Pteridophytes

Pteridophtyes are a phylum of plants. They are the vascular plants (those having xylem and phloem tissues) that reproduce by releasing spores rather than seeds, and they include the highly diverse true ferns and other grace­ful, primarily forest-dwelling plants. There are about eleven thousand dif­ferent species of pteridophytes, making them the most diverse land plants after the flowering plants (angiosperms). Pteridophytes may represent the closest living relatives (sister group) to the seed plants. (Seed plants include the angiosperms, the conifers, and a smaller assortment of other plants.)

Ferns reproduce by releasing spores rather than seeds.

As in seed plants, the greatest diversity of pteridophytes is found in the tropics, with only about six hundred species adapted for life in temperate cli­mates. Species living today are relics of ancient lineages that once dominatedthe land surface. There is a rich fossil record showing that pteridophytes have ancestors dating back nearly four hundred million years. Before there were seed plants, there were pteridophytes such as large, treelike (up to 36.5 me­ters [120 feet] tall) Lepidodendron, an ancestor of modern club mosses (which are no more than .30 meter [1 foot] tall), and shrubby Sphenophyllum, a fore­bear of today’s horsetails. Some of the ancient predecessors of modern ferns were also preserved, but there are comparatively few fossils for interpreting relationships among the approximately eleven thousand species of true ferns. These relatively young species probably arose along with the other most re­cent lineages of vascular plants, the angiosperms or flowering plants, another group lacking an extensive fossil record.

Pteridophytes range greatly in size. There are tiny floating ferns used as “green fertilizer” in rice paddies because they partner with bacteria that pull nitrogen from the air and “fix” it in chemical compounds that other plants can use. In some tropical forests, the largest plants are tree ferns that can be up to 30 meters (100 feet) tall and have huge spreading leaves up to 4.5 meters (15 feet) in length. Pteridophytes also show a transition from simple to complex leaves. Some pteridophyte groups, including the club mosses and horsetails (classes Lycopodiopsida and Equisetopsida), have sim­ple microphyllous leaves, featuring a single, unbranched vein and modest vascular supplies that do not cause breaks or gaps in the stem vasculature. The true ferns (class Filicopsida), however, have larger, more complex macrophyllous leaves whose veins are usually extensively branched, placing such large demands on the plant’s vasculature that distinctive gaps form in the xylem and phloem of the stem.

All pteridophytes have a true alternation of generations, in which a dom­inant sporophyte generation produces spores through meiosis, and a free- living gametophyte generation forms gametes (egg and sperm) by mitosis. Ferns can be used to illustrate the life cycle stages common to all pterido- phytes. Diploid (2n) fern sporophytes are familiar to most people and are often found as quiet accompaniments in floral arrangements. When mature, the undersides of fern leaves produce clusters of capsular structures called sporangia, within which meiosis forms the haploid (n) spores. These spores are released from the sporangia, often when dry wind currents cause the ac­tive snapping of the capsules, lofting the spores into the air.

Spores that are wind-borne to shady, moist habitats germinate and yield multicellular, but microscopic, gametophytes, the sexual stage of the life cy­cle. These short-lived, delicate plants mature and produce egg-forming archegonia and sperm-producing antheridia. When water is present, multi­flagellated sperm swim from mature antheridia, are chemically attracted to the necks of the archegonia, and fertilize the eggs. Although frequently bi­sexual (hermaphroditic), in most cases the sperm produced by a gameto- phyte cannot successfully fertilize its own eggs and must swim to archegonia on neighboring, genetically different gametophytes. The diploid zygotes, produced by the fusion of haploid egg and sperm, divide mitotically and dif­ferentiate into mature sporophytes, completing the life cycle.

Although most pteridophytes are homosporous (produce spores that are all the same size), a few groups are heterosporous with large megaspores and small microspores. The megaspores produce megagametophytes that only form eggs, and microspores only produce microgametophytes and sperm. Heterospory evolved independently in several groups of vascular plants, including all members of the orders Selaginellales and Isoetales and those in a few fern groups (the families Marsileaceae and Salviniaceae of the class Filicopsida). The most successful origin of heterospory ultimately re­sulted in the great diversity of seed plants.

No pteridophytes are cultivated as crop plants, but the leaf buds (“fid- dleheads”) of some ferns are commercially harvested and canned or frozen. Fern leaves used in floral arrangements are a major industry in Florida, and in some cultures tree fern stems are used to make elegant, naturally sculpted bowls. The contrasting colors of the vascular tissue in the stems and leaf bases of these plants create complex and pleasing designs. In the past, club moss spores provided the powder used to coat rubber gloves and prophy­lactics, and photographers used masses of these same spores as flash pow­der, since they could be easily and quickly ignited.

References

Jones, David. Encyclopaedia of Ferns. Melbourne, Australia: Lothian Publishing Co., 1987.

Raven, Peter H., Ray F. Evert, and Susan E. Eichhorn. Biology of Plants, 6th ed. New York: W. H. Freeman and Company, 1999.

Tyron, Rolla, and Alice Tyron. Ferns and Allied Plants. New York: Springer-Verlag, 1982.