46 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 
These observations were supplemented by many others taken with 
the ordinary soil thermometers at a depth of 2 inches below the surface. 
Readings of these were made from January 25 to May 20, 1907. Within 
this period the lowest and highest readings at station I were 62° and 
119° F., and at station II 54° and 112° F., the lowest in each case being 
in the shade and the highest in the sun. It is to be observed that whether 
in sun or shade, the temperature of the soil at a depth of 2 inches was 
always higher at station I than at station II. The difference on some 
cloudy days was no more than 3°, but on sunshiny days a difference of 
10° to 15° F. was frequently noted, and in some cases as much as 20° 
COV25 0? 
Readings of the black-bulb thermometer were made from January 26 
to May 20, 1907, the temperature being taken at 1 to 2 inches above 
the surface, in nearly all cases in the middle of the afternoon, but part 
of those in April and May at about 10 a. m. 
From the table on page 98 it is seen that: 
(a) The temperature of the air at 1 or 2 inches above the surface, 
from January 26 to May 20, averaged about 3° higher at station I than 
at station II (readings in each case in the shade), but frequently the 
difference was as much as from 8° to 12° F, 
(6) Differences in readings at points in sun and in shade on either 
side of the gulch are nearly the same, running much of the time from 
January 26 to May 20, between 20° and 35° F., though coming up several 
times to 40° and in one instance to 42° F. 
Summarizing the observations of temperature for the two sides of the 
gulch, it appears that on the right side, with southerly exposure, the 
soil temperatures have a much wider range than on the opposite side 
and in the daytime are much higher, the average observed maximum 
for 3 months being some 22° F. higher than that of the left side at a 
depth of 1 inch. These differences become less at greater depths, but 
still at a depth of 1 foot the soil is from 4° to 7° F. warmer on the right 
than it is on the left side, the difference being greater in winter than in 
summer. The roots of plants growing on the right side of the gulch, 
except as they penetrate to considerable depths, are in a soil always 
warmer, and in the winter, in the daytime, very much warmer, than 
that of the left side, but they are also exposed to much greater extremes 
of temperature. 
The air temperatures (in the shade) are also higher by 5° or more on 
the right side than on the left (often much more), but the greatest dif- 
ferences of temperature on either side are those of full sunlight and shade, 
running from January to May for the most part between 20° and 40° F. 
As the left side is the shady and the right side is the sunny one, during 
all the cold months of the year, these differences are greatly in favor 
of plants sensitive to cold that grow on the right side. When the sun is 
