SUMMARY. 141 
desert vegetation than some more highly specialized forms. ‘The remark- 
able adaptations of the sahuaro are in strong contrast with the simple 
structures of the creosote-bush, but the latter ranges far more widely 
than the former, and stubbornly endures vicissitudes to which the sahuaro, 
even in the areas of its best development, inevitably succumbs. 
(7) It has become evident that the relations of desert plants to each 
other are not less important than their relations to their physical envi- 
ronment. The commonly received view that desert plants are engaged 
in a struggle with their environment, and not with each other, requires 
material modification in view of what has been observed to take place 
on Tumamoc Hill, where vigorous competition between different species 
and different individuals of the same species goes on from year to year. 
There is also a mutual accommodation, in certain cases, by which plants 
with root-systems reaching different levels are enabled to live advan- 
tageously in close proximity. It is plain that no general statement of 
the relation of desert plants to each other can be formulated at present. 
Each species requires investigation in its relation to its immediate asso- 
ciates, and when, as must happen, both competition and accommodation 
are involved the complicated nature of the problem is manifest. 
(8) Passing to matters pertaining to general movements of desert plants, 
it may be said that the agencies and structures operative in the dissem- 
ination of seeds and propagative bodies of plants on the domain of the 
Desert Laboratory present no special features, so far recognizable, and 
no unusual interpretation of their action is necessary to account for the 
presence of the species found here. 
The plants which have been brought by various efficient agencies are 
of widely different geographical origin. ‘The analysis of the flora, par- 
ticularly as regards the genera, indicates for some of them northern 
Mexico as their center of dispersal, for others tropical or subtropical 
America, for another contingent the northwestern United States, and 
for another the high northern regions of either hemisphere, while still 
others, including miscellaneous introduced species, have come from vari- 
ous parts of the world by the most diverse routes, some of which have 
been satisfactorily traced. 
(9) It is noteworthy that between 400 and 500 species of plants, of 
such diverse geographical origins, should have been able to establish 
themselves within the narrow limits of the Tumamoc Hills and the adja- 
cent valley and accommodate themselves to the somewhat trying cli- 
- matic conditions prevailing here, since the effectiveness of general climatic 
factors in limiting the range of species is well known. Important in 
this connection are the observations of Professor Thornber on the relation 
between the annual distribution of rainfall and vegetation in southern 
Arizona, according to which it appears that grasses are favored by the 
preponderance of summer rainfall to the east and south of Tucson, in 
contrast to preponderating winter rainfall to the west. 
