No. 512] PRESENT PROBLEMS IN PLANT ECOLOGY 



481 



parral, which, however incapable of normal develop- 

 ment in the desert, hold their own ground, where the con- 

 ditions are less strenuous, so tenaciously and completely 

 that the desert species make no headway against them. 



This, of course, is an interpretation merely, hut with 

 such an accumulation of evidence we arc now in a posi- 

 tion to proceed with the problem along definite lines with 

 the expectation of definite results. Sowing together 

 seeds of desert and other plants, the transference of in- 

 dividuals to denuded areas beyond their natural limits, 

 and multiplied comparative observations of the deport- 

 ment of different species on the "edge of the desert" are 

 simple and obvious methods of procedure at the outset. 

 Some of this work has already been done, enough to con 

 vince those engaged in it that in general the problem 

 of the successful occupation of a desert habitat involves 

 the recognition of actual competition on the part of its 

 would-be occupants, a competition severe enough in some 

 quarters to set up a barrier beyond which, in the midst of 

 otherwise entirely favorable environmental conditions, 

 they can not pass. 



In their relations to each other, desert plants fre- 

 quently exhibit not merely competition but accommoda- 

 tion. This has been clearly shown by recent studies of the 

 root systems of certain cacti and other plants by Dr. \\. 

 A. Cannon. To take a striking example— superficial ob- 

 servation of the association of the sahuaro (Cereus gi- 

 ganteus) with one of the palo verdes ( P<ir1;i)is<,,uii 

 ■micro phylla) and some other shrubby perennials gives 

 no satisfactory clue to the reason of this relation, and 

 the common explanation that they are plants of similar 

 biological requirements, and therefore grow together, is 

 altogether inadequate and in part misleading. The 

 careful study, however, that has been given to the root 

 systems of these plants brings out the important fact 

 that they grow close together by virtue of simple accom- 

 modation, which enables them to utilize to the utmost the 

 scanty rainfall. The roots of the sahuaro are spread 



