Seedless Vascular Plants

The Lycophyta, Equisetophyta, and Psilophyta are collectively referred to as the fern allies because, like the ferns (Pterophyta), they reproduce by single-celled spores released from sporangia (spore sacs). They do not produce flowers or seeds and both ferns and fern allies contain well-developed conducting tissues to transport fluids within the plant. Fern allies, however, differ greatly in appearance from ferns because they generally bear small, simple leaves with an unbranched vein, whereas almost all ferns have larger, often lacy cut leaves called fronds that contain branching veins. The fern allies include some of the earliest known land plants, many of which are long extinct. Today, there are probably fewer species of fern allies than there were many millions of years ago.

The life cycles of the fern allies and ferns are similar. Alternating generations of sporophytes (diploid plants producing spores) live independently of gametophytes (haploid plants producing eggs and sperm). The sporophyte is the dominant of the two generations; it has two sets of chromosomes per cell and is larger and more conspicuous. Through meiosis in the sporangia (spore sacs), the number of chromosomes in some cells is reduced by half; these cells develop into spores. If a spore lands on a suitable site it will germinate to form a gametophyte, usually less than 1 centimeter across. The resulting gametophyte, with a single set of chromosomes per cell, produces an egg in each archegonium (vase-shaped structure) and sperm in each spherical antheridium. Sperm are released from antheridia and, in a drop of water, they swim to an egg and unite with it to create another sporophyte with two sets of chromosomes.

Lycophyta

In the Carboniferous period (over three hundred million years ago), Lycophyta included large trees that are now extinct, but which have left remains preserved as coal. This division of fern allies is represented today by three distantly related families of small herbaceous plants called club mosses, spikemosses, and quillworts. The club mosses are homosporous (producing spores of one size) while spikemosses and quillworts are heterosporous (producing spores of two sizes). These plants generally grow to less than 20 centimeters high, rarely up to 1 meter. They have branched or unbranched stems that are erect, creeping, or hanging, and covered with simple, one-veined leaves. Their roots branch with equal forks. Sporangia are borne singly in the upper angle formed between leaf and stem. Leaves associated with sporangia are clustered in zones along the stem or packed into terminal cones. Heterosporous lycophytes are distinctive for the small flap of tissue on the upper surface of each leaf called a ligule.

Club Mosses (Lycopodiaceae).

There are about 375 species of club mosses distributed worldwide, especially in mountainous tropical habitats. These large mosslike plants, whether terrestrial or epiphytic, have branching stems that are densely covered with small, narrow leaves. Unlike spike-mosses and quillworts, club mosses are homosporous and have kidney-shaped sporangia that open like clams. The sporangia may be clustered in zones or packed in terminal cones. The small, disc- or carrot-shaped gametophytes associate with fungi for assistance with the uptake of nutrients. Princess pine (lycopodium sp) is a common lycopod in eastern forests.

Spikemosses (Selaginellaceae).

Most of the approximately 750 species of spikemosses occur in tropical and subtropical regions where they occupy a variety of habitats ranging from rain forests to deserts. These mosslike terrestrial plants typically are less than 2 centimeters high. Like the club mosses, they have branching stems densely covered with small, narrow leaves. The sporangia of most species are packed in four-sided, terminal cones. They are heterosporous, usually producing four megaspores in each megasporangium and hundreds of microspores in each microsporangium. Upon germination, the tiny megagametophytes produce eggs and the minute microgametophytes release numerous sperm when the spore wall opens.

Quillworts (Isoetaceae).

There are probably over two hundred species of quillworts distributed worldwide in a range of habitats including lakes, streams, roadside ditches, and soil pockets on exposed rocks. The slender leaves of these terrestrial or aquatic plants can grow up to 50 centimeters long, and some can grow up to 1 meter. The short and squat to globose stems are covered with long, thin leaves, giving the plants the appearance of a tuft of grass. The sporangia are embedded in a basal cavity of the leaf. They are heterosporous, usually producing tens to hundreds of megaspores in each megasporangium and thousands of microspores in each microsporangium. Upon germination, the tiny megagametophytes produce eggs and the minute microgametophytes release four sperm when the spore wall opens.

Equisetophyta (Horsetails and Scouring Rushes)

These plants are distinctive for their tubular, grooved, and jointed stems. Although more diverse in the fossil record, today they are represented by only fifteen species, which are distributed nearly worldwide in moist to wet, often-disturbed habitats including shores, roadsides, marshes, and woodlands. Silica in the stems makes them useful for scouring and sanding— hence, one of their common names is "scouring rushes." These plants are usually less than 1 meter tall, but on occasion can grow to several meters. Their stems range from horizontal to erect and can be branched or un-branched. They bear whorls of leaves fused along their edges to form a slightly expanded sheath at each joint. In cross section, stems are seen to have a large central canal and smaller canals under the grooves and ridges. The sporangia hang from six-sided, umbrellalike sporangiophores, which are packed into terminal cones. The plants are homosporous and the spores are notable for the tiny, straplike elaters that coil and uncoil to aid in their dispersal. The small, lobed, cushionlike gametophytes initially produce either eggs or sperm. Gametophytes of some species, initially producing archegiona, later develop antheridia.

Psilophyta (Fork Ferns)

The Psilophyta have long been thought to be among the most primitive of all living vascular plants because of their similarity in form to some of the oldest land plant fossils. Recent studies, however, indicate that they may be more closely related to the ferns than to the fern allies. There are about seventeen species, growing mainly in the tropics and subtropics. Most grow as epiphytes on tree fern trunks. They are called fork ferns (or whisk ferns) because the leaves associated with the sporangia (sporophylls) are forked, whereas their other leaves are simple or absent. Fork ferns grow less than 0.5 meters high and are without roots. They have horizontal, erect, or hanging stems that may be branched or unbranched. Their leaves are scale-like and with or without a vein. They are homosporous, bearing two- or three-lobed, fused sporangia on or above the sporophylls. The spores produce small subterranean gametophytes that associate with fungi for assistance with the uptake of nutrients. Gametophytes look similar to the underground branches of the sporophyte.

see also Bryophytes; Epiphytes; Evolution of Plants; Ferns; Vascular Tissues.

W. Carl Taylor

Patricia A. Batchelor

Bibliography

Gifford, Ernest M., and Adriance S. Foster. Morphology and Evolution of Vascular Plants, 3rd ed. New York: W. H. Freeman and Company, 1989.

Judd, Walter S., Christopher S. Campbell, Elizabeth A. Kellogg, and Peter F. Stevens. Plant Systematics: A Phylogenetic Approach. Sunderland, MA: Sinauer Associates, Inc., 1999.

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

Walters, Dirk R., and David J. Keil. Vascular Plant Taxonomy, 4th ed. Dubuque, IA: Kendall/Hunt Publishing Company, 1996.

Seedless Vascular Plants

When one walks through a contemporary forest, all of the surrounding trees are vascular plants. Wood, which is made up primarily of xylem , and bark, which contains phloem , are the major structural elements of the trunks and stems. These trees produce seeds, whether they be formed within the cones of the pines or within fruits, such as the winged samaras of maples or the fleshy cherries of the cherry tree. In the Carboniferous period a similar hike would also place one in a forest of woody trees, many as large as 98 feet (30 meters) tall, but there would be no seeds produced. These plants were seedless vascular plants, which were propogated by spores. Ultimately they became extinct, leaving behind expansive fossil fuel deposits.

The differences between spores and seeds are extensive. Seeds are multicellular structures that provide a protected place where the egg (n) was to be fertilized by male gamete nuclei (n) in pollen. The resulting cell, called a zygote , is diploid (2n), the same as the original plant that produced it. It repeatedly divides mitotically, while still within the protection of the seed coat, to form an embryo. The embryo may remain dormant for a significant time period but ultimately emerges from the seed when it germinates.

The seedless vascular plants do not have this protection. Their gametes are produced mitotically by a gametophyte (n) that lives independently. There are often many vase-shaped archegonia on these small plants, and the unfertilized egg is inside the base of this structure. The embryo formed following fertilization is not as well protected as one located within a seed. It grows and emerges from the archegonium, where it is exposed to the environment.

These embryos survive best to develop into young sporophyte individuals when they are in moist habitats, whereas seeds can endure more severe conditions and therefore can have wide habitat type distributions. The sporophyte eventually develops rhizomes, underground stems, roots, or rhizoids that serve to absorb water and nutrients and allow independent survival. In the case of the plants of the Carboniferous forests, this development was extensive.

The simplest type of spore production in living seedless vascular plants is found in the leafless whisk fern, Psilotum, a member of the phylum Psilotophyta. This sporophyte (2n) plant, really little more than a branching twig, has many sets of three-fused sporangia, which produce spores through meiosis . These haploid (n) spores grow into minute plants about the size of a small piece of macaroni. These plants have archegonia and small gametangia (which produce the male gametes) to complete sexual reproduction and make new sporophytes.

Many variations on this basic spore-producing alternation of generations life cycle are found in the seedless vascular plants. The carboniferous trees, which are now recognized only through the study of their fossils, most likely had common ancestors with a variety of present-day organisms including the ferns, horsetails, and lycopods.

The modern ferns, phylum Pterophyta, have leaves of varying sizes and shapes and still occur as trees in tropical areas. The descendents in the other groups do not presently attain treelike stature. Horsetails or scouring rushes, phylum Sphenophyta, have ancestors stretching as far back as the Devonian era. A ribbed, silicon-impregnated stem has branches in many species that are whorled and give the plant the appearance of a bottle brush or animal tail.

The phylum Lycophyta includes club mosses, sometimes called ground pines, which in some cases have stems with reduced leaves (microphylls) fused together to look like the foot of an animal. The name lycopodium means "wolf foot," and most likely originated because of this morphological analogy. The selaginellas, which often superficially resemble mosses, are in fact very different from mosses. Their small, leafy green stems are sporophytes (2n) and have vascular tissue. In contrast, the small leaves of mosses are gametophytes (n) and no vascular tissue is present. Shifts like this, from a predominant gametophyte generation to a predominant sporophyte generation represent one of the major trends of evolutionary advance in the plant kingdom.

see also Alternation of Generations; Bryophytes; Evolution of Plants; Plant; Pteridophytes

Dean Cocking

Bibliography

Perry, James W., and David Morton. Photo Atlas for Botany. Belmont, CA: Wadsworth Publishing Company, 1998.

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

Van De Graff, Kent M., Samuel R. Rushforth, and John L. Cawley. A Photographic Atlas for the Botany Laboratory, 3rd ed. Englewood, CO: Morton Publishing Company, 1998.