NATURE OF PLANTS 35 



Plant hairs do not always serve this purpose. Desert plants 

 and those living in dry localities are frequently characterized by 

 hairy coatings. Such plants are frequently termed xerophytes 

 in contradistinction to aquatics or hydrophytes and to plants 

 living in moderately moist soils, which are called mesophytes. 

 The hairy coatings of xerophytes give them their familiar silvery 

 or hoary appearance as seen in the sage-brush, dusty miller, and 

 mullein. The hairs are usually empty tubes or cells which re- 

 flect a large portion of the intense light and heat and consequently 

 materially lessen the loss of water, just as straw sprinkled upon 

 the ground keeps the earth below moist and cool. In many in- 

 stances plant hairs develop mucilage which protects as with a 

 varnish growing regions as buds or young shoots and leaves of 

 birches, alders, poplars, and peach ; or oils and poisonous sub- 

 stances are formed which repel by disagreeable odors or tastes 

 the attacks of animals. The nettles are shunned because of the 

 burning sting produced by the plant hairs. The upper portion 

 of these hairs (Fig. 25) is practically a delicate glass tube since 



FIG. 25. Stinging hair of nettle: A, portion of epidermis bearing hair. 

 B, tip of hair enlarged, showing easily detachable knob. I. D. Cardiff. 



the walls are filled with silica. The basal portion of the hair is 

 composed of soft, yielding cellulose so that the fluid collecting 

 in the hair distends this portion of it. The knob at the end is 

 fastened 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 is 

 transformed into a veritable hypodermic needle which easily punc- 



