Equisetales

Equisetales is an order of primitive, vascular, seedless plants within the class Equisetopsida. It encompasses the family Equisetaceae and the living genus Equisetum, commonly known as horsetails or scouring rushes. The fossil record also includes numerous extinct genera, making the group one of the oldest lineages of living land plants.

Extant members, represented solely by Equisetum, are perennial, rhizomatous herbs with jointed, hollow stems. The stems are ribbed and often photosynthetic, while the leaves are reduced to small scales arranged in whorls at the nodes. Branches are typically whorled, giving the plant a brushlike appearance. The tissue contains substantial silica, contributing to a rough texture. Reproduction occurs via spores produced in terminal strobili; the spores are homosporous and disperse by wind. The life cycle alternates between a free-living, photosynthetic gametophyte and the sporophyte, which is the dominant plant form.

Habitat and ecology: horsetails are found worldwide, often in damp or disturbed habitats such as wetlands, stream banks, and nutrient-poor soils. They favor moist environments and require water for the development of their sexual stages.

Evolution and fossil record: the group dates back to the late Silurian or early Devonian, around 420 million years ago. In the Carboniferous, tree-like relatives such as Calamites were abundant in swamp forests, but the modern lineage comprises herbaceous plants with a broad, though reduced, geographic distribution.

Taxonomy and diversity: today Equisetales contains one extant genus, Equisetum, with a relatively small number of species (roughly a few dozen worldwide, commonly cited as about 15–20). Extinct genera enrich the historical diversity of the order.