124 THE NATURE AND WORK OF PLANTS 



Animals, on the other hand, are constantly striving 

 to use the bodies of these plants for food without 

 injury from their weapons, and some of them suc- 

 ceed even with such well-protected forms as the 

 cactus and the thistle. Some grazing animals may 

 even eat poison ivy without harm. 



171. Branches used as leaves. — A species may- 

 find itself in a location to which its leaves are wholly 

 unsuited, and it may cease to develop these organs 

 in the usual way. In such instances branches are 

 sometimes modified to carry on the work that should 

 be done by the leaf. An example of this may be 

 seen in the " smilax " of the gardener, which is used 

 so profusely for decorative purposes. The slender, 

 thin bodies having the appearance of leaves are 

 really short branches, and the true leaves are to be 

 found as small colorless or brownish bracts at the 

 lower side of the base of these leaflike branches. 



172. The part of stems in the struggle for exist- 

 ence. — If the surface of the earth were level and 

 plain, and the number of plants were not so great 

 as to fully occupy this space, probably all species 

 would form short stems which would simply lift the 

 leaves and flowers from the ground. 



