460 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1909. 
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 accommodation, which enables them to utilize to the utmost 
the scanty rainfall. The roots of the sahuaro are spread just beneath 
the surface of the ground, where they take up and promptly pass on 
to the storage cells of the trunk the water brought to them by every 
light rain. The roots of the palo verde, on the other hand, extend 
much more deeply into the ground, and are in a position to utilize 
the water which soaks down to lower levels after heavier rains. Thus 
the sahuaro profits by all rains, hght and heavy alike, while its con- 
stant companion, the palo verde, is free from all competition on its 
_part for the water which penetrates to lower levels. Much the same 
thing is seen on the flood plain of the Santa Cruz and other rivers, 
where the mesquite, with its deep roots reaching to the water table, is 
associated with Bigelowia and other plants, the roots of which extend 
to relatively shallow depths. In short, it appears that just as in a 
tropical forest the vegetation occupies successive “ stories,” so here the 
root systems of various plants habitually reach to different depths, 
and thus enable at least some species that would otherwise compete 
with each other to live in close and advantageous association. 
From what has been said it is evident that in the successful occupa- 
tion of a desert habitat the mutual relations of the associated species 
play a highly important part. It is not quite easy at this stage of 
progress to point out the exact steps by which these complicated re- 
lations are to be determined and estimated; meantime the homely and 
effectual method of patiently gathering the data that are obtainable 
by careful observation is open, and as far as it has been pursued has 
yielded valuable results. 
The broad general problem of the local distribution of desert plants 
is necessarily approached along the several lines that have been 
indicated. As we have seen, atmospheric conditions, whether of 
intense insolation or extreme dryness, that obtain in arid regions are 
limiting factors which many plants successfully meet, but to which 
many others succumb. There has been great need of more practical 
methods of determining and estimating the influence of atmospheric 
factors, and it is a matter of congratulation that the methods devised 
by one of the participants in this discussion, already widely in use in 
different parts of the United States, promise to meet this need to a 
degree that could not be hoped for at an 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 
