No. 512] PRESENT PROBLEMS IN PLANT ECOLOGY 



477 



most instructive data yet derived from the study of 

 desert plants. 



But little reflection is needed to arrive at the conclu- 

 sion that the classical question regarding the relative 

 importance of physical constitution and chemical compo- 

 sition of the substratum to plant growth— though like the 

 poor it promises to be always with us— does not, and can 

 not reach the heart of the problem. For every plant 

 which successfully holds its place in a true desert habitat, 

 there is a delicate balancing of the regulation of trans- 

 piration, the power of absorption, the capacity ot the 

 conducting system, the presence or absence of storage 

 tissues, and, we may well believe, the possession of proto- 

 plasmic properties which contribute to its powers of en- 

 durance. This being the case, it would seem that in 

 future, investigations of the habitat relations, of desert 

 species especially, must be directed mainly to the plant 

 itself. The advantage of a thorough knowledge of sods 

 is too obvious to call for comment, but it must be re- 

 membered that we are as yet only at the threshold of a 

 greater and more promising work, namely, the investiga- 

 tion of the physiological requirements and capabilities 

 of plants that can grow in a true desert habitat as com- 

 pared with those that can not. In such comparative 

 study lies, as it seems, the hope of real progress. Tt is 

 impracticable for any investigator, at the present time, 

 to mark out a straight path for others to pursue,^and it 



suggestions that may be offered. 



In the first place, important results have already fol- 

 lowed the simplest experiments and observations when 

 these have been conducted with exactness and with a 

 definite end in view. To refer to a specific case, -Pro- 

 fessor Thornber, of the University of Arizona, under- 

 took a few years ago to compare the habits ot certain 

 desert plants as regards germination. It was found that 

 while the seeds of some species germinated at a given 



