PLANTS IN ARID REGIONS—SPALDING. 459 
the experiment in the laboratory and to tabulate the results statisti- 
cally, but it could hardly add to the conclusiveness of the demonstra- 
tion. The same is true of the manifest competition of species with 
species, as seen for example in the occupation of relatively extended 
areas by some of the perennial grasses which, but for their presence, 
would certainly be covered, as the adjacent areas are, by a thick growth 
of other plants. Here the actual advance of the grasses from year to 
year may be observed, and such observations for the sake of more 
definite statement are now in progress on the Desert Laboratory 
domain. Convincing evidence of competition is thrust upon one’s 
attention in passing from the desert to areas beyond its borders, and 
if the transition is abrupt, as for example on the western edge of the 
Salton Basin, where the desert abuts almost upon a mountain wall, 
the case is all the more striking. In this instance a straight course 
of less than 5 miles brings one from the actual desert, with its char- 
acteristic sparse growth of salt bushes, creosote bush, galleta grass, 
and the like, to the chaparral of the mountain side. Along the way 
the desert species fall out one by one, and are replaced by elements 
of the chaparral. As far as can be judged by their habits elsewhere 
and from their known range in altitude, there is absolutely no reason 
for this, except their inability to compete with plants of the cha- 
parral, which, however incapable of normal development in the 
desert, hold their own ground where the conditions are less strenuous 
so tenaciously and completely that the desert species make no head- 
way against them. 
This, of course, is an interpretation merely, but with such an 
accumulation of evidence we are now in a position to proceed with 
the problem along definite lines with the expectation of definite 
results. Sowing together seeds of desert and other plants, the trans- 
ference of individuals to denuded areas beyond their natural limits, 
and multiplied comparative observations of the deportment of dif- 
ferent 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 convince 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 be- 
yond which, in the midst of otherwise entirely favorable environ- 
mental conditions, they can not pass. 
In their relations to each other, desert plants frequently exhibit 
not merely competition but accommodation. This has been clearly 
shown by recent studies of the root systems of certain cacti and other 
plants by Dr. W. A. Cannon. To take a striking example, superficial 
observation of the association of the sahuaro (Cereus giganteus) 
with one of the palo verdes (Parkinsonia microphylla) and some 
