346 PLANT-LIFE 



animal in existence. The spineless, or less distinctly 

 spiny, variety of Rest-Harrow grows in situations where 

 transpiration does not need a severe check, and hence 

 where nutrition is normal. But if we have accounted 

 for the spines in this matter-of-fact fashion, there is yet 

 room for mild rhapsody, for the adaptation is remark- 

 able. The armoury of plants has been called into 

 existence by physiological necessity; herein we dis- 

 cover its primary significance; but it happens that it 

 serves more than its essential use, for it chances to be 

 a means of defence against animals a not unimportant 

 factor in survival. We have seen that in the Cacti 

 (p. 263) the stem is green and succulent, but the leaves 

 have been reduced to spines. This is a purely physio- 

 logical adaptation to desert conditions, but it happens 

 to have an important secondary use, for browsing 

 animals would need to be sore distressed by hunger and 

 thirst before they would attempt to satisfy their appe- 

 tite at the expense of sore and irritated mouths. These 

 desert plants survive because they have evolved a form 

 which is doubly useful; it conserves the water-supply, 

 and at the same time keeps animals at bay. 



Kerner, in his Natural History of Plants, directs atten- 

 tion to the manner in which young trees of Beeches, 

 Oaks, and Larches, if they grow where they can be 

 reached by oxen, sheep, and goats, have their young 

 shoots and the attached leaves eaten by these animals. 

 It happens that the portion of a shoot left on the tree 

 after its mutilation dries up, but its hinder part remains 

 alive. One effect of the pruning by these animals is a 

 vigorous production of buds on the living part. Shoots 

 arise from these buds next spring, and the pruning by 



