STEMS 123 



jack-in-the-pulpit preserves it from the ravages of 

 grazing animals, and the well-known qualities of the 

 poison ivies serve a similar purpose. The active 

 substance in the latter case is an oil secreted by the 

 leaves and stems, making even proximity dangerous 

 to the animal. 



The prickles of roses and other shrubs are exam- 

 ples of weapons coming from the epidermis of the 

 stems, and which may come away in the body of 

 the animal which comes into contact with the stems. 

 The thorns of many species, including the well- 

 known honey locust (Gleditschid), are branches which 

 have altered their method of growth in such manner 

 that they are very effective weapons for defence. 



The edges of the leaves of some grasses are cut 

 into saw teeth, and these are edged with silica so 

 finely that they cut the flesh like knives. The mar- 

 gins and surfaces of thistle leaves and stems are 

 drawn out into spines, the protective value of which 

 may be easily seen. 



The members of the cactus family protect their 

 bodies to a great extent by sharp spines, many of 

 which are barbed. These spines are modified leaves 

 which have lost the original function of such organs, 

 and have become solely organs of defence. 



