EDGE OF TllK COLORADO Dl.SllRT 213 



sists of mesquite, catclaw, l.arrea, and Atriplex, in as true 

 a piece of desert as can he imagined. Thus within a distance 

 of five miles we have passed consecutively through, first, the 

 mountain chaparral, next, a mixture of chaparral and desert 

 forms, and lastly, into a true desert vegetation. 



A more perfect knowledge of physical conditions that 

 have resulted in the remarkable differences of vegetation 

 within such short distances is greatly to be desired, though 

 it is not likely that this would furnish a complete explanation 

 of all the facts. At present it can only be said that precipi- 

 tation diminishes \ery rapidly from the mountain to the 

 desert, that the air here is much drier, the temperature higher, 

 and the hot, dry winds from the desert inciuce an especially 

 high rate of e^'aporation. 



It l^ecomes, then, for plants from outsicie its borciers, 

 a test of endurance, and one of the most significent facts im- 

 pressed on the attention in making the short journey of five 

 miles from the mountain to the desert is that the great 

 majority of the species constituting the chaparral ha\e 

 dropped out bv the time that less than a dozen character- 

 istically desert species ha\e come in. Only the chamisal 

 and a few of its associates are capable of enduring the con- 

 ditions that exist on the edge of the desert, and even these 

 fail to maintain themselves beyond the point where the 

 creosote bush and the guietta grass appear. 



I'here is, in this case, e\'ery indication that the species 

 which drop out as the desert is entered are prevented by 

 adverse physical conditions from advancing farther, rather 

 than by competition on the part of the desert species which 

 meet them here. Desert plants are notoriously poor fighters 

 where the conditions are favorable to mesophytes, and, while 

 it is bv no means to be inferred that competition does not 

 exist among themselves, it does not appear that it is their 

 presence that prevents the ad\ance of other plants upon their 

 ground. I he latter are simply unable to cope with the con- 

 ditions under which desert plants make a normal de\elopment. 



