THE PLANT WORLD 257 



well lead to some positive evidence upon the general method of origin of 

 species, and if the entire effort of the Desert Laboratory inquiry resulted 

 in bringing to light positive evidence as to the actual origin of two species 

 of plants its energy would have been well expended by reason of the 

 value of such results to evolutionary science in general. 



The long standing questions as to the causal facts in regard to the 

 various protective structures of xerophyles have advanced but little 

 toward their final solution in the last decade. Are the spines, thorns, 

 prickles and poisons of desert plants really the results of efforts on the 

 part of the plant for self-protection ? So many of the special features of 

 xerophytic vegetation have been interpreted in the spirit of an enduring 

 optimism that leans on the future for a confirmation of its conclusions, 

 which have been received with patient credulity by the botanical world, 

 that it is necessary to ask ourselves the most elemental questions about the 

 best known and most apparent features of the vegetation of the desert. 



Not the least interesting of the results to be obtained from studies of 

 the plants of arid regions, are those which may be expected as to the 

 physiological and mechanical causes accountable for the remarkably low 

 densit)' of distribution, one of the most highly characteristic features of 

 the flora of the desert. 



