44 



DESERT BOTANICAL LABORATORY 



1 1 per cent greater than at the sea level in a humid atmosphere, and 

 that the intensity is 26 per cent greater at an altitude of 15,600 feet, on 

 the summit of Mont Blanc, than at sea level. It is to be seen by refer- 

 ence to the curve of humidity that the amount of moisture in the air in 

 the locality under discussion is at times extremely small, which would 

 further intensify the insolation. It would be entirely safe therefore in 

 a comparison of this summit with that of Mont Blanc to say that the 



Fig. 4. Meteorological data from San Francisco mountain, Arizona, Aug. 8 

 to Aug. 19, 1898. The upper curve, traced from the hygroscopic record shows 

 variations in relative humidity. The lower curve shows the corresponding air 

 temperature in the shade. The instruments were in the shelter shown in Plate 

 XXIX. 



intensity of the sun's rays is at least 25 per cent, greater than at sea 

 level and about 12 per cent, more than on the plain below, which by its 

 altitude and other conditions would in turn be about 1 2 or 13 percent, 

 greater than at sea level. 



The species of plants inhabiting this and all alpine situations are al- 

 most altogether perennials, passing the unfavorable season in the form 

 of a fleshy or woody rootstock. The aerial shoots are generally pros- 

 trate and lie close to the ground, a device generally conceded to be 

 due to the thermotropism of the plant whereby it receives all possible 

 advantage of the higher average temperature of the soil than of the 

 air. So far as the structural features of alpine plants are concerned it 



