42 



PART I. ORGANOGRAPHY. 



The absorption of nutriment, however, is not their only use. 

 Like the other organs studied, they have frequently become 

 modified and adapted for uses quite differant from the normal, 

 and like them, also, in some instances, they exist merely as 

 abortive and functionless organs. 



One important use, different from the normal, which many of 

 them subserve, is that of protection. To this end they are vari- 

 ously modified in form and structure. In the Opuntia Cactus, 

 Fig. 162, they become hardened, sharpened and barbed, and in 

 the western Mentzelia ornata, Figs. 163, 164 and 165, are barbed 

 and strongly silicified, so that, if animals feed upon either of 

 these plants, their lips and tongues will be so painfully irritated 

 that they will not be likely to repeat the experience. In the Black- 



Fig. 138. Fig. 139. Fig. 140. 



Fig. 141. 



Fig. 142. 



Fig. 138. — Portion of Rose stem showing recurved prickles. 



Fig. 139. — Portion of stem of Lantana, showing hooked prickles, ordinary hairs and 

 glandular hairs. 



Fig. 140. — Young prickle of Lantana, enlarged, showing how, in this case, the prickle 

 is formed by the development of the papilla at the base of an ordinary hair. 



Fig. 141. — Stinging hair of a species of Nettle, magnified. 



Fig. 142. — Stinging hair of Wigandia urens, magnified. 



berry, Greenbrier, Gooseberry and Rose, Fig. 138, some of the 

 hairs develop into hard, sharp prickles, which cause herbivorous 

 animals to avoid them. In the Nettle and Wigandia urens they 

 are modified into still more efficient protecting organs, namely, 

 stinging-hairs, which consist of slender, rigid tubes, swollen at 

 their base into a flexible-walled sac filled with an irritant fluid. 

 These hairs are so constructed that, when touched by an animal, 

 their tips, which are very fragile, are liable to be broken off, and 

 the sharp, tubular shafts driven into the skin, at the same time 



