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.