320 ODD HOURS WITH NATURE 



mentalist has taken the generalization that a 

 mechanically protected plant is a food plant as 

 a working hypothesis for experiment. Thus he 

 is reported to have laboured successfully to pro- 

 duce a spineless cactus, in the belief that by means 

 of such a plant, containing in its substance both 

 food and drink, large areas of scrub and semi- 

 desert might be turned to account for the grazing 

 of animals. It is hardly likely, however, that the 

 animals reared under such condition would be 

 worth the trouble of providing them with the food 

 supply. But it is possible that the experimenter 

 might make something useful out of the thistle if, 

 by depriving it of its defences, he constrained 

 it once again to take up its old place as a food 

 plant, for most members of the family have the 

 power of growing on ground on which the food 

 plants of cultivation cannot thrive. 



