Lycophytes are vascular plants in the class Lycopodiopsida, a division of vascular plants known as Pteridophytes (the ferns and their allies). The class Lycopodiopsida is divided into three subclasses: the Lycopodiidae, Selaginellidae, and Isoetidae.
Like other pteridophytes, the lycophytes have an alternation of generations, consisting of two generations of morphologically different plants. The larger, longer-lived generation is diploid (having both sets of chromosomes) and known as the sporophyte stage. This stage produces structures known as sporangia. The sporangia produce haploid spores (having one set of chromosomes) that can be aerially disseminated into the environment. If a spore lands on a suitable, moist substrate, it will germinate and grow into an independent, haploid structure known as a prothallus (or gametophyte). The prothallus is typically about 0.08-0.12 in (2-3 mm), long and contains both male (antheridia) and female (archegonia) sex organs. Mobile, haploid, flagellated sperm are produced in the antheridium, and these swim to an archegonium to produce a fertilized zygote. The zygote can develop into a new, diploid sporophyte, thus completing the life cycle of the pteridophyte.
The subclass Lycopodiidae consists of one family, the Lycopodiaceae, containing 2-5 genera and 450 species . The most familiar genus is Lycopodium, also known as clubmoss or ground pine. These are terrestrial, perennial, evergreen plants, which grow rooted in the soil or forest floor, and have creeping or erect stems and numerous small, scale-like leaves. Their haploid spores are produced in a club-like structure known as a strobilus. Species of Lycopodium are found on all continents (except Antarctica ) and many oceanic islands. Some species familiar to North America are the ground pine (Lycopodium obscurum), ground cedar (L. complanatum), and running clubmoss (L. clavatum). No substantial economic products are obtained from species of Lycopodium. The spores are rich in a volatile oil, and have been used to make explosive powders. Rhizomatous strings of some species are sometimes collected and used to make evergreen Christmas wreaths.
The subclass Selaginellidae consists of one family, the Selaginellaceae, containing one genus and 700 species. The single genus is Selaginella, or the spike-moss. The spike-mosses are small, evergreen plants of moist terrestrial habitats, although a few species occur in drier places, and others are epiphytes (they live attached to tree limbs, but do not obtain any nourishment from their host). Spike-mosses grow erect or creeping, and they have numerous tiny, scale-like leaves arranged in a spiral on their stem. Unlike species in the Lycopodiaceae, spike-mosses have two kinds of spores, larger megaspores and smaller microspores. Species of Selaginella occur almost world-wide, but are most diverse and abundant in the humid tropics. A widespread species is Selaginella selaginoides, which occurs in boreal and temperate regions of both North America and Eurasia.
The subclass Isoetidae consists of one family, the Isoetacae, containing two genera and 77 species. The most widely distributed genus is Isoetes, or the quillworts. These are aquatic or moist-terrestrial plants, usually growing as a rosette of leaves emerging from a central, corn-like rhizome , from which numerous wiry roots emerge. The leaves are long and grass-like (or quill-like). The sporangia occur on the inside of the inflated leaf-bases, and the plants are heterosporous (having megaspores and microspores). Species of quillworts are widely distributed on all continents, most commonly growing on the bottom of freshwater lakes and other surface waters. A representative species is Braun's quillwort (Isoetes braunii) of boreal North America and Eurasia.