No. 512] PRESENT PROBLEMS' IN PLANT ECOUHiY 1S.'5 



the influence of atmospheric factors, and il is a matter of 

 congratulation that the methods devised by one of the 



earlier period. But it is never to be forgotten that under 

 the same atmospheric conditions, and with equal chances 

 in other respects, the deportment of two plants side by 

 side, their capacity for adjustment let us say, is so differ- 

 ent that the essential problem lies first of all in the 

 physiological capabilities of the plant itself. 



More strikingly true, if possible, is this seen to be the 

 case when the relation of desert plants to the soil is con 

 sidered. It is well that so much soil work has been done, 

 that we have soil maps, that determinations of water ca- 

 pacity and other physical as well as chemical character- 

 istics have been ascertained in so many habitats, and that 

 we have a growing literature embodying observations of 

 the relations of plants to underlying rocks, in short that 

 the substratum has been the object of so long and so 

 thorough study; there is no danger that we shall have 

 too much of this, but there may be danger that we may 

 sometimes forget to j>lace the emphasis where it belongs, 

 namely, on the fact that every species and every variety 

 of plant is a law to itself in its relations to rock or soil. 

 It is true enough that the different percentages of alkali 

 salts at different distances from the center of a salt spot 

 stand apparently in causal relation to the growth of dif- 

 ferent plants in corresponding concentric zones, but it is 

 equally true that this zonal arrangement is also the vis- 

 ible expression of the capacity of these different plants 

 to cope with the conditions there existing, and of this 

 capacity, if it is to be expressed, as some day it must, in 

 physical measurements, how inadequate is our knowl- 

 edge. How greatly we need to really know the physio- 

 logical constants, not of one but of many desert plants. 



It is in the same line of thought, and with the same 

 purpose, that I have referred to the inadequate concep- 



