Phylum Lycophyta: Club Mosses and More
The lycophytes are the oldest of the seedless vascular plants that have living representatives. They constitute one of the two major lines (clades) of vascular plants, which split probably in the Silurian Age, but at least by the Devonian. For the last 400 million years, therefore, they have developed independently from the rest of the vascular plants. During this time, they evolved from small, semiaquatic herbaceous plants to huge trees that dominated the Coal Age forests for 40 million years and then, as continental masses shifted and the climate dried, they declined in importance until most became extinct by late Carboniferous‐early Permian time. Their structural features show convergence with taxa on the line leading to the flowering plants. Leaves, wood, trees, and reproductive structures that resemble seeds evolved in both lineages.
There are about 1,200 species today in three lycophyte families: Lycopodiaceae, Selaginellaceae, and Isoetaceae. Both of the latter two families have only one genus each— Selaginella with about 700 species and Isoetes with about 100. None of the lycophytes is over a meter or so tall, even in the tropics where they flourish and are the most abundant. Many are epiphytes growing high in the tree crowns. The temperate zone plants are small, trailing, evergreen plants that once were collected in quantity to place as crudely woven evergreen “blankets” on graves in cemeteries. SomeSelaginella species are known as “resurrection plants” because they grow in arid sites and shut down metabolically during dry periods, rolling their aerial stems into tight balls and appearing lifeless. When moisture is available, they uncurl and flash green leaves into the sun, making and storing sufficient photosynthates to weather the next dry period. A number of lycopods are present in the arctic flora, and many form a groundcover on the forest floor in the northern and montane conifer forests.
The distinguishing features of the lycophytes are the arrangement of their vascular tissues and their leaves—microphylls with only a single vascular strand. The sporangia on the modern plants are kidney-shaped, like those of the ancestral forms, and borne on sporophylls clustered in strobili. A distinguishing ligule (scale-like outgrowth) is present in the Selaginella-Isoetes group.
The life cycles of members of the three groups vary. The lycopods are homosporous and the spores give rise to bisexual gametophytes, which in some species develop underground and live with the assistance of a mycorrhizal fungus; others develop on the ground surface. The gametophytic phase of the cycle may last several years (15 in some) before gametes are released and zygotes produced. The embryo develops slowly into the sporophyte, and the latter may remain attached and drawing sustenance from the gametophyte for a long period.
Selaginella species are heterosporous with two kinds of gameto phytes. The megagametophyte (female gametophyte) develops within the megaspore, and when the spore wall breaks the archegonia are exposed. The microgametophyte (male gametophyte) develops biflagellate sperm, which also are released by breakage of the spore wall. They swim to the nearby archegonia; after fertilization the embryo develops within the archegonium, which is retained in the megagametophyte, a situation not unlike that in the flowering plants—except that no integuments and subsequent seed coat grow around the embryo of Selaginella. This is a forerunner of a seed, but not yet one.
The third group of lycopods, the Isoetes line, also is heterosporous with sporangia borne on the quill-like sporophylls that cluster around the corm. A distinctive feature of these plants is a cambium that produces secondary tissues and is located in the corm.