8 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 
(2) THE RIVER BANKS; ASSOCIATION OF COTTONWOODS AND WILLOWS. 
The banks of the river are lined with willows (Salix nigra) and cotton- 
woods (Populus fremontiw), which constitute their conspicuous vegeta- 
tion, while the arrow-weed (Pluchea sericea) is of frequent occurrence. 
Another plant conspicuously present where the banks are sandy is Aster 
spinosus, which also extends out on the flood-plain to some distance. 
As in similar situations elsewhere, a considerable number of annual plants 
succeed in 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 specifically distinct from Salix nigra, 
which ranges from New Brunswick to Florida and westward to California. 
The cottonwood (Populus fremonti), according to Bray (1904), appar- 
ently takes the place of Populus deltovdes 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 Potamogeton 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, barely one. 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 plants possess, it is not 
unlikely that various species have been brought here time and again that 
have thus far failed to gain a permanent foothold. It is quite probable, 
however, that various species not found here would thrive if they could 
once get a start. Seeds of willows, for example, may often have reached 
the river when there was not sufficient moisture in its bed to result in 
their germination, and even when started they would still find conditions 
far less favorable for survival than along the banks of streams in regions 
of abundant rainfall and more humid atmosphere. 
In its physical features, then, and in its flora (and fauna as well) the 
Santa Cruz is a characteristic river of the region through which its course 
runs. However luxuriant may be the growth of the few species that 
have established themselves along its banks, and however its waters may 
locally be choked at times with dense masses of one or more aquatic spe- 
cies, yet as regards both physiognomy and life it is essentially a river of 
the arid Southwest, presenting the same general characters as those of 
the Salt River, the Gila and others of this region, and no botanist could 

