1904] 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 50-60%" 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 are 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™, 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 
Mo. Bot. Garden 
1904. 
