Phosphate
33 Concentrated Super-
34 Manure Salts
35 Mariate Potash
36 Sulfate Potash
37 Sulfate Potash Magnesia
39 Oalcitio Limestone
41 Copper Sulfate
42 Iron Sulfate
43 Maanganese Sulfate
44 Zinc Sulfate
45 Magnesium Sulfate
46 Aluainan Sulfate
B. Secondary Nutrients?
Calcium -- a major constituent of various limestones, shells, slags,
phosphate rock, superphosphates, and gypsum.
Promotes early root formation and growth
SImproves general plant vigor and stiffness of straw
SInfluences intake of other plant foods
Neutralizes poisons produced in the plant
SEncourages grain and seed production
SIncreases calcium content of food and feed crops
Is an important constituent of cell walls
Magnesium -- found in dolomitic limestone, magesium sulfate,
sulfate of potash-magnesia, magnesium oxide, and
Is an essential part of chlorophyll, which gives the
green color to leaves
Is necessary for the formation of sugar from carbon
dioxide and water in sunlight
SRegulates uptake of other plant foods
SActs as carrier of phosphorus in the plant
Promotes the formation of oils and fats
SPlays a part in the translocation of starch
(Magnesium is generally deficient in soils low in calcium
Until recent years little importance was given to the importance
of secondary, and trace elements ("C" under this question). Now,
however, it is known that poor yields are often due to deficiencies
of one or more of these elements.
Sulphur -- sources for crop use: natural sulphur and sulphur obtained
from fertilizer materials, such as gypsum, ordinary super-
phosphate, ammonium sulfate, and potassium sulfate.
SGives increased root growth
Helps maintain dark green color
Promotes nodule formation on legumes
Stimulates seed production
Encourages more vigorous plant growth
C. Trace (micro) Elements?
The trace elements, sometimes called micro elements, are boron,
manganese, copper, zinc, iron, chlorine, and molybdenum. Although very
small quantities are needed, all are necessary for plant growth. Much
attention is now being given to these elements and their importance.
Unsatisfactory plant growth in many areas in the United States, es-
pecially on sandy and muck soils, is traceable to the lack of one or more
of these elements. Except in the "panhandle" of Florida, the soil has
come from sandy or calcareous materials at one time under the sea. Be-
cause of this origin, the porous nature of the soil, and the plentiful
rainfall, deficiencies in the micro elements, except chlorine, are fre-
quently encountered in Florida. Such deficiencies can usually be cor-
rected by the addition of the needed elements to the soil in mixed
fertilizers or as separate elements. In some cases sprays containing
the micro element may be effective.
Note: Color slides showing nutrient deficiency symptoms in plants are
available on a cost basis from the National Plant Food Institute, 1700
K Street, N. W., Washington, D. C., and from the National Agricultural
Supply Company, Fort Atkinson, Wisconsin. Florida Department of Agri-
culture Bulletin No. 93-MALNUTRITION SYMPTOMS OF CITRUS has in it
numerous photographs of typical deficiency patterns in citrus.
4. What principles are involved in fertilizer use?
Certain general principles involving the crop, the soil, and fertilizer
materials, influence fertilizer usuage -- kinds, amounts, and placement.
A. Plant Food Removal by Crops. This is a principle involved in determining
kinds and amounts of fertilizer to apply.
One of the principles of fertilizer usage is that of the amount of
plant food removed by the different crops. The amount contained in the
plant can, in most instances, be used as one of the guides to the kind and
amount of fertilizer to be applied.
The data in Table 1, page 45, shows the amount of nine nutrients
removed by crops common to this state. Higher or lower yields than shown
will remove nutrients roughly in proportion to the variation in yield
from that stated in the table.
From a study of the information in Table 1 several things will be
(1) Fertilizer removal is associated with the kind of crop. In other
words, different crops respond differently to fertilizer treatments. While
some crops require large amounts of a particular nutrient, others seem to
do well on a limited supply of the same nutrient. This difference in re-
quirements for certain nutrients is due primarily to the differences in the
foraging or feeding ability of plants. Some are heavier feeders than others.
Table 1. Approximate Pounds Per Acre of Nutrients Contained in One Crop of the Size Shown
(These figures may vary with soil type, season, and fertility of soil.)
Corn (Grain) ....
Corn (Stover) .
Oats (Grain) .
Oats (Straw) ...
Rye (Grain) .
Sorghum (Grain) .
Wheat (Grain) ...
ACRE AIOS POTASSIUM MAC-
ACIE NITROGEN PHORUS S CALCIUM SULFUR COPPER MANGANESE ZINC
YIELDAS PO AS KO NESIUM
40 bu. 35 15 10 1 2 3 0.03 0.03 0.06
Stone 15 5 30 8 2 4 0.01 0.32 0.05
100 bu. 90 35 25 6 6 7 0.04 0.06 0.10
3 tons 70 25 95 12 5 5 0.03 1.05 0.21
80 bu. 50 20 15 2 3 5 .0.03 0.12 0.05
2tons 25 15 80 8 8 9 0.03 ... 0.29
30 bu. 35 10 10 2 3 7 0.02 0.22 0.03
1.5 tons 15 8 25 8 2 3 0.01 0.14 0.07
60 bu. 50 25 15 4 5 5 0.01 0.04 0.04
3 tons 65 20 95 29 18
40 bu. 50 25 15 1 6 3 0.03 0.09 0.14
1.5 tons 20 5 35 6 3 5 0.01 0.16 0.05
FRUITS AND VEGETABLES
Apples ...... .. ........... ... 500 bu.
Beans. Dry ...... ..... ............. 30bu.
Cabbage .......... .. 20 tons
Onions ...... .......... 7.5 tons
Peaches ........... ........ 600 bu.
Potatoes (Tubers) ............ .... 400 bu.
Spinach . . ...... 5 tons
Sweet Potatoes (Roots) .. ........ 300 bu.
Tomatoes .. . ......... 15 tons
Turnips (Roots) . . . ... 10 tons
Cotton (Seed and Lint) ... ....... 1500 Ibs.
Cotton (Stalks. Leaves & Burs) .......... 2000 bs.
*Peanuts (Nuts) ................. ..... 1.25 tons
"Soybeans (Grain) ..... ........ 40 bu.
Sugarcane .. .... .. 30 tons
*Legumes normally get the greater part of their nitrogen from the air.
Courtesy: Smith-Douglass Company, Inc.
. . . . . . .
.. ........ .
. . . . . . . .
. I . .. .. . . .
(2) The importance of conserving crop residue -- stalks, leaves,
roots, etc. When properly used, the residue has a high fertilizing value.
When wasted, there is a loss to the farmer.
(3) Legumes remove considerable amounts of nutrients other than
nitrogen from the soil. (Most of the nitrogen comes from the air.) If all
of the aboveground growth is removed then legumes are soil-depleting
rather than soil-building crops. Therefore, nutrient removal by legumes
is not a principle that can be used to determine the amount of nitrogen
fertilizer to be applied.
B. Other Principles Involved in Determining Kinds and Amounts of Fertilizer
Even though the principle of nutrient removal by crops is, with most
crops, important as a guide to fertilizer usage, it is by no means the only
principle that is to be considered. If only the nutrients contained in the
crop had to be given consideration in determining the kinds and amounts
of fertilizer to use the matter would be comparatively simple. Other
principles are the following soil factors:
(1) Chemical condition.
(2) Organic matter content.
(3) Physical condition, and
(4) The degree of alkalinity; or acidity (pH).
The economic value of the crop itself, leaching, and erosion are additional
In the case of nitrogen, the organic matter in the soil is a possible
source of this nutrient for all plants. Also, legumes can obtain the greater
part of their nitrogen from the air. Therefore, legumes usually require
little or no nitrogen in fertilizer form except for small amounts to start the
plants grown on soils low in fertility or low in organic matter. Thus, as
previously stated,the nitrogen figures for legumes cannot be considered as
a guide to the nitrogen fertilizer needs of these crops. So far as the other
crops are concerned, the nitrogen figures are important in showing the
relative needs of crops for this nutrient.
Phosphate for crops, for all practical purposes, comes entirely from
the soil supply or fertilizer and manure additions. The utilization figures
for this nutrient, however, probably have the least significance so far as
determining fertilizer applications are concerned. This is due to the fact
that a large part of the phosphate applied to soil reacts with materials in
the soil in such a way that it is "fixed" or made insoluble and then is only
very slowly available to the plant. Therefore, the amount of this nutrient
applied must be large enough to take care of the soil's "fixing-power" and
then leave some extra for the plant. Fixation is usually much less on light
sandy soils than on heavy -- clay -- soils. The use of comparatively large
amounts of phosphate over a period of years gradually fills the soil's
deficiency for phosphate, so that on sandy soils or on soils well fertilized
with phosphate in the past, the phosphate needed in the fertilizer tends to
approach that given in Table I. On the newer and especially on heavy
soils, relatively large amounts of phosphate must be used.
In the case of potash, the sources for the crop are the natural soil
supply, and additions ot fertilizer or manure. Since most of the potash in
the soil is not available to plants, little, if any, attention should be paid
to the total potash content, so far as fertilizing any one crop is concerned.
The available potash content of the soil is of great importance to the crop.
If it is low, compared to the amount needed by the crop, then more potash
must be applied for best yields and quality.
Sandy soils are usually low in potash, while the heavy soils are
usually better supplied with this nutrient. However, the removal of potash
by cropping without sufficient replacement by fertilizer and manure results
in the depletion of the available potash even in those soils naturally well
supplied. On some heavy sol Is "fixation" or absorption of this element
takes place, rendering it unavailable to the immediate crop. Peat and
muck soils are usually very low in potash.
The figures for nitrogen and those for potash in all the crops are the
most important and significant from the fertilizer standpoint. However,
the longer soils are farmed, and the lighter -- sandier -- the soil, the
more fertilizer usage tends to approach the figures given for all three
major nutrients -- nitrogen, phosphate, and potassium.
Note:Refer to FIGURE 10 for information on the amount of nutrients
that plants usually utilize from a sack of 12-16-16 fertilizer.
C. Principles Involved in Fertilizer Placement
No doubt, many farmers have applied the amount of plant nutrients
recommended for high yields and been disappointed with their crop's re-
sponse. Not understanding the failure, they probably blamed the ferti-
lizer itself -- jeopardizing their farming future through antagonism toward
a particular material that did not actually cause their troubles.
Many such experiences come from improper application or place-
ment of fertilizer. Indications now point to the ranking of proper place-
ment with proper amounts of fertilizer.
Proper placement simply means putting the fertilizer in the soil in
a way that minimizes injury to the seedling or plant and permits maximum
plant nutrients to be absorbed at the time they are needed by the plant.
Just where and how fertilizer should be placed to meet these two conditions
depends upon several factors -- the crop being grown, the character of the
soil, and the fertilizer materials themselves.
Placing fertilizer in narrow bands is one way of avoiding fixation
of phosphorus and potash by the soil. The fertilizer only touches a small
part of the soil and can satisfy the fixing ability of this small part of the
soil. If the same amount of fertilizer were to be mixed throughout the
plow layer it would be in contact with more of the soil and therefore more
apt to be fixed in a form unavailable to the plants. If only a small amount
of fertilizer is to be used, the plants can obtain a larger portion of the
fertilizer if it is placed in a band one to two inches to the side and below
the seed. If large amounts of fertilizer are applied the band should be
placed farther from the seed -- 4 to 6 inches -- to prevent injury to the
seedling by the strong fertilizer solution.
What is meant by fertilizer analysis?
The word "analysis" or "grade", as related to fertilizers, refers to the
percentage of each plant nutrient contained in a sack of fertilizer in the case
of solids, or to the percentage of nutrients in liquid fertilizers. For example,
the figures 12-16-16 on a 100 pound bag of complete fertilizer means that the
manufacturer guarantees that there are 12 pounds of nitrogen (N), 16 pounds of
available phosphoric acid (P205), and 16 pounds of potash (K20) -- in that
order -- in the bag. In the case of phosphorus and potassium the percentages
are those of the oxide form shown. The actual equivalent amounts of pure
phosphorus and potassium are less than the amounts shown. However, standard
fertilizer recommendations are stated in terms of P205 and K20.
Another example of fertilizer analysis, is that of a 100-pound. bag of ammon-
ium nitrate showing 33.5 percent nitrogen content. This simply means that the
manufacturer guarantees that 33.5 of the 100 pounds is nitrogen.
In the near future fertilizers may be labeled with an analysis based on the
pure element. Such labeling will not affect the crop-producing capability of
the fertilizer. An analysis, for example, of 14-14-14, based on N, P20O, and
KO as we now buy it would become 14, 6.1, 11.6 based on N, P, and K as
proposed. This would be the same sack of fertilizer and have the same effect
in crop production.
The following drawing, using a 100-pound sack of 12-16-16 will be helpful
in understanding fertilizer analysis. It will, also, provide information as to
approximate amounts of the major plant food elements that are, under average
conditions, utilized by plants.
WHAT IS IN THE FERTILIZER BAG?
HOW MUCH DOES THE CROP GET?
This means 12% nitrogen.
If it is a 100-pound bag,
it contains 12 pounds of
The crop will probably
get about 50% of this
This means 16% available phosphoric
acid (as P205). If it is a 100-pound
bag, it contains 16 pounds of phos-
This means 16% potash (as K20).
If it is a 100-pound bag, it contains
16 pounds of potash.
The crop will probably
The crop will probably get about 50% of this
get about 20% of this potash.
FIGURE 10 Amount of N-P-K In Fertilizer Bag, and Its Utilization by Crops.
The approximate content of the principal nitrogen, phosphate, and potash
Name of Material Nitrogen Content
Ammonium nitrate------------- 33.5
Ammonium sulfate----------- 20.5-21
Ammonium nitrate -- Limestone
Mixtures ---------------- 20.5
Monoammonium phosphate------ 11
Diammonium phosphate ----- 16 -21
Ammonium phosphate-sulfate---- 13 -16
Sodium nitrate --- ------ 16
Urea --------------------- 42 -46
Calcium cyanamide------------ 21
Calcium nitrate -------------15
Anhydrous ammonia ------------82
Aqua ammonia -------------- 16 -25
Name of Material
Name of Material Phosphate Content
Muriate of Potash ------------ 60-62
Sulphate of Potash ----------- 50-53
Sulphate of Potash -- Magnesia 22
Note: Additional information on the composition of fertilizer
materials is given on pages 44-45 of the publication
entitled "OUR LAND AND ITS CARE", 1962, published
by the National Plant Food Institute.
6. What Is a legume? How do legumes extract nitrogen from the air? What is
their economic importance?
Legumes are plants that bear seeds in pods. They have the ability, when
inoculated with nitrogen-gathering bacteria, to obtain nitrogen from the atmos-
phere and deposit it in the plant. This is known as a nitrogen-fixation process.
(Nitrogen is indispensable to life because it is the key ingredient in
protein, a vital element in all living organisms, animals, and plants. Protein
is an essential part of any diet.)
Approximately 80 percent of the air, by volume, is nitrogen. Each acre
has 35,000 tons of nitrogen above it. But plants are unable to use this nitro-
gen by merely being in contact with it. It may be taken from the air, com-
bined with other elements and be added to the soil in either a liquid or solid
state. A small amount of nitrogen is brought to the soil in rainfall. In
some manufacturing processes nitrogen gas is combined chemically with hydrogen
to produce ammonia (NH"). The ammonia is then "burned" to produce nitrogen
dioxide (NO2) which is the nitrite form of oxygen. Ammonia and nitrije may be
combined to form the commonly used ammonium nitrate fertilizer (NH4NJ ). Cer-
tain non-symbiotic, or free-living, soil organisms gather nitrogen in one way
or another and leave it in the soil. As previously stated, it can also be
taken from the air by legumes which must be inoculated.
Inoculation is discussed in the following paragraphs:
a. Symbiotic nitrogen fixation bacteria can grow best on the roots of
leguminous plants, but the bacteria can "rest" in the soil for a few years if
no leguminous crop is growing.
b. When the leguminous plant starts to grow, the young roots are invaded
by bacteria which multiply rapidly in the root and form nodules -- small round
growths on the roots. The bacteria use food from the host plant for their growth.
c. These special strains of bacteria posses an enzyme system which
permits them use of the food -- probably sugar, which they take from the host
plant as a source of chemical energy to change the pure nitrogen from the air,
which moves into the soil, into a protein-like substance. This process is some-
what similar to the chemical process described above. Both the bacteria and
the host plant can use this new form of nitrogen to make the proteins and other
nitrogen-containing materials necessary for their growth. Since the bacteria
are in nodules attached to the root of the host plant, the host plant has a
ready source of usable nitrogen which is translocated through its conducting
tissue to all parts of the plant.
d. If the proper bacteria are present, most legume crops seldom need
any nitrogen fertilizer. To be sure the proper bacteria are present in the soil,
seed of legume crops should be inoculated if the legume crop to be planted is.
being planted for the first time in a field. If several years have passed since
the crop was last grown, the proper strains of bacteria in the soil may have
died and the seed should be re-inoculated. Inoculum can be purchased for all
legume crops. The inoculum is merely a dust or wettable powder which is pri-
marily resting cells of bacteria. This inoculum is mixed with the seed before
planting so that bacteria can immediately attach themselves to the roots of the
e. This arrangement between leguminous plants and bacteria requires
that the host plant furnish food for the bacteria. This is "repaid" to the host
plant in the form of usable nitrogen compounds. Such a system, which benefits
oth the host and the bacteria is called symbiosis, a term describing the living
together in more or less intimate association of two dissimilar organisms. In
other words, a definite partnership is established with the legume plant furnishing
the energy needed by the bacteria, who in turn use this energy to change the
free nitrogen of the atmosphere into a form that the plant can assimilate and use
to build protein.
The total amount of nitrogen that legumes take from the air in the
soil cannot be known with any exactness since the type of legume being grown,
and the conditions under which they are grown, influence the relative amounts
taken from the air and the soil. In other words, such factors as the kind of
legume being grown, the effectiveness of the legume bacteria, soil conditions,
and the presence of necessary plant-food elements -- except nitrogen --
determine this amount. (In soils highly fertile in nitrogen little or no fixation
may occur, as the bacteria seem to use this available nitrogen rather than to
fix additional nitrogen from the air.) Studies show that the weight of roots of
legumes generally is about one-third that of the tops and that the percentage
of nitrogen in the roots is slightly less than that in the tops. These studies show
that the percentage of nitrogen in roots of a number of legumes that have been
analyzed ranges from 1.40 to 2.30 percent and in tops from 2.10 to 2.80 per-
cent. It is generally assumed, so they say, that more than half the nitrogen in
legumes is taken from the air. Assuming these figures as a guide, it might be
expected generally that an acre yielding 3 tons of aboveground leguminous
plant material would contain from 126 to 168 pounds of nitrogen, while the root
content would range from 28 to 46 pounds. Since the amount removed from the
air is thought to be more than one-half this amount it might be assumed that the
plants "fixed" from 77 to 107 or more pounds of nitrogen in the plants. But,
aspreviouslystated, the total amount of nitrogen that legumes can "fix" from the
air cannot be known with any exactness because of varying conditions. Under
ideal conditions some crops, such as alfalfa, may "fix" up to 200 to 250 pounds
of nitrogen per acre.
The amount of nitrogen returned to the soil would depend, of course, on
the amount of the plant material -- roots and top residue -- returned.
With cheap nitrogen fertilizers available, the use of the legumes to
supply nitrogen primarily may be too expensive; however, other benefits from
legumes are to be had, namely:
a. the reducing of soil erosion
b. the addition of organic matter, and
c. the providing of foods and feeds higher in protein than non-legumin-
VI. Plant Diseases
Why are there so many Irishmen in the United States? What contributed
greatly to the downfall of the German empire in World War I? What caused
South America to become the coffee-producing center of the world? Why are
so few of this country's once numerous chestnut trees living today? And why
do so few stately American elm trees now line the streets in the northeastern
section of this nation? The answer to these questions? Plant diseases
One of the tragic events in history was the Irish famine in the middle
nineteenth century. The people of Ireland were almost wholly dependent on
their potato crops for food. During 1845 and 1846 their potato crops were
almost entirely destroyed by late blight, a fungus disease. As a direct result of
the disease outbreak Ireland lost a third of its population. A million died from
starvation, and an additional million and half emigrated. Many of them came
to this country.
Late blight is said to have had a place in the downfall of Germany in the
First World War. In 1917 the disease destroyed almost a third of Germany's
potato crop, which made up a large part of her wartime diet. Reduction in the
already scanty food supply contributed to the breakdown of her people's morale
and physical endurance, thus leading to the end of the war.
Ceylon was once the coffee-producing center of the world. Then the
coffee rust invaded its plantations. The disease could not be controlled. It
spread throughout the East and yields dropped so low that the industry could not
maintain itself. South America, particularly Brazil, thereupon became the
coffee empire of the world.
Chestnut trees once thrived in the eastern section of this nation, providing
food, shade, and wood for man. Today few Irees of this species remain.
Chestnut blight destroyed them.
Stately elm trees once lined the streets of cities and towns in the north-
east section of this nation. Few remain today. The Dutch Elm disease has killed
most of them and is still causing many to die.
Plant diseases are continually causing losses. In dollars, this amounts
to roughly two to three billion each year. Without present control measures the
loss would be even greater. Great strides have been made by Plant Pathologists
in developing new and better ways of controlling diseases.
1. What is a plant disease?
Disease, literally "dis-ease", is any abnormal condition of a living plant.
This results in a lowered efficiency or breakdown in a plant's function -- a dis-
turbance in growth and maturation -- or even death of the plant.
2. What are some of the characteristics of plant diseases?
There are a number of different diseases of plants. Some are of such a
mild nature that losses are negligible; others occur sporadically but affect the
plant in more serious ways -- reducing yield, etc.; and there are still others
that are spectacular in occurrence and result in disastrous losses.
Diseases can occur on any part of a plant: roots, stems, leaves, flowers,
or fruits. Some diseases involve all these parts while some are confined to one
structure but yet may affect the others.
The reaction of the affected plant to the cause -- causal agent or patho-
gen -- of the disturbance produces the various symptoms that man recognizes as
the disease. Leaf, flower, or fruit diseases can usually be recognized because
dead spots, blights, rots, or mottling clearly indicate a diseased condition.
Diseases of roots or stems, however, are perhaps more difficult for the layman
to recognize. Symptoms of disease of these structures are usually expressed by
(a) sudden or gradual wilting or drying of leaves on one or more branches,
(b) yellowing of leaves followed by wilting or shedding, (c) stunting of the
entire plant, as in the case of the Corn Stunt disease, or (d) a combination of
two or more of these.
3. What are some of the characteristics of the causative agents?
Diseases do not just happen; they are the result of a cause. They are
caused by (a) parasites such as bacteria, fungi, viruses, and eel-worms or
nematodes, and by (b) non-parasites such as deficiencies of certain required
elements, unbalanced physiological processes, or injuries.
Some people believe that certain conditions, such as rainy or cloudy wea-
ther, cause diseases. This is not true; however, such conditions often affect
disease development. In other words, they render conditions more favorable
for fungi and bacteria to develop rapidly and attack plants. Mineral nutrition,
also, sometimes affects the development of diseases. An example of this is
Stewart's disease -- a bacterial disease -- of corn, which increases in destruc-
tiveness when higher rates of nitrogen are used in fertilization.
Some of the characteristics of the causal agents of diseases are discussed
in the following paragraphs.
A. Bacteria -- small and mighty.
Bacteria are an improtant part of the world in which we live. Some
are beneficial, others are harmful. They are microscopic, single-celled
plants that multiply by dividing. Under favorable conditions the cells may
divide as often as once every twenty minutes. Bacteria which cause plant
diseases are more or less short and cylindrical in shape, and are described
as rod-like. None are spherical like the coccus forms, which cause some
animal or human diseases. Bacteria gain entrance to plants through injured
tissue, natural openings such as stomata and hydathodes, or are borne inside
Table 2. Kinds of Plant Diseases and Some Crops on Which They Occur
Group Disease Some Crops on which Diseases Occur
Corn, alfalfa, tomato, potato
Crown gall of many crops
Fire blight of pear, bacterial blight of
snapbean, angular leaf spot of cotton
Tomato, potato, sugarcane
Cotton, cowpea, cucumber, canta-
loupe, bean, watermelon
Grain crops, grape, onion, spinach,
Grain, curcurbit, legume, rose,
Cotton, tomato, sweetpotato,
Apple scab, rose blackspot, and
numerous other different leafspots
Cotton, beans, tomato, potato, rice,
peanuts, carrots, chrysanthemum
B. Fungi -- perhaps the most common disease pathogen.
Fungi are living plants such as molds, mildews, and mushrooms.
They have no roots, stems, leaves, or flowers. Instead of these organs
they consist of mycelium -- collectively called mycelia -- which is a
tubular, thread-like structure. Fungi reproduce by spores, and under
favorable conditions reproduction is rather rapid. They contain no
chlorophyll; thus, they are unable to manufacture their own food as green
plants do, but obtain it from decaying organic matter or from living plants
and animals. In other words, they grow at the expense of the environment
in which they exist. Fungi that derive their nutrients mainly from living
plants are called parasitic fungi; those that grow on mainly dead and de-
caying organic matter are called saprophytic fungi.
All fungi are not detrimental. Examples of beneficial fungi are those
that give rise to the desirable flavors of our choicest cheeses, provide
sources of antibiotics such as penicillin, and those that aid in the decay-
ing and rotting of dead plant materials. Many are edible, such as mush-
rooms that are used in steak sauces, soups, and in other dishes of the
C. Viruses -- a scourge of mankind.
Viruses are infectious agents of molecular size which cause many
diseases of plants, animals, and man. In fact, they are so small that they
are invisible under the optical microscope. Some of them have been photo-
graphed under the electron microscope, where they are highly magnified --
30,000 to 50,000 times their actual size. They are not considered as
living organisms, although they have the capacity to reproduce. They
cannot grow outside of living cells, but some can be carried over a year
or more in dead plant tissue.
Most viruses produce symptoms in plants which are usually defined
into two groups: (a) those that cause mottling or spotting of leaves,
flowers, or fruits, and (b) those that cause yellowing, leaf abnormalities,
dwarfing, or excessive branching.
D. Nematodes -- microscopic eel-worms.
Nematodes are very small animals that feed on cell sap in certain
plant tissues, thus interfering with plant growth. They also can cause in-
juries which permit invasion by fungi and bacteria. For example, some of
the root rot organisms gain entrance to plant roots in this manner. Nema-
todes that cause plant diseases may be divided into two groups: (a) Root
nematodes, or those which attack roots, and (b) Foliar nematodes, or those
which attack foliage.
E. Other causes of disease -- non-parasitic.
Non-parasitic, or non-infectious diseases,are caused by:
a. Mineral deficiencies or toxicities
b. Mechanical injuries
c. Chemical injuries
d. Insect injuries
e. Lightning or other climatic injuries
f. Abnormalities caused by genetic disturbances In the plant
4. How are plant diseases spread?
Most pathogenic organisms are dependent to a great extent on various
agents for their spread. Some of the ways in which they are spread are listed
in the following paragraphs.
a. Seeds. A number of diseases are seed-borne. The causal agent occurs
on the seed coat or inside the seed.
b. Plants. Remains of diseased plants sometimes are sources of infection.
Vegetative propagation occasionally results in the spread of diseases. When
diseased buds or scions are used the new plants nearly always become diseased.
Likewise, diseased tubers, crowns, and other vegetative parts used for propaga-
tion carry pathogenic organisms.
c. Insects. Insects are important agents of transmission of bacteria,
fungi, and viruses. Those that have sucking mouth parts -- aphids, thrips,
leafhoppers, etc. -- transmit more viruses than do insects having chewing
mouth parts. Insects not only actively transmit pathogenic organisms but often
wound plants by sucking or chewing plant parts, and such wounds may provide
easy access to the plant for pathogenic organisms. In some instances insects
may hasten rotting in trees or logs by boring into the heartwood thus producing
entrance for wood rotting fungi, or they may even carry the pathogen into the
d. Wind and Water. These are perhaps the most common agents.
Pathogenic bacteria and fungi are very often spread from one plant to another
by rain, running water, dew, or wind.
e. Others. Man spreads pathogenic organisms by various means. For
example, pruning equipment used on disease-infested trees or shrubs and later
used on healthy plants, or cultivating equipment. Likewise, birds are vectors
of causal agents, and even cattle grazing in pastures very often spread patho-
gens. Many fungi even shoot their spores out into the air or onto plant sur-
5. How are plant diseases controlled?
Control of plant diseases is actually a matter of prevention; therefore,
control measures should be initiated before diseases become apparent. Control
measures applied after diseases appear sometimes can prevent further spread
of the disease organisms by killing them and by protecting healthy plant parts.
There are several methods of control, one or more of which can be applied to
most plant diseases. These are discussed in the following paragraphs.
a. Use of Resistant Varieties. Disease resistance is the only practical
control measure; however, resistance to many diseases has not been, or cannot
be, obtained. Some of the advantages of growing disease resistant varieties
are (a) that with some diseases greater protection is offered, (b) chemicals do
not need to be purchased and applied, and (c) there is no chance of injury
due to fungicidal action.
Many varieties of various species of plants have been developed with
resistance to certain diseases -- and only to those diseases specified. For
example, Resistant Detroit cabbage was developed for resistance to a disease
known as yellows; but this variety is not resistant to blackrot or any other
disease of cabbage. Another example is the Contender variety of bush snap-
bean, which is resistant only to common bean mosaic.
b. Use of Disease-free Seed. Since a number of diseases are seed-borne,
the use of disease-free seed is one way that control can be effected. The
causal agent can occur on the seed coat or inside the seed. Some seed are
produced in areas where certain seed-borne diseases do not occur, or seed have
been obtained from plants which were disease-free and are certified as such.
Whenever possible such seed should be used. If seed that are certified as to
being disease-free are used, it is a good policy to save the certification tag
until the end of the growing season.
c. Seed Treatment. This refers to treatment of seed to eradicate the
fungus which has infected the seed and Is established within the seed coats or
in more deep-seated tissues. Hot water treatment is used in some instances.
If disease-free seed are not available, then it is to the grower's advantage
to treat seed to kill pathogenic organisms present.
d. Seed Protection. Seed protection is simply coating the seed with a
fungicide which will help prevent infection by soil organisms to which the
plant is particularly vulnerable during the germination stages of growth. Treat-
ment of seed with chemicals to control seed-rot and damping-off increases the
possibility of a good stand by protecting seed from these diseases.
e. Soil Fumigation. Fumigating the soil prior to planting will control
nematodes and damping-off fungi in seed beds. Common weeds and grasses are
also destroyed by soil fumigation. Fumigation is not practical except for
plant-producing beds, garden spots, or fields of high income crops.
f. Rotation, Sanitation, and Good Drainage. Many plant disease organ-
isms live on rotted or unrotted plant material in the soil for one or more years.
If the same crop is planted in the same field two or more years in succession,
a build-up of disease organisms is likely, thus rendering the crop more liable
to attack. However, if unrelated crops are rotated two or more years, chances
for a build-up of inoculum are greatly decreased. Sanitation involves keeping
fields clean of diseased plant refuse, keeping out of fields when plants are
wet with dew or rain, not carrying diseased material from one field to another,
etc. Good drainage is necessary because the development and activity of
some soil-borne disease organisms are greatly favored by water-logged soil.
g. Spraying or Dusting Plants with Fungicides. This is a very important
disease control measure and, in some instances should be considered just as
important a cultural practice as preparation of soil or application of fertili-
zer. For most diseases spraying is much more effective than dusting; however,
there are a few diseases -- downy mildew of cabbage, bean rust, etc. -- where
dusting is more effective than spraying. Plants should be sprayed when they are
dry; dusted when they are wet.
h. Use of Proper and Balanced Fertilizer. The use of the proper amount
of a balanced fertilizer produces vigorously growing plants, which ordinarily
are not attacked by certain pathogenic organisms.
6. What precautions should be observed when using chemicals to control plant
When using chemicals to control plant diseases carefully follow all
directions on the container with respect to:
a. Use of Recommended Amounts of the Chemical. Excessive amounts
may result in injury to seed or plants, while inadequate amounts will likely
give poor control.
b. Coverage of Seed or Plant Surfaces. Be sure there is complete cover-
age of all seed or plant surfaces. Inadequate coverage will give poor pro-
tection and therefore poor control.
c. Application Schedules When Treating Plants. The number and fre-
quency of applications of the chemicals to plants is very important. Spray or
dust according to recommendations.
d. When Seed Should be Treated. Note recommendations as to when
seed should be treated. Seed of some plants should not be treated far in advance
of planting time.
e. Hazards to the Handler. Fungicides may cause irritation of the skin
or the mucous membrane of the respiratory tract of man; therefore, avoid undue
exposure when handling these chemicals.
Four Keys to
READ THE LABEL ON EACH PESTI-
CIDE CONTAINER BEFORE EACH
USE. Follow instructions; heed all
Cautions and warnings. Why read
the label each time? Because
chemical nature of pesticides and
'': 'their uses vary greatly. You should
S refresh your mind each time on
the material's specific uses.
STORE PESTICIDES IN THEIR ORIG-
INAL, LABELED CONTAINERS. Keep
them out of the reach of children
and irresponsible people. They i
cannot be properly identified un-
less they are in original labeled
containers. Lock pesticides in a
shed away from feed, seed, and
other farm supplies.
APPLY PESTICIDES ONLY AS DI-
i RECTED. Apply them only to the
crops specified, in amounts speci-
'l fied and at times specified in label
instructions, or by your agricul-
DISPOSE OF EMPTY CONTAINERS
SAFELY. It is almost impossible to
remove all material from a con-
tainer. "Empty" containers con-
tain small amounts of pesticides
which could harm children or ani-
mals who might get into them. It .
is best to dispose of empty con- ,- .
tainers by burying them at least
18 inches deep in an isolated area
provided for this purpose away
from water supplies.
VII. Plant Insects (Common to Farm Crops)
Insects are important competitors of man for food and fiber; however,
progress in insect control has been phenomenal in recent years. A quarter of a cen-
tury ago it was estimated that insects destroyed approximately 25 percent of farm
production, compared to today's figure of 12 percentofa much higher agricultural
output. Who knows but what this figure may eventually be reduced to one of in-
significance? Only time will tell the outcome.
1. What is an insect?
Some people call any small crawling or flying animal a "bug". This
isn't always so! First, true bugs are members of only one order of insects. Then,
what some psople call a "bug" may not be one at all -- as in the case of spiders,
and mites. Then, what is an insect? An insect is defined as any of numerous, small
boneless animals, including ants, bees, flies, boll weevils, bollworms, aphids, and
a multitude of others, whose bodies are divided into three sections or body regions.
These sections are (a) the head, bearing one pair of antennae and mouth parts,
(b) the thorax, bearing three pairs of legs, and often wings, and (c) the abdomen,
which has as many as eleven segments, but never bears legs.
However, the above description of a true insect does not include all of
the "bugs" that attack crops. As stated previously, mites and spiders are not
classified as true insects. Yet these, and others not classified as true insects, will
be considered as "insect-type" pests in this publication because of their economic
importance in plant production.
2. How are the major common insects classified?
Insects may be classified in different ways. Two ways in which they are
classified are: (a) according to the way they feed on plants, and (b) as harmful or
A. Classification According to the Way They Feed on Plants.
When classified in this manner they are known as (a) chewing
insects, or (b) sucking insects.
a. Chewing Insects. This group bites off, chews, and
swallows plant parts. Colorado potato beetles, blister beetles, Mexican
bean beetles, peach tree borers, armyworms, cutworms, and cotton boll-
worms* are examples of chewing insects.
b. Sucking Insects. This group either rasps or pierces the
epidermis -- outer layer -- of the plant and sucks sap from the cells.
Aphids -- 6ften called plantlice, plant bugs, leafhoppers, chinch bugs,
thrips, squash bugs, mealy bugs, and scales are examples of sucking in-
(*Many insects, feed on plants by chewing on them during
one stage in their life-cycle, and sucking on them during
another stage. An example of this is the cotton bollworm,
which chews out the inner part of the boll during its larva
stage, and then feeds during Its adult -- moth, or reproduc-
tive -- stage by sucking juices from other plant parts. Since
insects are classified as being either of (a) the chewing-
type, or (b) the sucking-type according to the stage in
their life-cycle when they do their greatest damage, the
cotton bollworm is, therefore, classified as being of the
(Some of the chewing-type insects are classified as internal feeding
types, and some of the chewing and sucking insects are classified as be-
ing subterranean types.
a. Internal Feeding Insects. Those of this group are of the chew-
ing type, which enter the plant and feed from within. The European corn
borer, cotton boll weevil, and corn earworm, are examples.
b. Subterranean Insects. This group includes both chewing and
sucking insects, as well as gall insects, and root borers, which attack
the plant below the soil surface. The corn rootworm, wireworms, and
white grubs are examples.)
B. Classification as Harmful or Beneficial.
Insects may also be classified as being harmful, or as beneficial. Both of
these types are discussed in the following paragraphs.
a. Insects. Throughout the day and night multitudes of harmful insects
are chewing, sucking, biting, and boring away at our crops, as well as our
homes, our livestock, and ourselves. These pests do untold injury -- reduce
yields, lower the quality, increase the cost of production and harvesting, and
require outlays for equipment and materials to apply control measures. Among
the many kinds of insects that attack crops are the "pesky" boll weevil, the
bollworm, aphids, and countless others, many of which you are already fami-
b. Beneficial Insects -- "friends" of man. While many insects are harmful
to plants, man, and animals, there are also a host of others which serve in bene-
ficial capacities. Some ways in which several of the "friendly" insects are
beneficial are discussed in the following paragraphs.
(1) Some Insects Improve Soil. By burrowing into the soil, ants, wild
bees, grubs, and beetles (a) let air into it, and (b) aid in improving the soil's
physical condition by bringing earth to the surface from the deeper soil layers,
and by burying decaying vegetable matter. Also, insects such as ants, termites,
and the grubs or larvae of many wood-inhabiting beetles, are constantly at
work tearing to pieces leaves, twigs, and trunks of fallen trees. By doing this,
these tree structures are returned to the soil to provide nutrients and organic
matter for growth of other plants. Insects also hasten the decay of animal bodies
and their return to the soil.
(2) Insects are Indispensible as Pollinizers of Plants. Many insects
serve in this way -- bees, wasps, flies, ants, beetles, thrips, and butterflies,
and others. Without these pollinizing agents many seed and fruit crops would
be less productive; other crops, such as white and red clover, onions, and most
varieties of apples and plums would be barren.
(3) Some Destroy Harmful Insects. They destroy harmful or "unfriendly"
insects, thus helping control infestations of the latter type. These we call pre-
dators and parasites. The predatory group are the lions and tigers of the insect
world. Some of them devour a large part or all of their prey; others, such as
the ant-lions -- commonly called "doodle bugs", merely suck the body juices
from those they attack. Predatory insects, such as the dragon flies and damsel
flies catch and eat mosquitoes and other softbodied insects. The lacewing
aphis-lions destroy both eggs and adult caterpillars; they also destroy all stages
-- egg, larva, pupa, and adult -- of many destructive insects such as plant
feeding mites, aphids, scale insects, and mealy bugs. Lady beetles, commonly
called "Lady bugs", are important predators of harmful insects. Possessing ha-
bits that are anything but lady-like, both young and adult beetles kill and
greedily devour the young of destructive aphids, scales, and other soft-bodied
insects. Not only do they destroy their adult insect enemies but other life
stages as well. Other predatory insects include the robber flies, ants, praying
mantis, and wasps.
The parasitic group of insects are less spectacular in their work than the
predatory ones, but are often more helpful to man. Parasitic insects attack
their insect enemies in all stages of their development -- egg, larva, pupa,
and adult. Usually, however, larvae enter the body of the larva, pupa, or
adult host, and feed on it until it -- the parasitic insect -- if nearly grown;
then the host insectdies. The parasitic insect may then pupate within the dead
body or emerge and pupate on or nearby the remains. Examples of parasitic in-
sects are Tachinid flies, which resemble large, bristly house flies; flesh flies,
grasshopper maggot, and wasps -- including mud-daubers, and yellow jackets.
Some of these direct their attacks against other parasitic insects; these are call-
ed secondary parasites.
It will pay you to be able to recognize the beneficial insects and to
learn what insecticides to apply, and when, in order to harm them as little as
possible. Friendly insects usually do not appear as early as the harmful ones.
Therefore, the low dosages of some insecticides required to control early-
season infestations of harmful insects normally cause little damage to the bene-
3. Why is a knowledge of insect identification and damages important?
Basic in any sound insect control program is the proper identification of
the specific insect or insects one wants to control. This i, absolutely necessary be-
fore one can determine the insecticide or insecticides to be used, the rate of appli-
cation to be made, and the application schedule to be followed.
Insecticides vary in their ability to kill certain types of insects. Fre-
quently, insecticides that control insects with chewing-mouth parts will not con-
trol insects with sucking-mouth parts. Also, certain species of insects often become
tolerant or resistant to certain insecticides.
Suppose aphids, spider mites, or boll weevils are damaging your cotton
crop, but you are unable to identify them as such. Yet, you go ahead and apply
DDT. What would be the redult? You would not control the insect or insects be-
cause they are not susceptible to DDT. Had you properly identified the insect or
insects as being one or more of those listed above then you would have been able to
select an insecticide that would have given good control, provided proper applica-
tion had been made. Keep in mind that it pays to know what insect or insects are
causing damage. This basic principle is a must
It is not only important that you be able to identify the insect or insects
that you want to control, but it is also important that you be able to recognize the
damage done by each specie of insect. In this way, if you are unable to find the
insect or insects causing the damage then you will still be able to identify the cul-
prits who need to be controlled. Also, it isn't always possible to find the insect
itself. For example, boll weevil infestations are not determined on the basis of
finding the boll weevil but largely on the basis of damaged squares.
Note: The following references will be helpful in identifying insects:
1. INSECTS, Yearbook of Agriculture, USDA, 1952
2. Handbook of The Insect World, Hercules Powder Company, Wilming-
3. Consult your Vo-Ag Teaching Aids Film Library catalog for available
4. What is the nature of the life-cycles of insects, and why is a knowledge of life-
cyc;es important in a control program?
a. Nature of Life-Cycles. You are familiar with how farm animals
row up, but do you know how insects grow up? And do you know that such a know-
ledge could mean more money in your pocket?
You probably know that many insects grow up from an egg. The egg
hatches. Larva emerge. It then pupates, and later grows into an adult. But did
you know that all these stages are not present in the ife-cycle of some species of
insects? And do you know that some insects do not come from eggs? They are live-
borne, as are some species of aphids.
Insects thrive because they are able to adapt themselves to the world as
they find it. This is clearly shown by their ingenious ways of reproduction. Insects
grow from an egg to an adult by one of six schemes; however, we shall only discuss
two, since most insects reproduce by only two of these schemes. These schemes in-
volve a series of stages in the life-cycle of insects, which are discussed in the fol-
In many insects the development from egg to adult includes several more
or less distinct stages. This series of stages in a life history is called metamorphosis.
The two types that most common "insect-type" bugs fall into are known as (a) simple,
sometimes called incomplete or gradual, metamorphosis; and (b) complex, or com-
Insects having life-cycles in which the young resemble the adults, as in
the case of such insects as some aphids, grasshopper, leafhoppers, thrips, and spider
mites, are said to have simple metamorphosis. There are three stages in their lives
(a) the eggs, (b) the nymph, and (c) adult. The nymph, as stated earlier, looks
like the adult; however, they are smaller, do not have well-developed wings and no
reproductive structures at the time. It has feeding habits usually similar to those
of the adult.
Insects having life-cycles in which the young do not resemble -- are not
at all like -- the adult, as in the case of the boll weevil, bollworm, flies and
beetles, are said to have complex, or complete, metamorphosis. There are four
stages in their lives: (a) the egg, (b) the larva, (c) the pupa, or resting stage, and
(d) the adult.
b. Importance of a Knowledge of Insect Life-Cycles. One of the
principles of controlling insects is the aiming of control measures at the "weakest
link" in their life-cycle -- the stage in which they are most vulnerable or suscepti-
ble to kill. In order to do this it is, or course, necessary to be familiar with their
life-cycle, so that the application of control measures can be properly timed. The
following is an example of how a knowledge of the life-cycle of one familiar species
FIGURE 11 Life-Cycle of a Boll Weevil
of insect is used to control it. Within 3 to 5 days after the female boll weevil de-
posits her eggs in cotton squares or immature bolls, the eggs hatch into white larvae,
or grubs. These grubs feed for 7 to 14 days within the squares or bolls in which they
hatch and then change into pupae. The adults emerge from the pupae in 3 to 5 days
and cut their way outside the squares and bolls. After feeding on blooms, squares,
or bolls for 3 to 4 days the females are then ready to lay eggs. Thus, if the female
weevils are killed before they lay their first batch of eggs their reproductive cycle
is broken qnd the infestation of weevils os reduced. This is the reason why, during
the season when cotton is fruiting, that applications of Insecticides should be made
every 4 to 5 days. Present day insecticides are normally potent for about two days.
Since emerging female adults begin to lay within 3 to 4 days after they emerge from
the pupal stage the use of 4 to 5 day applications kill them before they begin to lay.
Even though the exact timing of applications of insecticides varies for
control of other insect species this principle is applicable in their control.
5. What methods are used to control insect enemies?
Some people think of an insect control program as consisting altogether
of control by chemicals. While the use of insecticides are often necessary to reduce
build-up of infestations, there are other control measures that are basic in any ef-
fective control program. The various control methods are discussed in the following
A. Cultural Control.
Cultural control methods, such as rotation of crops, tillage,
clean culture, etc. can be effective in killing many insects, or slowing
down their multiplication, often to the point that later infestation levels
cause little economic damage. The use of such practices makes environ-
ments unfavorable for insects that live in the soil, harbor in plant residue,
etc. A knowledge of the life-cycles of insect species is an important
factor in cultural control. Some of the cultural methods are discussed in
.the following paragraphs.
a. Rotation. Planting two similar crops in succession tends
to aid in the build-up of insect populations, while the rotation of unre-
lated crops often kill or aid in reducing infestations. Such rotations iso-
late such pests as white grubs, wireworms, and other long-cycle pests
from their food supply.
b. Clean Culture. Removal of plant residues, burning chaff
stacks, and disposal of volunteer plants are measures that also help in
controlling insect pests. These measures are particularly helpful in con-
trolling such insect beetles and caterpillars which hibernate in plant de-
bris. Tillage operations reduce populations of soil-inhabiting insects by
changing the physical condition of the soil, burying a stage of the insect,
eliminating host plants of the pests, and hastening growth or increasing
the vigor of the plant.
c. Location of Related Crops. The location of relabted4 crops
close to one another contributes to a build-up of insect infestations.
d. Planting Trap-Crops. Planting trap-crops and then ap-
plying chemicals to these crops and/or turning the trap-crop under, is
often of value in reducing infestations.
B. Natural Control.
a. Climatic Factors. Weather conditions, especially tem-
perature, play an important role in the build-up or reduction of infesta-
tions. Direct killing of some insects may result from extremely low tem-
peratures. (Winter killing is modified by the amount and kind of protection
available to insects, and the inherent resistance to cold of the hibernating
stage.) Insects usually seed sheltered places to over-winter -- under
bark, in weeds, and grasses, under plant debris, or in the ground. Snow
even affords some protection to hibernating insects by insulating them
against extreme low temperatures.
Weather conditions may govern the number of generations of
of a pest in a given season. For example, hot-dry weather aids in slowing
up the multiplication of the cotton boll weevil, while cloudy-rainy con-
ditions aid in their multiplication. Moisture and temperature also in-
fluence the condition of the host plant, thereby affecting the plant-feeding
b. Soil Type. This is a mojor element affecting the suita-
bility of an area to some pests. For example, certain wireworms live in
light, sandy soils, while other pests live only in heavy, poorly-drained
soils. Soil type also influences plant distribution which in turn influences
the distribution of particular plant feeding insects and mites. An example
of this is the cotton boll weevil, which feeds only on cotton plants; there-
fore, this insect is found only where cotton is grown.
c. Control by Natural Enemies. Birds, small mammals,
fish, disease organisms, and predatory and parasitic insects are natural
enemies of insects; they often play a greater role in reducing insect pop-
ulations than given credit. (Refer to section "B" under question number
2 in this unit for information on beneficial insects.)
C. Chemical Control.
As previously stated, cultural practices, and natural control
practices are basic in any sound insect control program; however, control
by these methods usually must be supplemented by the use of insecticides.
Recent discoveries of new synthetic insecticides have re-
sulted in major break-throughs in the control of insect enemies.
(Refer to the following paragraphs for additional information
regarding chemical control.)
6. What is the meaning of the terms "pesticide", and "insecticide"?
A pesticide is a poisonous chemical that is used to control pests, either
animal or plant. For example, insecticides are used to control insects, herbicides
are used to kill herbs -- or weeds, rodenticides are used to control rodents -- rats,
and mice. The suffix "cide" means killer; for example, insecticide means insect
7. How are insecticides formulated?
Insecticides are seldom applied full strength. They are formulated in
ways to dilute, extend, and make them easier to apply. The most common formula-
tions are (a) dusts, (b) granules, (c) insecticide-fertilizer mixtures, (d) fumigants;
and other formulations, which are used as spray mixtures: (e) wettable powders,
(f) soluble powders or solutions, (g) emusiflable concentrates, and (h) aerosols.
8. How are insecticides classified?
Insecticides may be classified in several ways. Two major ways of classi-
fying them are as follows. (a) They are commonly classified as stomach poisons, con-
tact poisons, and fumigant poisons. This grouping is based on the mode of entry of
the insecticide into the insect's body, and may have no significance as to its mode
of kill. (b) Another classification may be based on the chemical nature of the in-
secticide such as inorganic poisons, organic compounds derived from plants, and
synthetic organic compounds. Both of these classifications are discussed in the fol-
A. Classification of Insecticides According to Mode of Entry into the
This classification system was widely used before the present
day synthetic organic insecticides were discovered.
a. Stomach Poisons. These poisons are materials which are
ingested by the insect. They kill primarily by action on or absorption
from the digestive system. Usually they are most important for control
of chewing insects, primarily foliage eating insects such as leafworms,
armyworms, and bagworms.
b. Contact Poisons. These poisons are absorbed through the
insect's skin or body wall, and act on the pest's nervous system. Insec-
ticides that function as contact poisons are usually effective in con-
trolling sucking insects, such as aphids, and many others. To be killed
the insect must come into direct contact with the insecticide by crawling
through it, or by the insecticide being applied directly to its body.
c. Fumigant Poisons. These poisons enter the insect's body
through the tracheal system in the form of a gas. Certain soil insects,
and grain pests that live within grain in storage bins or other enclosures
are readily killed with fumigants.
B. Classification of Insecticides According to Their Chemical Nature.
Many of the present day widely used synthetic organic insec-
ticides enter the insect's body in more than one way. To avoid the over-
lapping under the system outlined in section "A", classif ication is now
based on the chemical nature of the insecticide: (a) inorganic poisons,
(b) organic compounds derived from plants, and (c) synthetic organic
Table 3. Classification of Commonly Used Insecticides According to Their Chemical Nature
Classification Effective Type(s) of Poison Examples of Insecticides
S. Inorganic compounds
2. Organic compounds de-
rived from plants.
3. Synthetic organic com-
a. Chlorinated hydro-
b. Phosphorus compounds.
c. Systemic phosphorus
Generally only as stomach poisons,
which controls primarily chewing-type
Mainly as contact poisons against
sucking-type insects; may also kill
as stomach poisons, and even as
Predominantly as contact poisons, but
usually function as stomach poisons, also,
and even have fumigating or systemic*
Predominantly as contact poisons, but
they also act as stomach poisons; some
act also as fumigants.
Contact, stomach, and systemic.
Systemic, and contact; some fumigating
Contact, and stomach; some systemic
Sulphur, lime-sulphur, lead arse-
nate, calcium arsenate, sodium
Nicotine sulphate, rotenone, py-
(Refer to breakdown "a" through
"e" under this division of classi-
DDT, methoxychlor, BHC, lindane,
toxaphene, aldrin, dieldrin, en-
drin, heptachlor, strobane.
Parathion, methyl parathion, Tri-
thion, ethion, malathion, guthion,
Di-Syston, systox, phorate or thi-
Carbon Disulphide, ethylene di-
chloride, ethylene dibromide, me-
*Systemic insecticides are those that are capable of absorption into plant sap and lethal to insects feeding on or within th
treated host plant.
**Dinitrophenols and derivatives, and Thiocynates were not included in this classification, since insecticides classified as
such are not commonly used. Also, attractants and repellants were not included because the use of these are still in the
compounds. The inorganic compounds are generally effective only as
stomach poisons. Insecticides of plant origin are effective mainly as
contact poisons, but may also kill insects as stomach poisons, and some-
times as fumigants. The widely used synthetic organic of today are pre-
dominately contact poisons; however, they usually function also as stomach
poisons, and some of them may even possess fumigating or systemic poison-
ing properties. Table 3 gives a breakdown in the classification of insec-
ticides according to their chemical nature, and additional information re-
lated to this type of classification.
9. What are some basic principles of insect control?
Basic principles of controlling insects have been discussed under questions
number 3 through 5, and 8; however, these principles of economical approach to
insect control are summarized as follows:
a. Scout or survey the crop frequently to determine the status of in-
b. Identify the specific insects) that need to be controlled. Utilize
your knowledge of insect life-cycles in carrying out control practices.
c. Whenever feasible rely upon cultural and natural control practices
d. Then if insect Infestations increase to the point that it is economi-
cally advisable to use chemicals to control them utilize the following basic
principles in selecting, mixing, and applying the insecticides:
(1) Select an insecticide that is effective against the insect
or insects to be controlled.
(2) Carefully follow directions on the container for mixing
-- and safe handling -- of the insecticide.
(3) Apply the insecticide at the right time, at the proper
rate, and in the correct form to gain maximum effec-
VIII. Weed Science
Weeds are the thieves and gangsters of the plant world. They steal from
desirable plants space, light, nutrients, water, and carbon dioxide. Indirectly,
they also cause other trouble. For example, they make it necessary to till the soil so
much that it is often compacted excessively, and more susceptible to surface erosion.
Weeds hinder harvesting operations, downgrade the harvested crop by polluting it
with weed seeds and trash, and serve as gracious hosts to insects and diseases --
that turn around for a second attack on crops.
Total annual losses in the United States to agriculture from weeds, insects,
loss of soil, animal diseases, etc. are estimated at eleven billion dollars. Authorities
believe that 33.8% of this amount is due to weeds, a percentage far higher than any
of the other losses.
1. What is a weed?
A weed is defined as a plant that grows where it is not wanted. A weed
may be a bitterweed that is infesting a pasture, dock in a small grain field, or
Johnsongrass in corn. It may even be a corn plant or some other plant in a field of
cotton -- a plant out of place.
2. How are weeds classified, and why is classification and identification so impor-
tant in controlling weeds?
The first step in effective weed control is knowing your weeds and their
growth characteristics. This section will help you define your weed problem before
you look for the method of control. Weeds are different from one another in more
than appearance. They differ also in life-cycles, root systems, rate of growth, and
chemical resistance, or tolerance. The life-cycles and longevity of weeds also vary,
dividing weeds into three groups -- annuals, biennials, and perennials. The impor-
tance of being able to recognize weeds and the group to which they belong are dis-
cussed in the following paragraphs.
A. Classification of Weeds.
a. Annuals. Annual plants complete their life-cycle in less
than one year. Annuals are normally considered easy to control, but they
are very persistent due to their fast growth and abundant production of
seed. Obviously, all methods of controlling annuals have one principal
purpose -- the prevention of seeding. They usually cost more to control
than perennials. Most common field weeds are of this group. Annuals are
further classified into two types: (1) Summer annuals, and (2) Winter an-
(1) Summer Annuals. Summer annuals germinate in the spring,
produce seed, make most of their growth during the summer, and
usually mature and die in the fall. Their seeds lie dormant in the
soil until the following spring, then they begin another life-cycle.
Cocklebur, morning-glory, common ragweed, pigweed, lambsquar-
ters, bitterweed, and crabgrass are examples of this group.
(2) Winter Annuals. Winter annuals germinate in the fall and
winter, and usually mature seed in the spring or early summer be-
fore dying. Cheat, henbit, little barley, and chickweed are ex-
b. Biennials. Biennial plants have a life span of 2 years.
The first years growth is purely vegetative; seed are produced during the
second year. Examples are bull thistle, common mullein, and wild carrot.
There is some confusion between the biennials and winter annual-group.
This is because the latter group normally lives during 2 calendar years
and during at least 2 seasons.
c. Perennials. Perennials live more than 2 years; some even
live indefinitely. Most produce by seed and many also spread vegetative-
ly. They are classed according to their method of reproduction: (1) sim "
pie, and (2) creeping.
(1) Simple Perennials. This type perennial spreads only by seed.
Injured or cut pieces may produce new plants, but they have no
normal means of spreading vegetatively. For example, a dande-
lion, dock, or plantain root cut in half may produce 2 plants. The
roots are usually fleshy and may grow very large.
(2) Creeping Perennials. These reproduce by seed, creeping
roots, stolons -- creeping aboveground stems, and rhizomes --
creeping belowground stems. There are many such perennials --
Bermudagrass, Johnsongrass, quackgrass, etc.
Some weeds, such as nutsedge -- nutgrass, maintain them-
selves and propagate by means of tubers, which are enlarged,
fleshy, usually terminal portions of a rhizome bearing buds.
B. Importance of Weed Identification.
Treatments of various kinds vary in their ability to control
different species of weeds, whether the treatments are cultural, or chem-
ical, or a combination of both.
Herbicides are available that are capable of killing most
species of weeds, but there is no herbicide available that will kill all
the species that are presently susceptible to herbicides. For example,
2,4-D is very effective, when properly used, against many species of
broadleaf annuals, biennials, and perennials; however, it is ineffective
against some broadleaf species within these groups, and is totally ineffec-
tive against a grass such as Johnsongrass. Consequently, it is absolutely
necessary that one be able to identify the weed, or weeds, that need io
be killed before he can select the effective herbicide(s) to be used.
The use of herbicides on weeds which are not susceptible to
these herbicides is a major reason why many weeds are not killed.
Because of the increasing recreational use of Florida's lakes and rivers,
the control of aquatic weeds has attained new importance.
3. How are weeds controlled?
We have two principal methods of weed control: (a) cultural practices,
such as conventional cultivation, flaming, mowing, mulching, crop competition,
crop rotation, and (b) chemical.
4. How are weeds controlled by cultural practices?
The use of cultural practices is basic in any effective weed control pro-
gram. These practices are discussed in the following paragraphs.
a. Conventional Cultivation. This involes the use of tools that physi-
cally lift weeds from the soil, cut off, or bury them -- hoes, cultivators, discs,
plows, and so on.
Tillage operations are effective on most small annual weeds, provided all
growing points are buried. However, burial is only partly effective against weeds
with underground stems and roots that are capable of sprouting--Bermudagrass,
Johnsongrass, and nutsedge--often called nutgrass. To control such perennials
they must be repeatedly cut--of or buired until the underground parts are killed
by carbohydrate starvation.
Tillage operations, such as shallow cultivation, that do not necessarily
bury weeds, but disturb their root systems, are effective in controlling some weeds.
These operations loosen or cut the root system sufficiently for the plant to dry out
before it can re-establish its roots. Small weeds are most easily controlled by this
method. It is most effective in hot, dry weather with dry soils. If soils are
moist, or if rain occurs soon after being cultivated, the roots may be quickly
re-established. In other words, the plants are merely transplanted with little
or no injury.
b. Mowing. Mowing is effective in controlling many tall-growing plants,
but not short-growing ones. Tall annual weeds are mowed primarily to prevent them
from producing seeds, and to reduce their competition with desirable plants. Re-
peated mowings may starve the underground parts. Tall perennial weeds, to be con-
trolled, must be mowed repeatedly for a period of 1 to 3 years--often enough to
prevent them from replenishing their underground stored food supply. The best time
to start mowing them is when their underground root reserves are low. For most
species this is in the late spring between full leaf development and first flower
How do repeated mowings deplenish a plant's food supply? New stems
and leaves grow at the expense of the belowground parts, hastening food depletion
When mowing annual weeds to prevent seed production mow when the
first flowers appear, because some seeds, if carefully dried, will germinate even
though the plant is cut soon after being pollinated. Some annual weeds, such as
the "pesky" bitterweed, sprout new stems below the cut. This growth can often be
controlled by cutting rather high at the first mowing and low enough with the second
mowing to cut off the sprouted stems. By this time "- the second mowing -- the stems
are often too hardy and woody to put out new sprouts below the cut.
Mowing does not control weeds that produce seed heads close to the
ground -- Bermudagrass, crabgrass, dandelion, plantain, etc.
c. Crop Competition. Competition makes full use of an old law of na-
ture -- survival of the strongest. Often it means using the best crop production me-
thods -- those so favorable to the crop that weeds are crowded out. Examples of
such methods include:
(1) Placing fertilizer sufficiently deep in the ground to prevent weeds from
getting it first, and
(2) Planting crops such as corn sufficiently thick to provide strong competi-
tion and shade. Rotating cotton and soybeans offers strong competition with
(3) Proper fertilizing and other good management practices to crowd out
many undesirable plants in pastures and lawns,
d. Crop Rotations. Rotations of crops is an efficient way of reducing
weed infestations. Some species of weeds are more prevalent in some crops than in
others. Usually a good rotation for weed control includes both (a) summer row crops,
and (b) winter or early spring grain crops, plus strong competitive crops grown in
each part of the rotation.
e. Mulching. Pine straw, plastics, and other mulching materials are
effective in controlling weeds -- as well as providing other benefits; however, the
use of these materials are usually economical primarily in mulching truck crops --
including home gardens, shrubbery, and flowers.
5. How are herbicides classified?
Herbicides -- weed-killing chemicals -- are classified in many ways. A
common way in which they are classified is according to (a) type, or the way they
kill: (1) contact, (2) growth regulators, and (3) soil sterilants. They are a-so class-
Tfied according to (b) their chemical structure -- phenoxy compounds, phenylacetic
acids, etc. These methods of classification are discussed in the following paragraphs.
A. Classification of Herbicides According to Type, or the Way They Kill.
a. Contact Herbicides. These kill primarily by contact with
plant parts rather than as a result of being translocated within the plant
and upsetting the plant's growth processes. Contact herbicides directly
affect only that portion of the plant it contacts. They are effective for
control of young seedling annuals. These plants die soon after coming in
contact with the chemical. Perennials, however, usually recover from
uninjured belowground parts. In other words, perennial weed tops are
merely "burned off", or literally, chemically mowed off.
Contact herbicides may be (1) selective, or (2) non-
selective, in their kill:
(1) Selective. Those that are selective kill or stunt either
broadleaf weeds, or grasses, but not both -- they have more toxic
action on some species than others. An example of a way in which
a contact selective herbicide is used it that of using DNBP in small
grain or legumes to kill chickweed, some of the mustards, and hen-
bit. It kills these weeds without injury to these crops.
(2) Non-Selective. These chemicals, such as diesel oil forti-
fied with DNBP, and pentachlorophenol, will kill all kinds of liv-
ing plant tissue, whether cotton, corn, dock, Johnsongrass, or
b. Growth Regulators. Growth regulators are sometimes
called growth modifiers, growth substances, and systemic or translocated
herbicides. These herbicides are absorbed by either the aboveground
parts or roots, and then moved or translocated through the phloem or
xylem, upsetting the plant's growth and metabolic processes. They have
a chronic effect upon the plant. In other words, full effects may not
result for some time -- a week or more after treatment.
Growth regulators are usually effective on certain plants
but not on others, thus making it possible to use these herbicides selec-
tively, killing certain plants without appreciably injuring others -- if
used correctly. (Herbicides of this type are usually -- but not always --
used in postemergence treatments.)
Examples of how growth regulators are selective in na-
ture: (a) 2,4-D used as a postemerge treatment to control cocklebur,
morning-glory, and other broadleaf weeds in corn, (b) 2,4-D used as a
preemerge or postemerge treatment to control bitterweeds in pastures,
(c) 2,4-D used as a postemerge treatment to control other broadleaf weeds
in pastures and lawns. These chemicals are selective in that they kill
certain broadleaf weeds, but do not kill grasses, unless the grasses are
very small unestablished seedlings.
c. Soil Sterilants. Soil sterilants are chemicals which pre-
vent the growth of plants in the soil. They are important for use in
greenhouse and potting soils, on ditchbanks, barnyards, fencerows, for
spot treatments of serious weeds in farm lands, and for many industrial
uses -- along railroad right-of-ways, etc. Normally they are uneconom-
ical for use on a large-scale basis on farm lands.
Sterilants vary in their residual action, according to the
nature of the specific chemical, the rate of application, type of soil,
rainfall, etc. Examples of soil sterilants are: methyl bromide, sodium
chlorate, simazine, atrazine, erbon, etc.
B. Classification of Herbicides According to Chemical Structure
Chemical structure Some examples of chemicals Classification
(Common name) (Type--way they kill)
a. Phenoxy compounds 2,4-D; 2,4,5-T; MCPA; Selective growth regu-
Sesone; 2,4,5-TP;4(2,4-DB) lator
b. Benzoic acids 2,3,6,TBA; Amiben Selective growth regu-
c. Aliphatic acids TCA; dalapon Contact, or selective
d. Heterocyclic nitrogen Simazine; atrazine; propa- Selective growth regu-
derivatives zine; maleic hydrazide; lator, or Soil sterilant
e. Substituted ureas Fenuron; monuron; diuron; Selective growth regu-
Neburon lator, or Soil Sterilant
f. Carbamates CIPC; CDEC; EPTC Selective growth regu-
g. Metal-Organic Sodium chlorate; ammonium Contact, or selective
sulfamate; DMA regulator
h. Inorganic salts Arsenic trioxide; sodium ar- Selective contact
senate; calcium arsenite
i. Hydrocarbons Herbicidal oils Non-selective contact
j. Organic halogens, Methyl bromide; carbon Soil sterilant
6. When are chemicals and other treatments applied?
A. Application of Treatments with Reference to Time.
Applications may be made with respect to the crop, or with
respect to the weed. Chemicals may be applied to control weeds as (a)
preplanting treatments, (b) preemergence treatments, and (c) as postemer-
a. Preplanting Treatment. Preplanting treatments are made before
the crop is planted. For example, methyl bromide may be used in
fumigating gardens to kill most weed seeds -- and soil diseases --
before planting. (Fallowing land and seed bed preparation are
cultural preplanting treatments.)
b. Preemergence Treatment. Preemergence treatments are those
made prior to emergence of a specific crop or weed. They are ap-
plied soon after the crop is planted, usually immediately behind
the planter. Examples of chemicals that are used in preemergence
application are: diuron, or monuron in cotton, simazine in corn,
2,4-D in pastures to control bitterweeds, and CDEC in some truck
crops. (Some of these chemicals are also used as postemergence
c. Postemergence Treatment. Postemergence treatments are those
made after emergence of a crop or specific weed. Examples are:
using diuron to kill emerged weeds in cotton, 2,4-D to kill emerged
broadleaf weeds in pasture, corn, small grains, etc. (Flaming,
used in such crops as cotton and corn, is also classed as a postemer-
B. Application of Chemicals According to Area of Application
Chemicals are applied in four ways: (a) on bands, (b) broad-
cast, (c) as directed sprays, and (d) on a spot basis.
a. Band Treatments. This consists of treating strips throughout the
field, usually a narrow strip centered in the top of the row. For
example, a 12" band on a 40" row. Such preemergence chemicals
as diuron used in cotton, simazine used in corn, and CIPC used in
some truck crops are applied as band treatments in order to con-
trol many weeds for at least several weeks until the crop has be-
b. Broadcast Treatment. These are blanket applications to an en-
tire area. They are more commonly applied to areas such as pastures.
c. Directed Sprays. This is the application of spray material to a
particular part of the plant, usually to the lower part of the stem
or trunk. Such applications are usually directed at or just above
the ground line, as in the case of applying herbicidal oils in cotton
and soybeans to kill small annual weeds that have emerged.
Directed spray applications, using dropped nozzles, are also
used to control weeds between row crops. Examples of the use of
directed sprays between row crops are the use of: (a) Diuron at
lay-by time in cotton to control small-seeded annual weeds, (b)
Two, 4-D at or after lay-by time in corn, and sorghum to control
Trees are often basally treated by directing the spray to the
base of the trunk. Examples are (a) the directed application of
chemicals 2,4,5-T to stumps to control sprouting, and (b) the ap-
plication of chemicals to kill the tree by chemically girdling it,
and to prevent sprouting growth.
d. Spot Treatment. Spot treatments are made to restricted areas
to control an infestation of a species requiring special treatment,
such as spots of Johnsongrass in cultivated crops, around buildings,
fencerows, etc. Another example is the use of soil sterilants in
fields and small areas to prevent the spread of perennial weeds.
7. Why should surfactants be added to some herbicides?
A surfactant is a material which facilitates and accentuates the emulsi-
fying, dispersing, spreading, wetting, and other surface-modifying properties of
herbicidal formulations. These are marketed under different trade names, and some
household detergents can be used as surfactants.
Water is repelled by the wax-like cuticles on plant surfaces. By adding
a surface-active agent in the form of a wetting agent -- surfactant -- the effective-
ness of a herbicide may be completely changed. The surfactant may increase or de-
crease the effectiveness of herbicidal sprays. Herbicides, when used to postemerge
weeds, should be absorbed readily. The addition of a surfactant to certain herbi-
cides increases "wetting" of the entire surface of plants, by preventing the water
droplets from forming a "ball" by causing the droplet to spread, thus covering more
leaf area and improving absorption. Examples of present usage of surfactants with
some herbicides to increase their effectiveness are:
(1) The use of diuron to treat weeds postemergence in cotton during early
growing season and/or at lay-by time.
(2) The use of DMA to control crabgrass and dallisgrass in Bermudagrass or
Zoysia grass lawns.
(3) The use of dalapon to control Johnsongrass.
When using growth regulating herbicides (refer to question number 5, A, b) surfac-
tants need not always be added. An exception to this is the use of dalapon-surfac-
tant mixture to improve wetting. Certain growth regulators esterss) do not require
the use of surfactants. The use of the amine salt of 2,4-d does not need extra sur-
factant for effectiveness. When using growth-regulating herbicides it is not usually
necessary that the entire plant be "wet", since this type herbicide is translocated in
the plant, and becomes effective at lethal plant sites, such as shoot and root tips.
8. What effect does the age of weeds, soil conditions, and climatic conditions
have upon weed control?
Timing is important in weed control. When herbicides are applied at the
right time more effective weed control is obtained with a smaller amount of chemical
under proper conditions. The factors discussed in the following paragraphs play an
important role in weed control.
a. Stage of Weed Growth. Generally, young, active-growing weeds
are easier to kill. The efficiency of herbicidal action decreases as plants approach
maturity. However, in the case of some perennial weeds, applications made at
certain later stages of development may be more effective than earlier treatments.
This is due partially to the larger amount of leaf absorbing surface and also to the
fact that a considerable amount of root reserve has been expended just prior to
blooming. Because of these reasons, Johnsongrass, for example, is easier controlled
with dalapon-surfactant mixture when application is made from the full leaf deve-
lopment stage to the time flowers first appear in the spring.
b. Soil Conditions. Warm, moist soil that favors rapid germination and
growth of seedlings is conducive to high effectiveness of applied herbicides. Drouth
conditions retard their effectiveness. For example, adequate moisture is necessary
to activate simazine when used to preemerge corn, etc.
The type of soil also influences the retention of herbicides used in
preemergence treatments -- and occasionally postemergence treatments. Leaching
is greater in light, sandy soils than in soils containing higher amounts of organic
matter and clay. For example, the rate of diuron, when used as a preemergence
treatment in cotton, varies from 0.17 pounds of technical material on medium sandy
loam soil to 0.42 pounds on clay loam. Some additional examples where soil tex-
ture determines the amount of technical material needed are:
(1) When using other preemergence herbicides in such crops as cotton, corn,
(2) When treating weeds postemergence in cotton with diuron, and
(3) When treating corn preemeigence with simazine or atrazine.
c. Rainfall. Rain occurring immediately following application of a
foliage herbicide may decrease its effectiveness by washing it from weed leaves be-
fore it can be absorbed. Later, however, rain may actually increase herbicidal
action by helping weeds grow faster, thus helping make them more susceptible to
kill. Excessive rainfall after application of preemergence herbicides may prove de-
trimental to some crops since it causes some herbicides to leach into the crop root
zone and cause injury. Light rain after application of preemergence herbicide on
the other hand, is beneficial.
d. Temperature. Higher temperatures, generally, speed up the action
of some herbicides, while low temperatures retard their action.
9. What is calibration, and why is proper calibration of spray equipment so impor-
tant in a weed control program?
The rate of application of a herbicide per acre is fixed by the rate of
spray delivery per nozzle, the spacing of the nozzles, and the speed of the sprayer.
80 Due oQ*.. Ret--#.
The rate of spray delivery per nozzle for any given liquid is fixed by the size of its
orifice and the pressure. The capacity of the pump must be such that it can supply
all nozzles at the required pressure. It should have enough extra capacity to supply
all the nozzles plus 3 gallons per minute for each 50 gallons of sprayer-tank capa-
city for hydraulic agitation requirements. Accurate calibration of spray equipment
is extremely important. Improperly calibrated equipment will either apply excessive
amounts that will prove costly and hazardous, or apply inadequate amounts which
might be ineffective.
10. What precautions should be observed when using herbicides?
Use only herbicides recommended by your state agricultural agencies.
Tentatively recommended herbicides should be used on limited acreage. Apply these
chemicals as recommended -- at the proper time, in the correct amount, and by
the correct procedure. Observe all precautions on the label with respect to
handling, including possible dangers to other crops.
11. How do the various methods of weed control compare as to cost, effectiveness,
No single method of control can be considered as best. The useful method
or methods depend upon the many factors or conditions prevalent in each case.
Often the most effective economic control involves the use of more than one method
-- a combination of both cultural and chemical treatments.
Experimental data and rapid adoption of chemical practices have advanced
chemical control methods. It should be emphasized, however, that herbicides, have
their limitations. They are not intended to entirely replace cultural management
practices, but to supplement them. Sound management including cultural practices
still play an important role in weed control programs. Weed control is a year round
job. It begins with plowing under the old crop to kill some of the weeds before
they mature, and continues with the use of weed-free seed, good seedbed prepara-
tion, and the use of mechanical or chemical methods, or combinations of both.
In selecting the method or methods to be used one should consider many
factors such as the kind of weed or weeds to be controlled, the effectiveness of the
various methods in controlling the weeds, costs, weather conditions, etc. For ex-
ample, to control weeds in cotton, a farmer's most economical weed control pro-
gram might, in addition to the use of preplanting cultural practices, include one of
the following programs -- dependent upon his individual situation.
a. Preemerge band treatment with diuron, followed by postemerge treat-
ments with herbicidal oil, flaming, and lay-by cultivation.
b. Preemerge band treatment with diuron, postemerge treatments) using
diuron as a directed spray during early season, another postemerge treatment using
diuron as a directed spray at lay-by time, followed by spot treatments with dalapon.
c. Preemerge band treatment with diuron followed with conventional