Lycopsiden

Lycopsiden, or lycopsids, are a lineage of ancient vascular plants within the division Lycopodiophyta. Today they comprise three living lineages: Lycopodiaceae (clubmosses), Isoetaceae (quillworts), and Selaginellaceae (spikemosses). In the deep past, numerous extinct lineages—most famously the giant arborescent forms such as Lepidodendron and Sigillaria—dominated humid coal forests during the Carboniferous and Permian periods.

Most lycopsids are herbaceous perennials with short, creeping or upright stems and simple, evergreen leaves called microphylls. Leaves are usually arranged spirally around the stem, and many species bear specialized reproductive structures called strobili. Reproduction occurs by spores rather than seeds. Lycopodiaceae are generally homosporous, producing a single type of spore, while Selaginellaceae and Isoetaceae are heterosporous, producing both microspores and megaspores.

Habitat varies by lineage. Clubmosses commonly inhabit forest understories and heathlands, often forming mats or cushions. Quillworts are typically found in moist, sandy soils or permanently wet habitats, including shallow water. Spikemosses occupy tropical to temperate environments, from soils to rock faces.

Ecology and evolution: Lycopsids occupied a pivotal role in Paleozoic ecosystems and contributed to large coal deposits. Their fossil record helps illuminate early vascular plant evolution.

Today, lycopsids remain important for understanding plant evolution, biodiversity in understories, and some species are used horticulturally as ornamentals or ground cover. Isoetes is sometimes cultivated for study in aquatic settings.