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Hairs (on plants) are modified cells of the epidermis, either on roots, stems, or leaves. Though each hair originates in a single cell, they often become multicellular, assume various forms, and perform several very different functions. The stems of mosses (q.v.) give off root-hairs which branch freely and bear gemmae. The sporangia of ferns, and perhaps the ovules in such cases of superficial placentation as poppies, are hairs or trichomes, as they are termed in general morphology. Hairs generally originate adventitiously, and those on the roots of flowering-plants are unicellular and exhibit nutatory movements. It is by means of these root-hairs that most flowering-plants take in their liquid food. Hairs occur generally on bud-scales and young stems and leaves, protecting them from radiated cold and perhaps damp, though such structures, as in the hazel and beech, often lose these hairs later, being, as it is termed, glabrescent. Hairs are particularly common in certain natural orders, such as the Labiatae and Boraginaceae, and they occur thickly on some plants characteristic of very dry situations and also on water-side plants, but not generally on actual aquatics. Thus it has been suggested that they act as a protection against unwelcome guests in the form of crawling, leaf-eating, or honey-stealing slugs or insects. Those which occur commonly on the outer surface of the calyx do probably perform such a function in addition to the protection of the young flower-bud. Among the chief forms assumed by hairs are the "glandular" or knobbed, which may be unicellular or multicellular, the moniliform, or necklace-like, the peltate and stellate, which, again, may consist of one or more cells, and the squamose, or scale-like, as in the chaff-like bodies on the leaf-stalks of many ferns. Hairs often serve as receptacles for certain bye-products of metabolism, such as the sticky substance exuding from those of species of Lychnis, Silene, Saxifraga, etc., or the formic acid of the nettle. The stinging-hairs of the latter are single cells with sharp hooked points, and pressed upon by surrounding epidermal and hypodermal cells. Hairy surfaces are termed villous if covered with scattered, long, weak hairs; silky, if with more numerous similar hairs, as in the Silver-leaf (Leucadendron argenteum) of South Africa; pubescent, if with numerous short hairs, as in the sage; or hispid, if with stiffer hairs, as in the nettle. Prickles and some bristles differ from hairs in not being exclusively epidermal in origin.

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