412 2 3-
Botany Circular No. 32 Fla. Dept. Agric. & Consumer Services
July/August 1996 Division of Plant Industry


Which Boston Fern Is It?
The Exotic Nephrolepis cordifolia (L.) Prefrston Science
Library
or the Native Nephrolepis exaltata (L.) Schott1
MAR 17 1997
Nancy C. Coile2
University of Florida
INTRODUCTION: Unfortunately, several species of Nephrolepis have been called Boston fern (Wunderlin
1982) and the general appearance of these species is very similar. The original "Boston fern," a mutant of
N. exaltata (L.) Schott (recognized ca. 1870), was cultivated for its graceful, broader fronds (Jones 1987) and
soon became the most cultivated fern. Numerous popular cultivars of N. exaltata are grown, including cvs.
Bostoniensis, Rooseveltii, Fluffy Ruffles, and Whitmanii Compacta (Huxley 1992). Nephrolepis cordifolia is
widely grown, especially the cultivars Petticoat, Duffii, and Plumosa (Huxley 1992; Jones 1987).

Small (1938) indicates that N. cordifolia (L.) C. Presl (sword fern) is an escape from cultivation and persists
especially in the crowns and boots of palm trees, in cypress swamps, around old homesteads and rubbish
heaps. Small (1938) gives the habitat for the native N. exaltata (wild Boston fern, Boston fern) as in the
hammocks of Lake County, FL and southward. Small's range agrees quite well with the distribution map
of N. exaltata (Wunderlin et al. 1995) which documents N. cordifolia in Lake, Citrus, Seminole and Brevard
counties southward and with outliers in Columbia, Duval and Leon counties, but excludes Orange,
Osceola, Manatee and DeSoto counties. Wunderlin et al. (1995) show N. biserrata from Broward, Collier,
Dade, Highlands, Manatee, Martin, Monroe and Palm Beach cos. and N. exaltata as occurring throughout
most of the state. Only N. exaltata and N. biserrata are considered to be native species (Small 1938; Nauman
1993).

Recently, there has been much concern about whether cultivated Boston-type ferns are the native N. exal-
tata or one of the non-native Nephrolepis species. This confusion about the species identity can easily be
cleared up by careful examination of Wunderlin (1982) and Nauman (1993) and by understanding the
terms used. Fern morphological terms which are important in understanding Nephrolepis include "peti-
ole" (as in flowering plants, the stem-like part of the leaf), pinnaee" (leafblade segments, or leaflets; pinna,
singular), "adxial costae of central pinnae" (the upper surface veins of a blade segment from the center
part of the blade), "sporangium" (tiny structure which contains spores), "sorus" (a group of sporangia;
singular, sori), "indusia" (the flap of tissue which covers several sporangia), "rachis" (the axis which bears
the leafblade segments), and "frond" (leaf blade).

IDENTIFICATION: Nauman (1993) cites four species of Nephrolepis and a hybrid as occurring in Florida:
N. cordifolia, N. exaltata, N. biserrata (Swartz) Schott, N. multiflora (Roxb.) F. M. Jarrett ex C. V. Morton, and
N. x avenri Nauman. The hybrid is between N. exaltata and N. biserrata and has been found only where the
two species occur together.

Wunderlin (1982) and Nauman (1993) report that while N. cordifolia may have tubers, the native N. exalta-
ta never has tubers. Nephrolepis exaltata and N. cordifolia lack hairs on the upper surface of the pinnae,
while N. multiflora and N. biserrata have short hairs on the upper surface of the pinnae. The leaflet tips of
N. cordifolia are blunt (Fig. 1A), while those of N. exaltata, N. biserrata and N. multiflora are attenuated to
long slender pointed tips (Fig. 1B). The pinnae bases of N. cordifolia overlap the abxial (lower surface) of
the rachis (Fig. 1A), but those of N. exaltata occasionally will also overlap the rachis. The presence of bicol-
ored scales on the upper surface of the rachis (Fig. 2) will distinguish N. cordifolia from all other Nephrolepis
species which lack the strong color difference. However, while N. biserrata has constant color scales on the
rachis, there may be bicolored scales on the petiole bases.


1 Contribution No. 37, Bureau of Entomology, Nematology and Plant Pathology Botany Section.
2 Botanist, FDACS, Division of Plant Industry, P.O. Box 147100, Gainesville, FL 32614-7100.











Fig. 1. A. Upper surface of N. cordifolia blade, showing blunt
tips for the pinnae. In the enlarged circle, the lower surface
of the pinnae are shown. Note the kidney-shaped sori; the
pinnae bases conceal the rachis of the lower surface. B. Life
size pinnae of N. exaltata, showing attenuated tips.
Illustration credit: W. D. Ross McClain.


Fig. lb.


Fig. 2. Scales on upper surface of the rachis of N.
cordifolia, Note that the attachment point of the
scale is dark-colored and the surrounding scale
tissue is pale. Use at least 10x magnifying lens to
view. The leaf segments in the middle of the
frond may be up to 9 cm long and 0.9 cm wide.
Photography credit: Jeffrey W. Lotz.


Fig. la.









Key to the Native or Naturalized Species of Florida Nephrolepis


Tubers p resent......................................................................................................................................... ...... N cordifolia
Tubers absent.
Hairs present on the upper surface of the pinnae; indusia less than 1 mm wide and circular, horseshoe,
or peltate.
Petioles have many dark scales with pale margins...................................................N. multiflora
Petioles have a few light brown scales with reddish to light brown margins.
Densely hairy......................... .. ...................... .......................... N .. biserrata
Sparsely hairy.................................. ............ ........... ................................... N x averyi
Hairs absent; indusia greater than 1 mm wide and shaped like kidneys, horseshoes, or half-moons.
Petioles with pale brown scales; rachis with pale to dark brown scales which have a dark
point of attachm ent..................................................................................................................N cordifolia
Petioles with pale brown to reddish brown scales; rachis with pale to dark brown scales
and sam e color throughout.............................................................. .......................... N exaltata






LITERATURE CITED

Jones, D.L. 1987. Encyclopaedia of ferns. Timber Press, Portland, OR. 433 p.
Huxley, A. J. (ed.) 1992. New Royal Horticultural Society dictionary of gardening, 4 vols. Macmillan
Press, London. 3240 p.
Nauman, C.E. 1993. Dryopteridaceae, No. 18. Nephrolepis, p. 305-308. In Morin, N.R. (ed.) 1993. Flora of
North America, Volume 2: Pteridophytes and Gymnosperms. Oxford University Press, New York.
475 p.
Small, J.K. 1938. Ferns of the southeastern states. Facsimile, 1964, by Hafner Publishing Company, New
York. 517 p.
Wunderlin, R.P. 1982. Guide to the vascular plants of Central Florida. University of South Florida Press,
Tampa. 472 p.
Wunderlin, R.P., B.F. Hansen, and E.L. Bridges. 1995. Atlas of Florida vascular plants. University of
South Florida, Tampa. Internet Address: http://www.usf.edu/-isb/projects/hb-atlas.html.