TRANSPIRATION AND TEMPERATURES 41 



No measurements were made of the intensity of the insolation, but 

 a comparison of the conditions with those prevalent in other localities 

 in which photometric work has been carried out would tend to the 

 conclusion that it is higher than that of 1.6 units recorded in the East 

 Indian tropics, an effect due to the low relative humidity of the desert 

 atmosphere. The intensity of the insolation may affect plants in three 

 ways of interest in connection with the purpose of this paper. The 

 action of the blue-violet rays may exercise a disintegrating effect upon 

 the protoplasmic constituents of the organism. The attainment of the 

 higher temperatures as a result of the conversion of light rays into 

 heat may reach the maximum limit of the activity of protoplasm ; also 

 the critical point at which chlorophyl begins to undergo chemical de- 

 terioration, and the action of the sunlight upon the soil would result in 

 the attainment of temperatures affecting the development and absorp- 

 tive activities of the root system. In order to obtain data bearing 

 upon these points the following observations were made : 



July 16. The bulb of a mercurial thermometer was pushed down 

 into the soil around the root tips of a clump of bunch grass to a depth 

 of 2 inches, and the glass stem of the instrument shaded from the 

 direct rays. The soil consisted of a mixture of volcanic sand and al- 

 luvial deposit. At 2.20 P. M. a temperature of 106° F. was recorded. 

 A few minutes later 108° F. with the air ranging from 91.4° to 93.2° 

 F. At 3 P. M. the black volcanic sand around the roots of Cleoine 

 serrulata Pursh showed a temperature of 111° F. with the air at 

 113° F. Professor Toumey cites the fact that the temperature of the 

 soil at the depth of one inch near Tucson reaches a temperature of 113** 

 F. with a mean average of 104.9° F. for the entire month of July. 

 Also that the average for the month of July at a depth of 4 feet was 

 82° F. with a maximum of 84.5° F. and a minimum of Si° F. The 

 writers are indebted to Professor Toumey for the statement that the 

 temperature of the soil near Tucson increases slowly during July, re- 

 mains stationary during August, and begins to decrease in September. 

 These observations are of great interest since the insolation would be 

 practically identical with that near Flagstaff, although the altitude of 

 the latter place is somewhat greater. The soil in which the observa- 

 tions at Tucson were made consisted chiefly of decomposed granite 

 with some mica. 



Mr. A. E. Douglass, of the Lowell Astronomical Observatory at 

 Flagstaff, Arizona, has communicated some observations indicating that 

 the sandy soil around the roots of small herbaceous plants in the Grand 

 Canyon, Arizona, on September 4, 1898, exhibited temperatures as 

 high as 148° F. 



