44 BOTANICAL FEATURES OF NORTH AMERICAN DESERTS. 
relationship of its flora. The extreme type of strict desert offered by 
the area in question points to the possibility of finding here the readiest 
solution of some of the more important problems presented by desert 
vegetation. 
TUCSON, THE SITE OF THE DESERT LABORATORY. 
In the original selection of Tucson as a laboratory site the following 
conditions were taken into consideration as being desirable: 
1. A distinctly desert climate and flora. 
2. A flora as rich and varied as possible, while still of a distinctly 
desert character. 
3. Ready accessibility. 
4. Habitability. 
Much of the arid region of the western United States is only partially 
or relatively arid and does not therefore contain those pronounced types of 
drought-resistant vegetation which it is the first object of the Laboratory 
toinvestigate. Such semi-desert areas are the western portions of Kansas 
and Nebraska, and the intramontane valleys of southern California. 
Another sort of location, to be avoided for a like reason, was a desert 
which was likely to be reclaimed by irrigation. The desert character of 
a small area, even though carefully reserved, might be seriously modified 
by seepage or other changes following irrigation development in the 
vicinity. 
Some of our deserts, such as the Mohave, the Colorado, and the lower 
part of the Gila, are of such extreme aridity that only a small number of 
vegetative types occur in them. The same paucity of vegetative types 
is usually characteristic of any flat area of desert as distinguished from a 
foothill, canyon, or mountain area, a broken and rocky soil giving a wider 
range of temperature and moisture conditions of both soil and air, and 
furnishing lodgment for a greater variety of plants. The yucca plains of 
the Otero Basin in New Mexico and the sage plains along the Snake and 
Columbia rivers in Idaho, Washington, and Oregon areexamples of deserts 
in which a pronounced paucity of woody species is correlated not with 
extreme conditions of aridity but with flatness of surface. 
The conditions of living at some spots in the desert suitable in other 
respects for laboratory purposes are so severe as to offer an obstacle to 
the best work. A period of such extreme heat as occurs in summer at 
some points of very low elevation, as for example, along the lower Colo- 
rado River or in the vicinity of Guaymas, Sonora, or the difficulty of get- 
ting pure water and good food, has been an effective argument against 
some otherwise good locations. 
Viewed from the standpoint of these primary requirements, Tucson 
has a climate of a thoroughly desert character, and a flora, including 
mountains and plain, rich in species and genera. In addition to its situa- 
tion in the heart of the desert of Arizona, it is centrally located, both as to 
