TutI MICIEOSTIUCTUJpI Of C[LUJLSI rElIRlS
-- /_, ,
No. V1432 --
UNITED STATES DEPARTMENT OF AGRICULTURE
FOREST PRODUCTS LABORATORY
In Cooperation with the University of Wisconsin
THE:: 2C'T'Tq':-c.F, CjIL .CSE FTP3VST 1
J, PI1T7R, Senior C'.e-'ist
This r-rcrt is a brief review of the literature on t:.e microstructure
of cellulose fibers durn;, the last three years.
:'.'.- first P-'rtion *-ves the ---,-ra]ly accrc-':i ce .ces r-rarrJing the
fiber 3truct ,re a. also the c r)--rversial concepts which were of signifi-
cance at the 1e:...-'u-.g of 1I.,. "e ideas and c..""-ciiat*r-s of old ideas
about the f-'..r strict ire are then disc.'sed in c,.-ection w"th the various
pnvers that have ar:ea-ed duriT l3"- \', inclusive.
I a r "iw of the literature sore articles are ge'e.rally V7.-sed,
Ace -r1.ngly, this revie-er wishes, at the outset, to ac'-c"..ed" the lieli-
hood of his missi:'..- articles that should be nclud, and to :c: the pardon
of an,,, author who has been overloc!>.
er.erally Ac e:tei- Fiber Ctruc--ures Pr vious to s t 3
A r-r-Mr.r of the articles on the microstructure of cellulose fibers,
which an-enred duri:- the five years previous to 1238, s--zs to i'.,e general
a;,reement on the foll'"'inr rtints,
'*in divisionss of Cell Wall
The cell 'wal is divided into a primary '.,d sec?:n',':rv layer and some-
t-s a tertiary la-er. Tr.e rrna ry layer is adjacent the .-.">le lamella
T.-.e secon.-1 ry lay':er is adjacent the inner face of -h? :rilnary; it is con-
siderably thicker than the primary layer cver: in thin-wallei fibers and much
thicker than the primary la'.-ir in thick-walle fibers. The tertiary layer
is adjacent the inner face of the secnnr-iry la:,y'r, d'i"".'ic.ilt to distinguish
in nrr'-Al fibers but easily distir-uished '., zo-? types of abncr.-.a fIbers;
itc prse:.ce in normal fi-ers is sometimes 'uestic:-i on acccuu:t of the
difficulty of reco i.ir it.
! iblis'he in TA.--I Tec-.h, Association i'Laers, Series -DNV, June 194"2
This paper forms a pnrt of the r-.rt, of the CT- it4ee 'n Cellulose ani
Allied SubE-tances of the Fivision of ihr-.i.strv an-. Che!cal technology
of t'K- National research >..r.ci. The pr,',,niis report a-:re.reed in the
F-'er Tr-.-e Journal 1?', nos. 21, 27, 24 ( i)
The secondary layer is divided into layers or sleeves. P-c number
of sleeves which have been reported vnry from approximately 8 to 12 in wood
and as high as 30 in cotton.
The primary layer of the wall and sleeves of the secondary layer a'r
to have been dissected into long slender fibrils. The presence of fibrils,
to this reviewer's knowledge, has not been definitely demonstrated in the
tertiary layer. In some abnormal fibers a coil springli'e structure loosely
associated with the secondary layer is discernible; some research workers
may consider that the coil-like structure demonstrates the presence of fibrils
in the tertiary wall.
Substructure of Fibrils
Microscopically visible substructures of the fibrils have been re-
ported. They comprise dermatosomes, ellipsoids, fusiforms, and spherical
units. These are considered substructures of the fibrils because they are
each isolated by a treatment of the fibrils under rather carefully controlled
conditions. The treatment selected produces substructures of at.- r,-x irately
the same magnitude and shape. They can be produced from the fibers directly
if the treatment is less severe than that ?Temployed for dispers ii the cellu-
lose. In dispersing the cellulose rather concentrated solvents are employed,
which quickly swell the fiber and disperse the cellulose without visibly
separating the cell wall into the fibrils and their visible substructures.
The largest submicroscopic structural unit of cellulose at the end
of 1937 was believed to be the micelle; since then the name crystallitee"
has been generally applied to the micelle on account of its crystal prop-
erties. The crystallite is assumed to be composed of long cellulose chains
which, in turn, are composed of arhydroglucose residues.
A preponderance of evidence favored orientation of the crystallites
in the primary wall to be approximately parallel with the perimeter of a
cross section of the fiber; also it favored orientation of the crystallites
in the secondary layer of the cell wall to be from 0 to l1 degrees to the
lonr axis of normal fibers.
Controversial Aspects of the Fiber Structure Previous to 1938
Previous to 1938 heated arguments had developed for and against the
presence of "cross walls" in cellulose fibers. The proponents of the cross
wills contcrQ.-.... that r-nrliko s.ruct'urps divided the cellulose fiber wall
ir.to segments a-'-r.:-. m-itely 40 micron. ir length. The structural units in
the cross walls wero -upr, -.'_- to be oriented *'ith their o,.nr axis parallel
witth cross'.'se pnri.r tV.'- fiber, Thi tnpe of arranremenrt v.ould
restrain th.e fi".-r fr-. shri.' I' and- swell".- transersely, If the fiber
were r, ated with a ."*ell'. :.r ~r'"t more dras'ic th- water, the cross walls
were r':'nsed to be res.7--:>ible f.rr 1 .e balc' iln'- pI-enn.renorn which developed.
?'.e presence of or-s walls also limi,. d the length of fi-rils to the length
of the fiber sere.ts bet *.**.. two such cross wal3 !1o infor:-.tior. as
re.'ar!.z the chemical co.'..:sition of tne crcr,-s walls was give:.. The idea of
cross walls eras op" ,'ed -.*' a larroe majority of research workerss 'who co':-
tended that the constrictions d.-.closed in the ballooc.in: phene:,.on were
ceus--r1 1-y the ;'rimary wall fibrils.
'.'i'.".rous controversies ':re carrie, on re-" r i-r- the che.-.ical nature
of the ceraentir'.- material between the -.- rious micro:.:t'ructurl. units e-
scrii',., above. 5or example, advocates of the cross-'.all co:c._-t a.r.itted
t. at no4 1. in- Was kownn about the cor.cr ,sition of the cross walls. They
further advocat-d the r,-sence of a -.'-t rious material .Trr-u:>Ji..g each of
th- layers, sleeves, fibrils, dcrmatosomes, and nmicelles. r:.us the largest
cellulose article e ':,ilt'". b"- itself as the micelle w-',ch *-j-n separated
frrni its *-.jhaent micelles by the myst.:rious cementin;: substance,
rt..er research workers advocated the presence of a recei.--l *o:e
material surr.--.'-in7 the e]oli'-.oidal -,,irticles.
HI--icelluloses were also advoca,- r! as be:'..- r-r"-er.t in the cell 'w-all
but no definite stand was taken re-r&Lnf its distribution .-m''.' he various
microstructural units of the cell wall.
Articles :',iblished -. ring l3-l:40 Inclusive
Bailey, I. 7'. Cell "--11 structuree of T-iL-her Flan.s. In.'. *ng.
Che-. 3':4:- (I-138). The cell wall is divided into a primary and a seco:.--
ary layer. :.'ch laver is composed of a coherent .matrix of p'rou!, aniso-
tropic cellulose whose micr-Structural units Zraffually iv'.inisl. to the
revclin- power of the rmicr': :c,-'"e (0.1 microns or less). Lig- n and :on-
cellulosic materials may be deorcsited in the interstices of the cellulose
so as to form tw-o continuous inter :-etrati. .haces. '7..e .'.a.e may be
removed without severin- the conti>..it" of the other. The stract',_re of the
secondary wall -iries (1) in c,.r-sity in successively formed parts, (2) in
the arr.n-ement of the ,''re.tes of the chain molecules, (3) i'. the distri-
bution of noncellulosic materials, r.i (4) in the pr,?se;.ce cf noncellulosic
layers. T'. secondary portion of the wall can be dissected into layers,
fibrils, fusiform bodies, dermatosomes, and 3:t}.or subst.ructures of varying
shares :,... sizes.
Fre,--'""'sIi'.., A. 2ubricrosc,?ic s ructure and maceration pictures
of native cellulose fibers. 3::1ier-'Thbr, 33:212 (l:38), The author *,i--ances
the micellar theory of IT&geli with the modification that the micelles are
held together by chain molecules which are a part of the two joined micelles.
Clark, S. F. Fine structure of the plant cell wall. Nature 142: 9
(1938). The principal idea of fiber structure conveyr!ed by the article relates
to the crystallite structure. It is suggested that the cellulose chains are
arranged parallel in zones and nonprallel at intervals between the zones.
In this manner the crystallites or micelles are a more or less continuous
mass consisting of regularly and semirerularly arranged cellulose. Lignin is
believed to exist in the intermicellar spaces. The author divides the cell
wall into middle lamella, pri.ar; layer, and secondary layer.
SFrey-'.yssling, A. The micellar theory explained by the ex:o-ple of the
fine structure of fibers. Yolloid-Z. 85:148 (1938). This article also
advocates the idea of the cellulose micelles and the intermediate substances
being attached to one another so as to form a continuous phase. The author
sur-:ests that the term "micelle" be retained in the literature. "icelles
are about 60 A. wide, whereas the spaces between them are 100 A. wide.
Hess, K'urt. Recent results of the investigr.tion of the structure of
the vegetable cell wall. Paoier-Pabr. 37:28 (1939). The article reports
findings obtained by means of r'.otomicrcgre.phs in which ultraviolet li .dt
VPs employed as the light medium. The article indicates that the results
obtained su-est a theory on fiber structure differinT from that put forth
Bailey, I. W. The microfibrillar structure of the cell wall. Bul.
Torrey Bot. Club 6S:201 (L-39). This article conveys the idea of two
structural phases in the cell wall, one composed of cellulose and the other
composed of noncellulosic materials, each r.,ase being continuous. The
author contends that the cellulose is regularly arran-e! in the microfibrils,
but that the microfibril arrai;e 'e.-.t fluctuates from layer to layer in the
secondary portion of the cell wall, thereby c-ausinr a phenomenon interpreted
as random crystallite arrangement. The remainder of the article is similar
to that published in Ind. E'-i.. Che-.. 30:40 (1938).
?.ailey, A. J., and Brown, R. '. Diameter variations in cellulose
fibrils. Ind. Eng. C:-.--. 32:57 (1940). The diameters of fibrils isolated
from various types of cellulose fibers were measured microscopically and
found to rcrn.;e from 0.928 to 0.9956 micron. Fibers can be divided into
fibrils and hydro.el by mechanically agitating the fibers in water.
Hock, Chas. .., and Seifriz, T. "'"icrodissection of paper pulp fibers.
Paper Trade J. 110, no. 5:31 (1940). Fibers fr.m ;'-er Tuin: were treated
with boilinF water and rhosphoric acid and examined microscopically. '
exa-ination revealed that fibl-ers are made up of mav fibrils arra,-:-9- parallel
to the long axis of the fiber. These fibrils are wrapped in an outer layer
of transversely arr'.nr-e fibrils and are bordered by an inner wL'-i ;.- which
subtends the lumen. Balloon formation is due to uneven weakening of the
interfibrillar forces of the outer wrapping.
T.. .", .. -. v,-. '...... 's.., F E. .i.is r..ce an ture of fiber
To ., :. , nC4
".c-}r-,' s.ns >'.*.ner ~ .' J. 1] no, ^t^,". ("t'4., j ^'"'-. r..^-:oc. P.....r-r .._.:4v
(A'v), Cellulose Ip 'bera cnta-'r. dif'- r't :p-re nt_- of li1r.in
weresdi.":'er i.. r r ...... ':,- "-.-iution, zr'.-u-,r..bly, t'-' i.nso:uble rezi-
?'e so o .. 7 [as considerm-r to a i'er r to th at
advocated Ldte. :.. residue recrrre'I frcn di.p--r.ing untleacied
PUl.: consIt? of more than half li czulph'.ic acid cESi,, prp1iL aricunts of
car-,''.v rates; the r osicues fr' r Ile.-c. ei p.1ps c.-onsi-ted of cor.oun'Es of
c.;'ner cd :r,'t,,i,.,drates,
"i :.',I, Ernst. :'--.n- the ir-.er ctrj.rct,.'re of cellulose fibers
visible by e:' -..* ". forei'-n su't'-.t r.ces. "'un:t-e 4 `e 22:3, 5. (l40). The
article describes 7relimi try trials on -. rrer~ntir. an rdr'ensi'-ing metals
in the nicr:.'tructure of fibers. The treatment cDr.:,i.ts of 4t7o stages,
one to swell the fiber va ard the other a di"Nuzicn of P. 2 ult solution
irnt, the fiber wall an den.-ition of the metal vy.ch is r.irozccpically
visible. '.'s teo .,iq fords a me- s of -.ear:-rin- th: r.-rituse of the
firn capill r' structure of fibers.
Vratky, F z., Y ., anrd Tre'r, '.. ."LCellar str.'ct-:re of native
celluloIse. '-"lz R -h u. "rrkstoff 2:4.., (r-39). The cell w-ll is impregra-
tot with a solution or, a noble metal salt an.: the metal salt subcsquently
reduced to finely divided meta.l wic'. is leor.ited in the- irterztices
e.'..e i the cellulose micelles. A, a-: roximation of the Ci-e of the inter-
ztices is made.
ireston, R. D. Wall of the conifer trac`.eid as a P.-i.F s-iral
c-:.lex. roc. T* :r >.l. Soc. 3, pa-rt 9:416 (1I3l.. The s.icellar
" rr' .ement of the primrar: layer is in t.-.ner. les?:trv.:.: i:- t'e fiber.
I:.e arr frement in the secon..ar-' layer .-3:s %..ular dJ 71rF ri -n.
Fund, B, C., and Irrston, R. c'. m.',crosc:-:c qtruct'.ire cf cell
walls. Proc. ,.yal Soc. (l :n on) 2126:214 (1l4:), The r-rtiele conveys the
idea that the cellulose chains are arrar.ed lewt;:ise i- !he primary layer
of t.e cell wall in contradistinction to the circ-il.r trarsverse e.rrarger-.ent
around the fiber as r ''-crep by other research '.or:ers.
a.rr, Th-.'a 7. Structure a, c -'o-ition cf pn'pn cell -e.'br.res.
)Tatre 14e;153 (-"4l). .e article is a brief review .*f the 'au.Ir's
c,-r:cepts !iven in previous articles. Cellulose ellii-ids about 1.1 by
1.5 -icrons are the mricro^tructural units of the cellulose Membrane. They
are covered and cemented t,- e-.?r by means of a pecti.-like material.