32 BOTANICAL FEATURES OF NORTH AMERICAN DESERTS. 
vegetative and reproductive cycle during the period of maximum pre- 
cipitation. Many interesting facts are also cited asto the uses of hairy 
coverings and resinous coating in the prevention of damage by extreme 
evaporation of water and intense radiation. 
GRAND CANYON OF THE COLORADO RIVER. 
The Colorado River comes down into the great Nevadan-Sonoran 
Desert through the deep canyon in northern Arizona, from which it 
emerges to be directly bordered on either hand by pronounced arid areas 
at low altitudes. An examination was therefore made to ascertain to 
what extent the xerophilous plants of the lower deserts had extended up 
along the shelves and terraces of the canyon at the elevation of the open 
deserts. Three visits in all were made to the canyon, one solely for the 
purpose of getting an impression of the range of vegetation from the 
timbered rim at 6,866 feet at the end of the railway leading to it, down the 
Bright Angel trail to the river at 2,436 feet. 
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. A very striking feature is extensive fields of a rosa- 
ceous shrub, Coleogyne ramosissima, which extends in an almost pure 
growth over the canyon terrace at an elevation of about 3,600 feet in 
a soil seemingly well supplied with lime (plate 29). There is a notable 
absence of many shrubs which would be present in the open desert at the 
elevation 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 producing 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 condense as it drifted across the 
canyon, whereas if it should drift off the plateau over a desert of low eleva- 
tion its precipitation would be greatly lessened or would cease altogether. 
Furthermore, the canyon exerts some influence upon the conditions 
affecting vegetation on its rim. The heated air from the lower warmer 
levels rises, expanding as it does so and increasing its relative humidity, 
with the result that during the daytime a current of air, cooler and moister 
than the air on the mesa, pours out of the canyon overitsrim. The effect 
of this is strikingly illustrated in the region of the Coconino Forest, where 
many Species not seen elsewhere on the mesa are to be found fringing 
the rim of the canyon. Razoumojfskya vaginata (Willd.) Kuntze is a 
loranthaceous parasite growing on the branches of the bull pine (Pinus 
scopulorum) throughout the transition zone. Itis most abundant, how- 
ever, along the margins of mesas, the rims of canyons, and certain hilltops. 
