| Taxonomic Classification: Kingdom: Plantae |
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| Swordfern Fig. 1 Ferns in a redwood stand
(Gerald and Buff Corsi, n.d., CalAcademy) |
San Francisco State University
Geography 316: Biogeography
The Biogeography of
the Sword Fern (Polystichum
munitum)
by M. Houston, student in Geography 316, Fall 2000
| Taxonomic Classification: Kingdom: Plantae |
|
| Swordfern Fig. 1 Ferns in a redwood stand
(Gerald and Buff Corsi, n.d., CalAcademy) |
This classification is that of the United States
Department of Agriculture (USDA, 2000), but it must be noted that fern classification is
subject to disagreement (Hickman, 1993; Jones, 1987). “Fern classification,” as
the Jepson Manual says, “is in rapid flux,” (Hickman, 1999).
Description of Species:
The sword fern, also known as the western sword fern, is a
member of a large plant group known as the Pteridophyta, consisting of not only all the
other ferns, but the fern-allies, such as horsetails and clubmosses, as well (Frankel,
1981; Mickel, 1979). All of the Pteridophytes lack flowers, fruits and seeds, and
reproduce by means of spores (Frankel, 1981). They are also vascular plants, meaning that
they have an internal circulatory system of specialized tissues grouped as bundles in the
roots, stems and leaves of the plant. One of these specialized tissues is called the
xylem, and it transports water from the roots to the leaves, while the other, the phloem,
transports nutrients and hormones within the plant (Jones, 1987; Mickel, 1979). The ferns
are distinguished from their allies by the presence of megaphylls: large, complex leaves
with many veins (Mickel, 1979).
Polystichum munitum is a typical fern in its morphology. It has an underground rootstock called a rhizome, which anchors the plant into the substrate, where, by means of small, hairlike roots, minerals and water are drawn from the soil (Frankel, 1981; Mickel, 1979). The sword fern’s rhizome is covered by reddish- brown scales, vestigial leaves one cell thick which serve to protect the rhizome (Jones, 1987; Mickel, 1979). The above-ground part of the plant is called a frond or leaf, and is divided into two major parts, a leafless stem called a stipe that reaches from the rhizome to the other and much larger part, the blade (Jones, 1987; Mickel, 1979). The stipe can be 6-55 cm long (Lellinger, 1985), is usually less than one quarter the length of the frond (Mickel, 1979), and is covered with scales both small and large, up to about 2 cm long (Lellinger, 1985). The stipe of the sword fern is bright glossy brown (Grillos, 1966). The sword fern has many fronds, sometimes as many as 75 to 100 (Grillos, 1966), 6-22 cm wide (Lellinger, 1985), and up to 1 meter in length (Keator and Heady, 1981). On the blade, the stem is called a rachis, and it is bright glossy brown in appearance (Grillos, 1966). Young fronds that have not yet unfurled themselves are known as fiddleheads. The fronds are compound—divided into separate leaflets—and each leaflets is called a pinna, plural pinnae (Frankel, 1981). The pinnae are lanceolate and variously serrated, and have a lobe at the base, near the rachis (Grillos, 1966). On the underside of the pinnae are brownish spore cases called sporangia, singular, sporangium, gathered into a group called a sorus, plural, sori (Keator and Heady, 1981). The sori of the sword fern are covered by a veil-like growth (the indusium) of the pinna which has small, straight hairs along its edge (ciliate) (Hickman, 1993).
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| Swordfern fig. 2 Parts of a fern (Grillos, 1966) | Swordfern fig. 3 Some parts of the swordfern (Grillos, 1966) |
Natural History:
Polystichum munitum, in common with other plants, can reproduce
vegetatively, that is, by the branching of its rhizome (Frankel, 1981). The sword fern can
also reproduce sexually, which it does with an alternation of generations (Grillos, 1966).
This is a reproductive cycle that consists of two generations, thus, the fern that we
might see in a shady forest grove is only half of the complete life cycle. This insouciant
forest dweller belongs to the sporophyte generation because it bears spores
in the sori on the undersides of the pinnae (Grillos, 1966). The spores themselves are
produced in the sporangia by meiosis (Keator and Heady, 1981). This results in each spore
possessing only one set of chromosomes (haploid) instead of the full complement of two
sets (diploid) that are found in the cells of the sporophyte plant. The sporangium
contains the spores until they are ripe. When this time comes, a ring of cells around the
sporangium called an annulus bursts open, causing the sporangium to snap
open, casting the spores upon the wind (Frankel, 1981). The spores eventually land, but
they do not produce a recognizable fern. If they land in supportive conditions, the spores
will grow into the other generation of the life cycle, the gametophyte
(Grillos, 1966). The gametophyte, or prothallis, is a flat, thin
heart-shaped plant (often mistaken for a liverwort) which is about 1 cm long and has
hairlike rhizoids on its underside to act as roots (Frankel, 1981; Keator
and Heady, 1981). Also on the underside are the sexual reproductive organs, the male antheridia
and the female archegonia (Frankel, 1981). Inside each antheridium are
sperms, coiled up and growing (Frankel, 1981). When they reach maturity, the antheridia
open and the sperms swim in a film of water toward the archegonia, drawn by a chemical
attractant released by the archegonia, each of which contains a single egg (Frankel,
1981). If a sperm manages to pass down the neck of archegonia and fertilize the egg, a
zygote with both sets of chromosomes is formed and grows, nourished by the prothallis
(Keator and Heady, 1981). The other sperms, meanwhile, die off (Frankel, 1981). When the
tiny fern grows its own root and frond, it has also grown independent of the prothallis,
which dies away (Keator and Heady, 1981).
Swordfern fig. 4 Sori on blade underside
(Charles Webber, 1956, CalAcademy)
Evolution:
Except in some tropical and subtropical places, the ferns and
fern-allies are no longer the common and dominant part of the vegetable landscape that
they were in the distant past. According to the fossil record, the ferns and fern-allies
began to be replaced by early seed plants like the cycads and conifers beginning in the
Mesozoic era, starting some 230 million years ago (Lellinger, 1985). Although the origin
of the ferns and fern-allies cannot be definitively placed, due to the incomplete fossil
picture, there is evidence of fern-like plants in the Devonian period, beginning about 395
million years ago (Lellinger, 1985). Lellinger (1985) further suggests that these
fern-like plants likely got their start in the late Silurian period, perhaps about 400
million years ago. The Silurian period would have been a time of great change for
these plants, for it was then that they made the shift from water to land, and such a
transition required extensive physiological and morphological change: to slow the loss of
water to the air, they developed an epidermis; to allow the exchange of oxygen and carbon
dioxide between the plant and the surrounding atmosphere, they developed stomata (pores);
they developed vascular systems to internally move water, minerals and nutrients; and, in
order to support themselves, they evolved the necessary physical structure (Lellinger,
1985).
The Carboniferous period, beginning some 360 million years ago, may have been the heyday of the ferns and fern-allies, for it was then that great coal deposits began to be formed (Frankel, 1981). These deposits are composed of the remains of ferns and fern-allies, which went through a process of being laid down in many layers as they died, and then being subjected to pressure and heat which served to compress the layers into the various forms of coal known as lignite, bituminous and anthracite (Frankel, 1981).
The significant gaps in the fossil record make it impossible to be clear about the evolutionary path of the sword fern, in taxonomic terms, except to say that it seems to be a descendent of the prehistoric class Coenopteridopsida, which descended from the even more ancient class Trimerophytopsida (Lellinger, 1985).
Distribution and Habitat:
Dryopteridaceae, the Wood Fern family, consists of
approximately 850 species distributed among 32 genera. Of these, half are found only in
the Old World, while the majority of those found in the New World are exclusively
tropical. Arachniodes, Ctenitis, Dryopteris, Tectaria, and Polystichum
are the main genera of Dryopteridaceae, and they can all be found in the United States
(Lellinger, 1985).
The genus Polystichum is large and occurs
throughout the world, examples being found on all continents except Antarctica (Tryon and
Tryon, 1982). The number of species is estimated to range from 135 (Lellinger 1985) to 160
species (Tryon and Tryon, 1982), with the majority of these located primarily in the north
and south temperate zones. The plants of this genus can usually be found growing in moist
forests, or in shrubby or grassy spaces, particularly among rocks, when at higher
elevations or latitudes (Tryon and Tryon, 1982). A small number of species can also be
discovered on cliff ledges or in crevices (Lellinger, 1985). About 55 species of Polystichum
occur in the American tropics, where they tend to be found in similar places as their
temperate cousins, as adjusted for the tropics: moist montane and cloud forests, stream
banks and ravines, the occasional cliff or rocky slope (Tryon and Tryon, 1982). Tryon and
Tryon (1982) report that the numerous and widespread nature of the genus seems to be not
only a function of its numbers, but also of a tendency for its members to freely hybridize
with each other.
Swordfern fig. 5 Distribution in the lower 48 states
Like the genus as a whole, Polystichum munitum, the
western sword fern, is distributed over a large area. It is abundant and common in
continuous populations in California, Oregon, Washington, Idaho, Montana, British
Columbia, Yukon Territory, and Alaska, and may also be found in disjunct populations in
the Black Hills region of South Dakota and on Guadalupe Island off Baja California
(Lellinger, 1985; Hickman, 1993). It seems likely that the occurrence of the sword fern on
Guadalupe Island is due to
sweepstakes dispersal. This may be the case for the Black Hills
distribution also, though I wouldn’t exclude the possibility that it is a climatic
relict.
Swordfern fig. 6 California distribution (Calflora)
In California, which is the location of emphasis in this essay, there is some difference of opinion as to the range of the fern, though Hickman (1993), Grillos (1966), and Keator and Heady (1981) are in general agreement. The distribution map indicates the widespread nature of the species in the state. Like others of the genus, P. munitum is common in moist woods and on shady slopes, frequently in association with California redwoods, Douglas firs or other trees. It can also occasionally be found on cliffs and rocky outcrops, and when found in the mountains of the Sierra Nevada, it is typically below 1600 meters in elevation (Hickman, 1993). As long as there is good drainage, the sword fern is adapted to numerous kinds of soils (Jones, 1987). It is also commonly distributed among floral arrangements.
Fun with the Sword Fern:
The sword fern grows easily, and once established, is very
hardy, making it ideal for the amateur pteridophile or ornamental gardener. Cultivars are
available through nurseries for those desiring a sword fern of their very own, but much
more exciting is to cultivate one from a wild plant. This can be done by means of either
vegetative reproduction or propagation from the spores; as these methods are somewhat
involved, particularly the spore method, I refer you to Frankel (1981), Grillos (1966),
Mickel (1979), and most especially, Jones (1987), whose aptly named Encyclopedia of
Ferns is a veritable treasure-trove of pteridophilia. All of these sources also
contain information on fern propagation, as well as numerous other fun fern activities,
ranging from pressing ferns to making various kinds of frond prints to candle making.
Bibliography
Calflora, 2000. Calflora Database. [Online]. Available: http://www.calflora.org/cgi/calflora_query?where_clarecnum6793&one=T&special=photo[October 10, 2000].
California Academy of Sciences, 2000. Manzanita Project. [Online]. Available: http://elib.cs.berkeley.edu/cgi/img_query?where-genre=Plant|Fungi&where-taxon=Polystichum+munitum
Frankel, Edward, 1981. Ferns, A Natural History. Brattleboro, Vermont. The Stephen Greene Press.
Grillos, Steven J., 1966. Ferns and Fern Allies of California. Berkeley and Los Angeles, CA. University of California Press.
Hickman, James C., ed., 1993. The Jepson Manual, Higher Plants of California. Berkeley and Los Angeles, CA. University of California Press.
Jones, David L., 1987. Encyclopedia of Ferns. Portland, Oregon. Timber Press.
Keator, Glenn, and Heady, Ruth M., 1981. Pacific Coast Fern Finder. Berkeley, CA. Nature Study Guild.
Lellinger, David B., 1985. A Field Manual of the Ferns and Fern-Allies of the United States and Canada. Washington, D.C. Smithsonian Institution Press.
Mickel, John T., 1979. How to Know the Ferns and Fern Allies. Dubuque, Iowa. Wm. C. Brown.
Tryon, Rolla M., and Tryon, Alice F., 1982. Ferns and Allied Plants, with Special Reference to Tropical America. New York, Heidelberg, and Berlin. Springer-Verlag.
United States Department of Agriculture, July 3,
2000. Integrated Taxonomic Information System.[Online]. Available: http://www.itis.usda.gov/plantproj/itis/cgi_bin/class_report.cgi?kingdom=Plantae&input=504530&input_type=T[October 10, 2000].
send comments to bholzman@sfsu.edu
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