36 The Plant World. 



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 va- 

 rious 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 species, 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 

 would for a moment fail to recognize this fact, especially as just 

 beyond its banks there is growing on every hand the mesquite, 

 the everywhere-present species of the Lower Sonoran zone. 



Thus even an abundant water supply, with the strong growth 

 of hydrophytes determined by it, altogether fails to reproduce 

 here, except in the most superficial way, the plant associations 

 of rivers in the eastern United States. There may be an ap- 

 proach to the mesophytic forest, where willows and cottonwoods 

 grow thick along the banks of the Santa Cruz, but not a meso- 

 phytic forest of the East or North. Other factors than the water 

 supply, however potent this may be, are to be reckoned with in 

 attempting to account for the wide differences of plant and 

 animal life that are here observed. 



(3) The Flood-Plain; Mesquite Forest Association. 



The flood-plain of the Santa Cruz River is essentially the 

 same in its physical characteristics as those of other rivers of the 

 Southwest. A deep, fine alluvium, closely resembling the Mari- 

 copa sandy loam of the Gila and Salt River Valleys, the product 

 of a long period of erosion and deposition, fills the valley from 

 the river-bank to the mesa-like slopes at the foot of the moun- 



