Plant Associations at the Desert Laboratory. 35 



gaining a foothold on the shifting sands of the river, but none 

 of these are distinctively characteristic of this special habitat. 

 It is the cottonwoods and willows that constitute the typical 

 vegetation of the river-banks. It may be noted in passing that 

 the willow of the Santa Cruz River does not appear to be spe- 

 cifically distinct from Salix nigra, which ranges from New Bruns- 

 wick to Florida and westward to California. The Cottonwood 

 (Populus Frcmontii), according to Bray, apparently takes the 



Fig. 4. Roots of mesquite ex- 

 posed where erosion of the banks 

 of the Santa Cruz River has taken 

 phice. 



Fig. 5. Dry bed of the river at 

 Tucson. Willows and cottonwoods 

 its banks. 



place of Populus deltoides west of the one-hundredth meridian, 

 extending to western California and into Lower California and 

 northern Mexico. 



The flora of the river and its banks is seen to be meager as 

 compared with those of regions with abundant rainfall. In the 

 Huron River, at Ann Arbor, Michigan — a fair example for the 

 eastern United States — there are over a dozen species of Pota- 

 mogeton as against the single species of the Santa Cruz River at 

 Tucson, and while along the banks of the Huron River, at the 

 place named, there are 10 or more species of willows, we have 

 on the banks of the Santa Cruz, at this point, but one or two. 

 It is evident that the total of conditions here is unfavorable to 

 the great majority of species of similar habitats in humid regions, 

 and with the admirable arrangements for dispersal which these 



