Horsetail

Horsetails, genus Equisetum, are the only living genus in the family Equisetaceae, a lineage of spore-reproducing vascular plants often called living fossils because their close relatives once dominated Paleozoic forests. A horsetail's hollow, jointed, ridged stem, usually carrying between 6 and 40 ridges, does most of the plant's photosynthesis, since its leaves are reduced to small scales fused into sheaths at each joint; spores form inside cone-like strobili at the stem tips, each spore fitted with four moisture-sensitive elaters that spring open to aid dispersal. About 18 species survive today, and the genus is nearly cosmopolitan, absent only from Antarctica, Australia, New Zealand and the Pacific islands, though its modern form dates only to the Early Jurassic.