Lycopodiidae 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, scalelike leaves. Their haploid spores are produced in a clublike structure known as a strobilus.
Lycopodium species are found on all continents (except Antarctica) and many oceanic islands. Some species common 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 Lycopodium species. 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— 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 worldwide, 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 quill-worts. 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- 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.