DEFENCES OF PL^^TS 345 



to climb ; they stand out nearly at right-angles to stems 

 and branches, and are not hooked. Doubtless, in con- 

 sideration of the open situations in which the Scotch 

 Rose grows, and the possible attentions of browsing 

 animals, it needs all its warlike, threatening prickles. 



We must not allow our imagination to run riot in 

 regard to these so effectively armed plants. We might, 

 in the exercise of mere fancy, come to the conclusion 

 that thorns, spines, and prickles have been deliberately 

 evolved as - a determined protection against animals, 

 but the facts do not justify us in so doing. Take, for 

 example, the two varieties of the Common Rest-Harrow 

 (Ononis) ; one grows in exposed places, in the haunts of 

 browsing animals, and is armed with spines ; the other 

 has few, if any, spines, and grows where " thieves do not 

 break through nor steal " — not in heaven, indeed, but 

 in a sheltered earthly paradise of its own. Are we to 

 say that the spiny variety has evolved spines as a 

 deliberate protection against browsing animals, and 

 that the spineless form dispenses with spines because it 

 does not need protection from animals in the haunts it 

 favours ? A poet might here find an opportunity for a 

 draught upon his inner consciousness and a moving 

 rhapsody on a singular design. The cold truth, how- 

 ever, brings us to earth. The spiny form is an adapta- 

 tion to an open environment and defective nutrition; 

 the spines are modified leaves, and an evidence of the 

 reduction of the plant's transpiring surface; in brief, the 

 modification of the leaves into spines effects a check on 

 transpiration, and the reduced transpiration current 

 leads to defect in nutrition. The spines would be 

 present on this account even if there were not a browsing 



44 



