260 EVOLUTION OF PLANTS 



not render the plants entirely immune, is shown, how- 

 ever, by the attacks of certain animals, especially insects, 

 which have apparently adapted themselves to these 

 peculiar conditions. Nevertheless, there is no question 

 that such plants suffer very much less from animals 

 than they would if these means of protection were 

 absent. It has been thought that the sharp needle-like 

 crystals or rhaphides, which occur so abundantly in 

 many Monocotyledons, may deter animals from eating 

 them, as many of them, especially the aroids, have an 

 excessively acrid taste, which is supposed to be due 

 to the mechanical irritation produced by these sharp 

 crystals. 



The presence of spines, thorns, and prickles, as well 

 as rough hairs upon the stems and leaves, is doubtless 

 mainly protective. They are usually most noticeable 

 in plants of dry regions, where the scanty vegetation is 

 peculiarly exposed to the attacks of herbivorous .animals. 

 The cacti are very perfect instances of this peculiarity. 

 The terribly sharp thorns of these plants render them 

 perfectly safe against the attacks of hungry animals, 

 which eat them greedily if care is taken to remove 

 their thorny armor. Where desert plants are not 

 thorny, they are usually ill-scented and thus distaste- 

 ful to herbivorous animals. 



But one more of the most remarkable cases of recip- 

 rocal relations between plants and animals will be cited, 

 namely, the peculiar arrangement known as myrmecoph- 

 ily, where ants inhabit certain parts of trees, to which, 

 in return for shelter, and sometimes food in the form 

 of honey-like secretions or peculiar albuminous fatty 

 bodies, they protect the plants from the ravages of other 



