A vascular cryptogam (plant that reproduces by spores) of the genus Equisetum that is a living fossil, being the only surviving genus in the family Equisetaceae that flourished during the Palaeozoic era. A horsetail has a hollow stem that photosynthesizes and leaves reduced to scales that usually do not photosynthesize. There are 25 species found, mainly in damp places, worldwide except for Antarctica.