Equisetophyta [key], small division of the plant kingdom consisting of the plants commonly called horsetails and scouring rushes. Equisetum, the only living genus in this division, is descended evolutionarily from tree-sized fossil plants. There are about 30 species, distributed in every continent except Australia and Antarctica and in every climate from the tropics to the arctic. The plants, which generally grow in moist places, have roots and ribbed green stems, the surface of which is impregnated with silica crystals. Their abrasive texture made them useful in former times for scouring, hence their common name. Most species have numerous whorled branches that lend the plant a plumed or feathery appearance, thus giving rise to their other common name, horsetail. The scalelike nonphotosynthetic leaves are joined together to form a fringed whorl that encircles the stem at regular intervals; the green stems and branches are the photosynthetic organs. The stem has no cambium or secondary growth. It consists of a silica-impregnated epidermis, a cortex, and a central structure called a stele that contains a ring of vascular bundles, consisting of xylem and phloem. The conspicuous plant form of Equisetum, which may be more than 3 ft (1 m) high in some species, represents the diploid sporophyte generation. A cone, or strobilus, at the apex of the sporophyte stem bears spore-producing structures. Upon germination, the spores produce a green, frilled, thumbnail-sized haploid plant form, the gametophyte; specialized structures on the mature gametophyte, the archegonia and antheridia, produce, respectively, eggs and sperms. As in mosses, the sperm swims to the egg through a film of water, attracted by specific chemical substances. A zygote, formed as the result of fertilization, develops into green sporophytes to complete the life cycle. The order Calamitales contains plants known only from fossil remains so abundant in coals and associated shales from the Carboniferous period that it is assumed that they formed a major part of the vegetation that later became compressed into coal. The plants of the genus Calamites may have reached a height of 100 ft (30 m).
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