How Plants Resist Hostile Forces Around Them 275 



but there are other instances where the actual adaptation seems 

 reasonably clear. Thus, in the desert the conditions of life are so 

 hard that plants can scarcely produce any surplus above their 

 own needs, while the very precious store of water laid up in their 

 stems, and essential to their own safety throughout the dry sea- 

 son, is particularly tempting to large animals. In such plants, 

 accordingly, we find the best development of features that ap- 

 pear to be most protective against animals, either distasteful 

 secretions, which are especially common and virulent in plants 

 of the deserts, or else a horrid armature of thick-set and dan- 

 gerous prickles and spines through which animals can penetrate 

 but painfully if at all. Furthermore, a few desert plants are 

 known which resemble so closely the background against which 

 they grow, either the rough gray surface of the soil (as in the 

 case of the half -buried flat-topped "Living Rock" Cactus of the 

 American Southwest), or else the drab pebbles in the beds of dry 

 water-courses (as in a number of plants described from the 

 peculiar flora of South Africa), that it seems as if such plants 

 must surely escape notice by animals, and secure a protection, 

 by this form of mimicry, though here again it may be true that 

 the result is incidental rather than adaptational. But while a 

 protective mimicry seems reasonable in plants of this kind and 

 habit, the same can hardly be said of those cases in the flora of 

 ordinary climates where some kinds have been claimed to secure 

 protection through their resemblance to Nettles, or Poison Ivy, 

 or other plants actually noxious. Indeed, so far as concerns a 

 protective function for the poison of plants like Poison Ivy, there 

 is a difficulty in the fact that the repelling effect is not felt until 

 long after the plant has been injured. I think we do not yet know 

 the meaning of the poisonous quality of plants. 



An injury done to the vegetative parts of plants does not ex- 

 tend to other parts, and is easily replaced; but damage to the 

 machinery of growth and reproduction is serious, in correlation 

 with which fact we find in those parts a good many apparently 



