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Encyclopedia Britannica - Main :: HOR-I25 |
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HORSETAIL (Equisetum) , the sole genus of the botanical natural order Equisetaceae, consisting of a group of vascular cryptogamous plants (see PTERIDOPHYTA) remarkable for the vegetative structure which resembles in general appearance the genera of flowering plants Casuarina and Ephedra. They are herbaceous plants growing from an underground much-4fejftt4ap jt. From Strasburger's Lehrbtah der Botanik, by permission of Gustav Fischer. Equisetum arvense. A, Fertile shoot, springing B, C, Sporophyllsbearing sporangia, from the rhizome, which which in C have opened. also bears tubers; the D, Spore showing the two spiral vegetative shoots have bands of the perinium. not yet unfolded. F, Dry spores showing the ex- F, Sterile vegetative shoot. panded spiral bands. (A, F, reduced. B, C, D, E, enlarged.) branched rootstock from which spring slender aerial shoots which are green, ribbed, and bear at each node a whorl of leaves reduced to a toothed sheath. From the nodes spring whorls of similar but more slender branches. Some shoots are sterile while others are fertile, bearing at the apex the so-called fructificationa dense oval
which open by a longitudinal slit on their inner side. The spores differ from those of ferns in their outer coat (exospore) being split up into four club-shaped hygroscopic threads (elaters) which are curled when moist, but become straightened when dry. In most species the fertile and sterile shoots are alike, both being green and leaf -bearing, but in a few species the fertile are more or less different, e.g. in E. arvense the fertile shoots appear first, in the spring, and are unbranched and not green. Any portion of the underground rhizome when broken off is capable of producing a new plant; hence the difficulty of eradicating them when once established. There are 24 known species of the genus which is universally distributed.The corn horsetail E. arvense, one of the commonest species, is a troublesome weed in clayey cornfields (see fig.). The fructification appears in March
appearance of the stem
fodder
weight
starch
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