Lycopodiophyta [key], division of the plant kingdom consisting of the organisms commonly called club mosses and quillworts. As in other vascular plants, the sporophyte, or spore-producing phase, is the conspicuous generation, and the gametophyte, or gamete-producing phase, is minute. The living representatives are all rather small herbaceous plants, usually with branched stems and small leaves, but their fossil ancestors were trees. Like other vascular plants, the axes of this group have epidermis, cortex, and a central cylinder, or stele, of conducting tissue. The spore cases, or sporangia, are borne at the base of leaves, either scattered along the stem or clustered into a terminal cone or strobilus. At maturity, the sporangia split across the top, releasing great quantities of spores. The spores germinate to produce small, nongreen, fleshy gametophytes, which bear both sperm-producing antheridia and egg-producing archegonia. The motile sperms swim to the egg through a film of water. The fertilized egg, or zygote, gives rise to an embryo and eventually to a mature sporophyte. The order Lycopodiales includes the common genus Lycopodium, the larger of two genera (the other is Phylloglossum) belonging to this order and containing some 100 species. The order Selaginellales contains only one living genus, Selaginella, with perhaps 600 species, although fossil forms resembling Selaginella are known from deposits of the Carboniferous period (see resurrection plant). The order Isoetales (quillworts) contains the small genus Isoetes, which grows in shallow water in lakes, ponds, and marshy places. The plants have a grasslike appearance and are therefore often not readily identified. The order Lepidodendrales contains members known only from fossil specimens dating from the Upper Devonian to Permian times. Lepidodendron, the most common genus, was of tree size.
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