PLANT ECOLOGY ANP FLOKISTICS OF SALTON SINK. 87 



but are seldom prominent, and then only over limited areas. Here they are both numerous 

 and well grown. There is an increasing amount of bare sand and gravel, but as the moun- 

 tains are near enough to draw from the clouds a certain amount of annual rainfall, these 

 intervals are often clothed in the spring with bright-flowered annuals. 



By another 450 feet of descent in a distance of 5 miles. Palm Springs railway station 

 is reached, and the Larrea zone has already been left behind. Here is a scene of desolation 

 indeed. A vast, apparently level, plain of gray, wind-swept sand stretches before the eye. 

 Picked up by the wind from the detrital fan at the pass, the sand is carried along with a 

 force that eats into wood and etches stone. Larrea remains the most abundant shrub, 

 but the individuals are separated by great distances of bare sand, and of many the form 

 has undergone a change. They lie prostrate on the sand, a long, narrow line of green, point- 

 ing away from the direction of the wind. By its varying action a hillock of sand may be 

 heaped about them, or much of their roots may be exposed. Here and there a sparse 

 fringe of Panicum urvilleanum serves to partially fix the sand, and where there is a little 

 shelter the surface may be thinly dotted with small tufts of Chamcesyce polycarpa, an inch 

 or two high. This sand desert continues for nearly 20 miles, until it becomes, along the 

 margin of the Sink, a series of dunes, each formed by an overwhelmed mesquite tree, whose 

 projecting ultimate twigs crown it with verdure.* 



Between the southeastern margin of the desert and the Colorado River the zonaliza- 

 tion is less extensive, since the dividing ridge which is here passed over has an altitude 

 of only about 400 feet. The zones are, however, well marked. 



The old sea-beach is bordered by mounds heaped about living mesquites, and smaller 

 ones in which are buried larreas, which do not long survive. Beyond is a stretch of drift- 

 ing hillocks, totally bare of vegetation. These are not "sand hills," as they are usually 

 called, but are dunes in form and in every respect, except that they are formed of fine 

 particles transported by the wind from the alluvial soil of Imperial Valley. 



Where a little basin has collected the water of a thunderstorm, almost always are to be 

 found mats of Pedis pappose, perhaps Chamcesyce setiloba, Lepidium lasiocarpum, Baileya 

 pauciradiata, or a few other annuals. 



Beyond these mounds the imperceptible dividing summit is occupied by an Opuntia 

 zone. To this succeeds a pebble-strewn plain of so slight an incUnation as to appear level, 

 scattered over with the tall, slender stems of the ocatilla {Fouquieria splendens), and con- 

 tinuing quite to the verge of the broad bottom-lands of the Colorado River, which are grown 

 up to mesophytic thickets of cottonwoods and willows. 



The general zonal disposition of the flora of the Colorado Desert, on a line drawn from 

 the summit of San Gorgonio Pass through Salton Sea, to the Colorado River, is shown 

 diagramatically below: 



Diagram of the floral zones of the Colorado Desert. 

 Tension Zone, Cottonwood-willow Zone, 



Yucca-Opuntia Zone, Fouquieria Zone, 



Larrea Zone, Opuntia Zone, 



Sand-drift Zone, Alluvial-drift Zone, 



Mesquite-dune Zone, Mesquite-mound Zone, 



Atriplex Zone of Salton Sink. 



FACTORS DETERMINING THE DISTRIBUTION OF THE SINK FLORA. 



The causes which determine the character and the distribution of vegetation are 

 general in their nature, and apply ahke to the parched desert and the humid valley, but 

 the resultant problems which they present are simple or complex in proportion to the 

 modifying influences which are involved. 



' The distances here given are along the Southern Pacific Railway; by an air-line they would be shorter. 



