Equisetopsida

Equisetopsida, commonly known as horsetails, is a class of primitive, seedless vascular plants that belong to the fern lineage. The class includes the living genus Equisetum, the sole extant member of the family Equisetaceae, and numerous extinct relatives dating back to the Paleozoic. The group has a long fossil record, with large arborescent forms in coal measures, such as Calamites, that helped shape ancient landscapes.

Modern horsetails are perennial, rhizomatous herbs with hollow, jointed stems. Leaves are reduced to scale-like sheaths encircling the stem at each node; the photosynthetic tissue is largely concentrated in the stems. Stems typically exhibit pronounced ribbing and silica content, making them rigid.

Reproduction and lifecycle: The sporophyte is the dominant phase. Reproduction occurs by spores produced in strobili at the tips of fertile shoots; each sporangium is borne on sporangiophores. Spores have elaters aiding dispersal. Fertilization requires free water, producing a subterranean gametophyte.

Taxonomy and diversity: The class Equisetopsida comprises the order Equisetales, the family Equisetaceae, and the genus Equisetum. Today about 15 species survive worldwide, usually in damp or disturbed habitats such as streamsides, wetlands, and moist woodlands.

Evolution and ecology: Once globally dominant in Paleozoic forests, horsetails declined after the late Paleozoic with the rise of seed plants and changing climates. Extinct relatives ranged from small to tree-sized forms and contributed to coal deposits. Modern species are relictual, representing one of the surviving lineages of early vascular plants.