PLANTS IN ARID REGIONS—SPALDING. 461 
by side, their capacity for adjustment, let us say, is so different 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 considered. It is well that 
so much soil work has been done; that we have soil maps; that de- 
terminations of water capacity and other physical as well as chemical 
characteristics 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 place 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 different 
plants in corresponding concentric zones, but it is equally true that 
this zonal arrangement is also the visible 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 knowledge. How 
greatly we need to really know the physiological 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 conception according to which the 
relations of desert plants to each other have been so persistently over- 
looked, or, at least, underestimated. It may now be set down as an 
established fact that over a large part of the arid or semiarid terri- 
tory of the southwest, competition on the one hand and accommoda- 
tion on the other have much to do with the association of plant species 
and the density of the plant cover. Far more, it would seem, than 
has usually been thought, the character of various associations in this 
region is determined not simply by the physical, but also by the living 
environment. More than ever, too, it is plain that the path of prog- 
ress lies in the direction of applying to the plant itself, in its natural 
surroundings, the experimental methods of the physical laboratory. 
Notable and fruitful beginnings have been made in this direction, 
but one who has attempted quantitative work with the sahuaro or 
ocotillo in the open need not be told that it involves difficulties not 
presented by seedlings of Vicia faba grown in pots, and that prog- 
ress will necessarily be slow. 
Thus far adjustment and adaptation have not directly entered into 
the discussion, although a moment’s thought shows that all the paths 
along which we have come converge right here. If one variety of 
