454 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1909. 
It will be necessary at the outset to understand what is meant by a 
desert habitat, since on this point the popular conception—and _pos- 
sibly that of some botanists—is not clear. There is as much differ- 
ence between habitats in the desert as in any other region, possibly 
more, and their definiteness of location and relative sharpness of 
demarcation form one of the most striking and characteristic features 
of arid regions. The rivers of the valley trough, such as the Santa 
Cruz, the Gila and Salt rivers in Arizona, though inconstant, are none 
the less the main drainage channels between the adjacent watersheds. 
Along their banks water-loving willows, cotton woods, and arrow 
weed find a congenial home. The adjacent flood plain, with its water 
table within reach of their roots, is the natural habitat of the mesquite 
and some other semimesophytic species. Within its limits the areas 
known as salt spots are inhabited by various halophytes, especially 
by species of Atriplex and Sueda. Just beyond the flood plain is 
the long slope, a most characteristic feature of desert topography, 
- which rises slowly to the foot of the mountains, often miles away, 
its soil and drainage conditions presenting a sharp contrast to those 
of the flood plain, and its vegetation being correspondingly different. 
The low outlying hills, in their turn, present quite as marked pecu- 
harities of soil, and furthermore introduce differences of aspect which 
are correlated with marked differences of vegetation. In short, the 
habitats of such a desert region as that of southern Arizona, as far 
as edaphic relations are concerned, present conditions which vary all 
the way from distinctly hydrophytic to extreme xerophytic, and all 
these may be in close proximity. 
For all these habitats the fact is to be emphasized that the general 
climatic conditions are the same, and it is important to note that 
not a few of the plants which grow where a sufficient or even abundant 
water supply is assured are nevertheless marked, as a rule, as plants 
of an arid region by their coriaceous, hairy or otherwise xerophilous 
leaf structure. The point to be specially noted here is that while 
plants of the arid or semiarid southwest grow in a great variety of 
habitats, some of which are by no means dry, all are subject to the 
severe conditions of a desert climate, especially intense insolation, 
low percentage of atmospheric moisture, and drying winds. The 
problem, therefore, of the occupation of any one of these habitats is 
successfully met only by those plants that are already adapted, or 
are capable of individual adjustment to the dry air and hot sun in 
which they must live; all others inevitably fail. 
This will be made clear by reference to the introduction, or 
attempted introduction, of various cultivated plants, a subject which 
presents a most instructive history. The yards of Arizona cities 
constitute an experiment station in which year by year, at private 
instead of public expense, the availability of one species after another 
