210 



ELEMENTS OF BOTANY. 



cient to insure the safety of the plants which have developed 

 them. But in desert or semi-desert regions the extreme 

 scarcity of plant life exposes the few plants that occur there 

 to the attacks of all the herbivorous animals that may 

 encounter them. Accordingly, great numbers of desert 



Fig. 189. — Stinging Hairs and Cutting Leaves. (All much magnified.) 

 3, stinging hairs on leaf of nettle ; 6, bristle of the hugloss ; 7, barhed margin 

 of a leaf of sedge ; 8, barbed margin of a leaf of grass. 



plants are characterized by nauseating or poisonous quali- 

 ties or by the presence of astonishingljr developed thorns, 

 while some combine both of these means of defense. 



254. Importance of Adapfireiiess in Plants. — It may be 

 inferred from the preceding sections that a premium is set 



