So. on 2] PRESENT PROBLEMS IN PLANT ECOLOGY 473 



Proceeding in a manner that will be indirectly a record 

 of personal experience, one of the first questions pre- 

 sented 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 prevenl its 

 occupation by others? 



It will be necessary at the outset to understand what 

 is meant by a desert habitat, since on this point the pop- 

 ular conception— and possibly that of some botanists- 

 is not clear. There is as much difference between habi- 

 tats 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 char- 

 acteristic 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 water 

 sheds. 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 mosquito and 

 some other semi-mesophytic species. Within its limits 

 the areas known as salt spots are inhabited by various 

 halophytes, especially by species of Atriplex and Suaeda. 

 Just beyond the flood plain is the long slope, a most char- 

 acteristic 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 corre- 

 spondingly different. The low outlying hills, in their 

 turn, present quite as marked peculiarities 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 con- 

 cerned, present conditions which vary all the way from 

 distinctly hydrophytic to extreme xerophytic, and all 

 these may be in close proximity. 



