Lycopodiophyta

Lycopodiophyta is a division of seedless, vascular plants that includes the living clubmosses (Lycopodiaceae), spikemosses (Selaginellaceae), and quillworts (Isoetaceae). This lineage traces back to the Silurian period and represents one of the earliest diverging lineages of land plants. Members are characterized by microphylls, leaves with a single vein, and a predominantly creeping habit, though ancient forms included large arborescent trees.

The extant lineages are divided into three families: Lycopodiaceae (clubmosses), Selaginellaceae (spikemosses), and Isoetaceae (quillworts). Lycopodiaceae typically form low mats or short upright shoots with numerous microphyll leaves; their sporangia are borne on leaf bases in terminal or lateral strobili, and most species are homosporous, producing a single size of spore.

Selaginellaceae and Isoetaceae are heterosporous. Selaginella produces microspores and megaspores in separate sporangia, leading to distinct male and female gametophytes, which are often subterranean or on the surface. Isoetes likewise produces two spore types and is usually found in wetlands or seasonally flooded habitats, with sporophylls that subtend a cone-like structure bearing the sporangia.

Ecology and evolution: Lycopodiophyta played a central role in Paleozoic forests, contributing to coal formation through fossilized remains. Modern members occur worldwide in temperate and tropical regions, with Isoetes favoring shallow waters and damp soils, Selaginella occupying diverse tropical and subtropical habitats, and Lycopodiaceae often found on forest floors.

Taxonomy and classification: In contemporary systems, Lycopodiophyta is treated as a division containing three living families and numerous genera; the fossil record includes numerous lycopsid species that were once large trees.