LOCAL DISTRIBUTION OF SPECIES. 57 
HABITS AND STRUCTURES RELATED TO DISTRIBUTION. 
It is not the plan of the present paper to discuss in detail adjustments 
and adaptations concerning which our knowledge, though delusively 
comprehensive as to certain classes of facts, is still for the most part 
without a solid basis of demonstration based on experiment. It is desir- 
able, however, to direct attention to certain habits and structures exhib- 
ited by some of the most characteristic plants of the Laboratory domain 
that have enabled them, with the aid of concurring agencies and in har- 
mony with environmental factors, to reach their present stations and to 
establish themselves there. 
A whole biological group of plants may be so conspicuously distin- 
guished by some one habit or characteristic relation to a single factor 
of its environment as to materially change at stated times the landscape 
of the region which it inhabits. This, as we have seen, is the case with 
the two biological groups into which the annual plants of the desert 
country of the southwest are divided. Reference to these has already 
been made in an earlier section, and it is only necessary here to emphasize 
the fact that the winter annuals on the one hand and those of summer 
on the other have a single fundamental difference, which is based on 
temperature relations. ‘Thus we have a well-marked case in which distri- 
bution in time is correlated with a single factor, namely, temperature. 
How far the previous geographical distribution of these groups of annuals 
is related to their present habits is still a matter of inquiry. 
While, however, such characteristics may be observed in biological 
groups taken as a whole, they are, in general, more satisfactorily studied, 
especially in an experimental way, in single species. A few of the plants of 
the Laboratory domain that have been subjected to more or less extended 
observation and experiment have given definite results as to the extent and 
promptness of response which they habitually make to the action of a 
single physical factor. 
One of these is the ocotillo (Fouquerta splendens), which during times 
of drought is defoliated, but puts out leaves with astonishing rapidity 
in rainy seasons or when it is artificially supplied with water. The his- 
tory of an individual of this species for a period of several months during 
which it was under observation has been given by Cannon (1905). Exper- 
iments by Lloyd (1906) the following summer show that leaf-formation 
in the ocotillo may be artificially induced by supplying the plant with 
water through the aerial parts, though incidentally the fact is brought 
out that the response is more prompt when water is obtained by natural 
processes through the root. From the work of these two observers it 
becomes evident that the plant in question responds quickly to the influ- 
ence of a single factor, water, and that the amount necessary to elicit this 
response is capable of close quantitative estimation, and with greater 
perfection of methods might be subjected to exact measurement. 
