. Nature and development of plants . y coatings. Such plants are frequently termed xerophytesin contradistinction to aquatics or hydrophytes and to plantsliving in moderately moist soils, which are called hairy coatings of xerophytes give them their familiar silveryor hoary appearance as seen in the sage-brush, dusty miller, andmullein. The hairs are usually empty tubes or cells which re-flect a large portion of the intense light and heat and consequentlymaterially lessen the loss of water, just as straw sprinkled uponthe ground keeps the earth below moist and cool. In many in-s
. Nature and development of plants . y coatings. Such plants are frequently termed xerophytesin contradistinction to aquatics or hydrophytes and to plantsliving in moderately moist soils, which are called hairy coatings of xerophytes give them their familiar silveryor hoary appearance as seen in the sage-brush, dusty miller, andmullein. The hairs are usually empty tubes or cells which re-flect a large portion of the intense light and heat and consequentlymaterially lessen the loss of water, just as straw sprinkled uponthe ground keeps the earth below moist and cool. In many in-stances plant hairs develop mucilage which protects as with avarnish growing regions as buds or young shoots and leaves ofbirches, alders, poplars, and peach ; or oils and poisonous sub-stances are formed which repel by disagreeable odors or tastesthe attacks of animals. The nettles are shunned because of theburning sting produced by the plant hairs. The upper portionof these hairs (Fig. 25) is practically a delicate glass tube since. Fig. 25. Stinging hair of nettle: A, portion of epidermis bearing , tip of hair enlarged, showing easily detachable knob.—I. D. Cardiff. the walls are filled with silica. The basal portion of the hair iscomposed of soft, yielding cellulose so that the fluid collectingin the hair distends this portion of it. The knob at the end isfastened to the hair by so thin a ring of wall, as seen in Fig. 25,B, that the least touch breaks it off. In this way the hair istransformed into a veritable hypodermic needle which easily punc- ARRANGEMENT F THE MES< (PHYLL hires the skin while the distended basal portion contracts andforces out some of the fluid. The burning sensation or stingthat immediately follows the puncture of the skin is caused bythe injection of formic acid while a variety of other poisons pro-duce the subsequent irritations. These poisons are so powerful in some of the East Indian nettles as to produce serious results. even tetanus or
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