A subdivision or class of * vascular plants containing only five living genera but having a rich fossil record, being especially abundant in the Carboniferous. The dichotomously branching sporophyte is differentiated into a shoot and root and the vascular tissue contains properly differentiated phloem. The shoot bears small usually spirally arranged leaves that leave no leaf gap in the stele. The sporangia are either borne singly in the axil of the sporophyll or on the upper surface of the sporophyll near the base. Both homosporous and heterosporous forms exist, the spores giving rise to a subterranean or terrestrial gametophyte, which is very reduced and short-lived. Embryogeny is endoscopic. The Lycopsida is divided into three extant orders: * Lycopodiales , * Selaginellales , and * Isoetales. There are two extinct orders: * Lepidodendrales ; and Pleuromeiales, known from the Triassic and intermediate in form between the Lepidodendrales and the Isoetales. |
