Equisetaceae
Christopher Noll
A worldwide family, except for Australia and New Zealand. Equisetum, with 15 species, is the only surviving genus of the subclass Equisetidae, which first appeared in the Devonian Period 350 million years ago and evolved dominant treelike plants such as Calamites in the Carboniferous Period around 300 million years ago. Plants are perennial from jointed widely creeping and branching dark rhizomes. Aerial stems are annual or perennial, jointed, and often roughened with tubercles (tiny bumps of silica). The nodes are very distinct, each with a fused sheath and a whorl of scale-like leaves ending in persistent or deciduous teeth. The internodes have vertical ridges alternating with grooves. The stem has a large hollow central canal (except in E. scirpoides) surrounded by a ring of several smaller vallecular canals (usually absent in E. fluviatile). Cones (strobili) terminate some stems. Sporangia form on the inner surface of each plate and produce many green spores (white in hybrids). Each spore bears four elaters (ribbon-like appendages) that coil or uncoil with changes in humidity. Gametophytes are unisexual (female larger than male) and develop on wet substrates into dark green ribbons that can persist for more than a year and range in size from 1 mm to over 3 cm, although they are rarely seen. The species are highly variable and many of the named variants are probably environmentally induced. Most species spread vigorously in open areas. The genus consists of two subgenera: subgenus Equisetum, usually with stems that have whorls of branches, and persist no more than one year; and subgenus Hippochaete, usually with unbranched stems that persist more than a year. Species within each subgenus often hybridize with each other.
