The ferns and their allies are vascular land plants. Two primary characteristics distinguish them from the bryophyes: 1) lignified cells in vascular tissues, and 2) dominance of the sporophyte rather than gametophyte phase of the life cycle. Unlike the gymnosperms and flowering plants, no members of the ferns or their allies produce seeds. The allied groups include psilophytes (whisk ferns), lycophytes (club mosses), and sphenophytes (horsetails). Also included are several extinct groups known only from their fossils. Only the extant groups are featured here.

A. Psilophytes

Sporophytes of early vascular plants probably consisted of stems only. Leaves and roots developed later. These organs are stem modifications usually specialized for photosynthetic and anchorage or absorptive functions, respectively. Few existing plants produce sporophytes that lack leaves and roots. Two genera, which together comprise the psilophytes, are among these. Both are restricted to tropical or sub-tropical regions.

Psilotum is the namesake genus of the group. Underground stems with unicellular rhizoids anchor this plant. Erect aerial stems branch in three dimensions (Fig.1). Stems have a simple solid core of vascular tissue. Photosynthetic tissues are near the outer stem surface. Surface stomates facilitate gas exchange. Sporangia are borne on short stalks. Spores within sporangia are of the same size. Small amorphous gametophytes that develop from spores grow underground. Multicellular male and female gametangia appear together on gametophytes.

Psilophyte sporophytes that exist today are very similar to fossil plants like Rhynia. Rhynia and its relatives originated 400 million years ago and flourished in the coal producing forest of the Carboniferous times. Included were many tree-like as well as herbaceous forms. This fossil group disappears altogether from the fossil record. Are the psilophytes today remnants of this extinct group? Evolutionary reduction from a simple fern ancestor has been suggested as an alternative possibility.

B. Lycophytes

Lycopsids constitute a small group (1000 species) of herbaceous plants. Some species are commonly called club moss. This name relates to the small leaves called microphylls that they produce. The "clubs" of these plants are clusters of sporangia aggregated at the tip of fertile stems. Sporangia in other species called fir moss (Fig.2) appear in leaf axes along fertile stems. Many lycophytes are tropical epiphytes. Temperate species form evergreen mats on forest floors. The quill-like microphylls of quillworts grow in clusters submerged on the edges of rivers and ponds.

Embryos in this group develop within female gametangia on small amorphous gametophytes. Gametophytes produce either bi- or multi-flagellated sperm that must swim to the eggs of female gametangia to accomplish fertilization. Advances over the simple leafless and rootless structures of mature psilopsid sporophytes include: 1) a more complex vascular tissue system for stems (various siphonostele types), 2) small photosynthetic leaves with a midrib of vascular tissue that connects to the vascular tissue of stem, 3) roots, 4) two types of sporoangia in some species, each producing spores of different sizes (Fig.3).

Lycophytes fossils extend back 400 million years in geologic strata. Trees as well as herbaceous forms flourished in coal forest of Carboniferous times. The group appears to have descended from an extinct assemblage of similar vascular plants called Zosterophyllophytes. Zosterophyllophytes disappeared from the fossil record at the end of the Carboniferous era.

C. Sphenophytes

The sphenopsids are represented by a single living genus – Equisetum – with only 15 known species. These few species are nevertheless widespread, particularly in damp, temperate or tropical habitats. Members of the group are readily recognized by the whorled arrangement of microphyll-like leaves on articulated aerial stems. Silica deposits on epidermal cells made temperate species useful for cleaning pots and pans during colonial times. Hence, the common name of scouring rushes. Sphenophytes are sometimes called horsetails. This name is derived from the appearance of fertile leafless stems in some species where sporangia cluster around the stem tip (Fig.4). Spores produced in sporangia have distinctive lateral appendages called elaters. Like the sterile cells intermixed with liverwort spores, these structures aid in spore dispersal. Germinating spores give rise to small thalloid-like gametophytes, usually in muddy locales. Multi-flagellated sperm are released from antheridial gametangia. Water is required for fertilization. Embryos develop within female gametangia (i.e., archegonia), but quickly outgrow parent plants (Fig.5). Even though Equisetum can produce sexually, asexual reproduction appears to be more common. Stems break apart at the joints just below the leaves. Stem fragments usually have embryonic roots that begin to grow soon after separation from the parent plant.

Fossils nearly identical to Equisetum date back 300 million years. Other extinct members of the group date back 400 million years. As with the lycopsids, tree-like forms and numerous other herbaceous species were prolific in coal forming forests of geologic times.

D. Ferns

For novices, ferns are probably a readily recognizable group. They grow in a variety of temperate and tropical habitats and generally produce large conspicuous leaves called megaphylls. Most of the 11,000 species are herbaceous (Fig.6). Some reach tree-like dimensions by means of adventitious roots that tightly encircle and add girth to stem trunks. At the other end of the spectrum are water ferns that produce small free-floating leaves (Fig.7).

Taxonomists use differences in sporangia of sporophytes to divide the ferns into three subgroups. In one of the smaller subgroups, layers of dividing cells on leaf surfaces yield large sized "eusporangia." In the majority of ferns, smaller "leptosporangia" are derived from single surface cells rather than cell layers. Most of the familiar ferns fall into the latter category. Most of the familiar ones are also characterized by an active dispersal of the small number of similar-sized spores in each sporangium (Fig.8). In the water ferns, leptosporangia are of two types: megasporangia that produce large spores and microsporangia that produce smaller ones. Water ferns are the only "heterosporous" ferns. This subgroup is also unique in the production of drought resistant "sporocarps" that bear sporangia clusters upon germination. In the more common "homosporous" leptosporangiate ferns, sporangia are usually borne in clusters on the undersurfaces or margins of vegetative leaves or on wholly fertile fronds.

A small "prothallus" is the most common type of fern gametophyte (Fig.9). It produces multi-flagellated sperm that swim to eggs produced in multi-cellular female gametangia. Water is still necessary for fertilization. "Apogamous" outgrowths from gametophytes can readily be induced however. This process results in the production of 1N "sporophytes," and serves as a fail safe for sporophyte production in the absence of fertilization. Self fertilization in ferns serves a similar fail safe function. Both processes generate interesting genetic challenges for the ferns that use them.

Fern fossils first appear as remains in the coal-swamp forests of the Carboniferous period 350 - 310 million years ago. Together with lycophytes and sphenophytes, they apparently dominated the vegetation in those forests. Unlike the former two groups, they remained abundant up to and including the present. Trimerophytes may have been the ancestors of ferns. Trimerophytes are a group of early land plants that flourished for a short period during earlier Devonian times and then became extinct.