PLANT LIFE IN DESERTS 23 



in the washes, the prevaiHng bush, and continues throughout the long 

 waste of desert to the Colorado river. A fuller description of the 

 vegetation of the Mohave desert and the areas to the north is given in 

 Coville's Botany of the Death Valley Expedition, cited in the bibliog- 

 raphy. 



THE GRAND CANYON OF THE COLORADO. 



A visit was made to the Grand Canyon of the Colorado with the 

 expectation that its lower elevations would afford lodgment for many 

 desert plants, and that a descent from the timbered rim, at 6,866 feet, 

 to the river at 2,436 feet, would permit the traveler to see in a brief 

 trip a wide range of desert vegetation. Although the descent is full 

 of botanical interest, and does carry one down through several dif- 

 ferent belts of vegetation, the comparatively limited number of 

 woody desert plants rendered the journey somewhat disappointing from 

 the standpoint of the main object of our trip. For the first 2,600 feet 

 of the descent the trees continue, but from that point to the river the 

 slopes are treeless and the vegetation of a desert character (Plate 

 XXVII) . One of the most striking features is extensive fields of a 

 rosaceous shrub, Coleogyne rafnosissima, which extends in an almost 

 pure growth over the canyon terraces at an elevation of about 3,600 

 feet in a soil seemingly well supplied with lime (Plate XXVIII) . There 

 is a notable absence of many shrubs which would be present in the 

 open desert at the elevations afforded by the lower parts of the canyon 

 and which have a seemingly good route for extension up the canyon 

 from the Mohave desert. The absence of these plants is presumably 

 connected with the narrowness of the canyon, which besides pr6\luc- 

 ing abnormal air currents and temperature conditions is responsible 

 for a rainfall greater than would occur at the same elevations in the 

 open desert. A cloud sheet precipitating rain on the 7,000-foot plateau 

 through which the canyon passes would presumably continue to pre- 

 cipitate as it drifted across the canyon, whereas if it should drift off the 

 plateau over a desert of low elevation its precipitation would be greatly 

 lessened or would cease altogether. 



Plant Life in North American Deserts. 

 The term desert may be applied to areas of the earth's surface 

 which support a sparse vegetation of a more or less specialized char- 

 acter owing to inadequate rainfall, or to the unsuitable composition or 

 lack of soil. Of these conditions, scanty water supply may be re- 

 garded as of the greatest importance, and it is to this factor that most 

 deserts owe their existence. Desert conditions arise in any region in 



