458 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1909. 
the only satisfactory explanation. The accumulation of physical 
data, however, has proceeded so far and so satisfactorily that the 
successful conduct of this line of investigation may be regarded as 
assured, but for the plant the relations are more. complicated, and 
their investigation correspondingly more difficult. It seems likely 
that in the study of ecological relations from the side of the plant 
we shall employ more and more the methods and conceptions of 
physics and mathematics, but the fact is too patent to call for argu- 
ment that neither now nor hereafter can these methods and concep- 
tions be employed exclusively. In fact there has never been greater 
need than at the present time for exact observation coupled with 
correct judgment, and these can never be replaced or superseded so 
long as this department of botanical investigation continues to be 
cultivated. This will receive additional emphasis in the following 
division of the present paper. 
The relations of desert plants to each other present a chapter the 
‘importance of which has been unduly minimized until the general 
impression, even among botanists, seems to be that desert plants are 
to be studied only in relation to their physical environment; they are 
thought to grow so far apart, in “open” associations, that they are 
quite uninfluenced by each other’s presence. Like other erroneous 
or incomplete conceptions, this may be true in part, especially where 
the most extreme desert conditions prevail, as for example in parts 
of the Colorado or Mojave deserts, but in the great semiarid region 
of the Southwest, taken as a whole, it is most misleading. The 
Desert Laboratory of the Carnegie Institution was located where it 
stands on account of the great natural advantages which the region 
and locality offer for the study of desert plants in place, yet I venture 
the assertion that over at least nine-tenths of the area of the labora- 
tory domain the establishment of a plant in the place which it occu- 
pies is conditioned quite as certainly by the influence of other plants 
as by that of the physical environment. It hardly needs more than 
simple observation to convince one that severe competition is the 
rule, though naturally its severity is heightened and the result 
hastened by the prevailing adverse physical conditions. 
Beginning with some of the most obvious cases, the winter annuals 
of southern Arizona present an instance of as unmistakable competi- 
tion of individuals with individuals and species with species as can be 
found in the eastern forest region of the United States. As the 
warmth of spring follows the winter rains the ground is thickly car- 
peted with Amsinckia, Pectocarya, Bowlesia, and various other her- 
baceous plants, which stand thick together and present to the eye the 
familiar crowding which is seen in a field of grain too thickly sown. 
Certain individuals dwindle and finally die, robbed of water, food, and 
light by their stronger competitors. It might be interesting to repeat 
