PRESENT PROBLEMS IN PLANT ECOLOGY :* PROBLEMS 
OF LOCAL DISTRIBUTION IN ARID REGIONS.? 
By Prof. VotNry M. SPALDING, 
Desert Botanical Laboratory. 
The physical conditions prevailing in arid regions are such as 
render it unsafe to admit without further investigation generaliza- 
tions regarding their plant life which have been drawn from studies 
conducted elsewhere. This is sufficient justification of an attempt to 
analyze certain problems which confront the student of desert ecology 
in his efforts to apply knowledge or principles drawn from previous 
experience. These problems have the advantage of a certain clear- 
ness of definition, which corresponds in a way with the sharp features 
of the desert and its characteristic vegetation. Their solution may 
involve great difficulties, and some of them, with our present methods, 
may be incapable of solution, but they are, at all events, capable of 
clear statement. 
In the attempt to present such a statement, which may or may not 
prove successful, I shall for the present limit the discussion to the 
desert country of the southwestern United States, for the sufficient 
reason that my own studies have been conducted in that region; and 
IT shall omit all consideration of the higher elevations of the moun- 
tains, which, though in the desert, are not of it; so that whatever is 
said at this time will be understood to apply to the floor of the desert, 
that is, the great plateaus and valleys which from Texas to California 
lie between the mountain peaks and ranges, together with the long 
slopes and low hills which border them on every hand and form the 
natural approach to the mountains. 
Proceeding in a manner that will be indirectly a record of personal 
experience, one of the first questions presented to a student of desert 
botany is this: What are the conditions that determine the successful 
occupation of a desert habitat by certain plants, but prevent its 
occupation by others? 
4A series of papers presented before the Botanical Society of America, at the 
Baltimore meeting, by invitation of the council. 
> Reprinted by permission from The. American Naturalist, vol. 48, No. 512, 
August, 1909. 
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