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 hfe exposes the few plants that occur there 
to the attacks of all the herbivorous animals that may 
encounter them. Accordingly, great numbers of desert 
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Fig. 189. — Stinging Hairs and Cutting Leaves. (All much magnified.) 
3, stinging hairs on leaf of nettle; 6, bristle of the bugloss ; 7, barbed 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 astonishingly developed thorns, 
while some combine both of these means of defense. 
254. Importance of Adaptiveness in Plants. — It may be 
inferred from the preceding sections that a premium is set 
