t 9 o 4 ] MAC DOUGAL— DELTA AND DESERT VEGETATION 49 



dead and dry, still retaining the seed pods, and progress through one 

 of these plantations was accompanied by a shower of seeds which 

 results from any disturbance of the plant. The clearings also fur- 

 nished suitable conditions for a plant with a deeply buried bulb, prob- 

 ably a Calochortus, which is eaten by the Cocopa Indians under the 

 name of "chech," and also forms an important article of food of the 

 sand-hill crane, and of the wild hogs that infest the tules. 



The forests of willow and poplar begin to lose density at a dis- 

 tance of 50-6o km from the Gulf, the willows extending farthest toward 

 salt water, a few being seen near the mouth of the Hardy branch of 

 the Colorado. Beyond these arc the mud plains, the portions not 

 actually subject to erosion being thickly covered with salt grass 

 (Distichlis spicata) and Cressa truxillensis, and bearing small clumps 

 and isolated specimens of salt bush (Atriplex), mesquite, and screw 

 bean. Such areas are inundated at the highest tides; consequently 

 the soil solutions are heavily charged with salts, and whitish alkaline 

 crusts appear during the winter dry season. 



The floods of spring and early summer from the rains and melting 

 snows of the headwaters region of the river raise the level of the 

 water until it flushes the innumerable old channels and covers the 

 greater part of the delta. Most of the herbaceous species make their 

 annual growth after the waters have subsided in July. Other species, 

 which are less affected by the lower temperatures and low relative 

 humidity of the winter season, are set in action by the favorable con- 

 ditions of March and April, and come into bloom at this time, thus 

 making two distinct seasonal groups of annuals. 



The main stream of the river cuts directly into the gravel plain 

 or mesa of Sonora at four points on the eastern margin of the delta, 

 and here are to be seen the striking contrasts of the isolated xero- 

 philous plants of the dry gravelly soil of the desert within a few 

 meters of the pure dense formations of the muddy soil of the alluvial 

 plain of the delta {fig. 2). In places the creosote bush (Covillea) 

 descends the gentler slopes to the margin of the moister soil near 

 the margin of the channel, accomplishing a growth which carries it 

 to a height of over 7 m , the maximum size for the species. 



The above description applies most directly to the eastern and 

 southern portions of the delta, which may be observed in the descent 



