TEMPERATURES OF PLANTS IN THE DESERT. fi 
TEMPERATURES OF PLANTS IN THE DESERT. 
No exact determinations have been made of the intensity of insola- 
tion in American deserts with respect to its effect upon vegetation, but 
it may be assumed to be high, partly on account of the low relative humid- 
ity. The green chlorophyl screens of the shoots, whether in leaves or 
green stems, are subject to the direct action of the rays and take on a 
temperature only modified by the cooling action of transpiration, which, 
however, must be slight in many instances, particularly in the cacti, 
which have been found to throw off water more rapidly in their native 
habitats by night than by day. Then again the action of the sun’s rays 
heats the surface layers of the soil, in whichthe roots of many species lie, 
so that in some cases at least the absorbing organs are embedded in a 
medium much warmer than the air. This point is illustrated by the 
following observations made near Flagstaff, Arizona, July 16, 1898: 
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 alluvial deposit. At 
22 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 3p.m.the 
black volcanic sand around the roots of Cleome serrulata 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 1 inch near Tuc- 
son 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 81° F. Professor Toumey states that the temperature of 
the soil near Tucson increases slowly during July, is 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 observations at Tucson were made con- 
sisted chiefly of decomposed granite with some mica. 
Mr. A. E. Douglass, of the University of 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. 
A pair of Hallock soil thermographs have been in operation at the 
Desert Laboratory since 1905, and it is found that these extreme tempera- 
tures are met only by the roots of species spreading in the surface layers 
of the soil. The records taken at 6 inches below the surface during the 
