58 The Plant World. 



gating ditches the species characteristic of salt-spots, and es- 

 pecially Suoeda Moquini, grow more luxuriantly than in the salt- 

 spots themselves, but this is a physiological response character- 

 istic of plant liie in general, and can hardly be taken in evidence. 



The question of soil preferences involves great difficulties, 

 due in part to the apparently contradictory deportment of many 

 plants that have been cited in evidence, and still more to lack of 

 sufficiently extended observation and experiment. The inter- 

 esting experiments of Kearney and Harter on the comparative 

 tolerance of various plants for the salts common in alkali soils 

 shows clearly the great complexity of the subject when the rela- 

 tion of any plant to a mixture of alkali salts is under investiga- 

 tion. On the other hand, an illuminating discussion of the soil 

 preferences of alpine plants by Fernald has brought forward so 

 many incontrovertible facts, established by years of careful 

 observation, that it no longer seems possible to doubt that the 

 chemical composition of underlying rocks has been an important 

 factor in determining the distribution of the remarkable eco- 

 logical group to which his study has been directed; the whole 

 question, however, calls for experimental investigation. 



(5) The Wash ; Palo Verde-Catclaw Association. 



At the foot of Tumamoc Hill, on the northwest, there is a 

 "wash," formed by the union of a number of similar ones which 

 come down from the long slopes to the westward, the system 

 presenting throughout its extent the usual features of a dry 

 watercourse, with sandy bottom, along which a dense vegetation 

 of shrubs or low trees marks its course even when seen at a dist- 

 ance. The wash with its branches constitutes a drainage system 

 for the west slope of Tumamoc Hill on the one hand and the east 

 slope of the nearby hills of the Tucson Range on the other, 

 forming the natural channel or run-off into which water runs or 

 seeps from the adjacent banks. Though its bed is usually dry, 

 there are abundant indications of the presence of water below 

 the surface, especially in the character and habits of its vege- 

 tation. In the first place certain species, notably the creosote 

 bush, attain here a size and vigor of growth in striking contrast 

 with the low, straggling individuals of the slopes nearby; and, 

 secondly, other species, particularly the mesquite, everywhere 

 taken as an indicator of water, are conspicuously present. Both 



