442 
Journal of Agricultural Research 
Vol. XXXI, No. 5 
air warmer than the soil for a greater part of the day, or ultimately 
for the entire 24 hours, as is shown in Table VI. An inspection of 
Tables I to V shows that on a clear day the air is maintained at within 
about 5° or 6° of the day's maximum for a period of from 5 to 6 
hours, and within 10° of the maximum for an average of 7 hours. 
Upon exposed surfaces the penetration of heat, when the maximum 
air temperature is 100° or upward, is terrific and the radiation of the 
accumulated heat from a brick or adobe wall can be felt long after 
sundown. Vegetation with unprotected surfaces often shows, 
within the tissues, temperatures several degrees above the sur¬ 
rounding air. Smith (23) observed interior temperatures in the 
giant bamboo, Dendrocalamus giganteus , 6° above the surrounding 
air, which he attributed to “ the intensity of the solar radiation inci¬ 
dent upon the growing oigan.” Even greater increment in tempera¬ 
ture has been observed in the tissue of the giant cactus upon the 
deserts of the Southwest. The different conditions found in the 
interior of the date palm are in most striking contrast. 
To sum up the evidence, then, the air temperatures, low or high, 
are retarded and largely intercepted by the protective layer of leaf 
bases and sheath fiber, before they reach the merisematic tissues of 
the growth center or phyllophore. This is a stabilizing effect, tend¬ 
ing to prevent the penetration of temperatures below the optimum 
for cell growth in cold weather, and conversely, retarding the penetra¬ 
tion of temperatures above the optimum during periods of extreme 
heat. 
THE SOIL TEMPERATURE 
The second factor, derived from the first, is the temperature of the 
soil at 2 to 3 feet deep, where the greater part of the young date palm 
roots are located. This determines the temperature of the soil mois¬ 
ture and must largely govern the temperature of the ascending sap 
current. 
In Indio the soil temperature at 2 feet deep is surprisingly constant, 
the diurnal range during the period of the experiment not exceeding 
0.5°, and during the heat of the summer not exceeding 1° to 1.5° F. 
The relatively low temperature of the Indio soil can be attributed 
in part to its peculiar physical character. The deeper strata are 
composed of the old lake bed silts, above which are alternating layers 
of fine silts from the widely dispersed shallow flooding from the 
Whitewater River and wind-blown fine sand with a high proportion 
of fine particles of mica. It has a high water capacity without the 
danger of becoming “puddled” or impervious to air as does a clay 
soil with repeated wetting. 
The ample irrigation practiced at this garden doubtless is largely 
responsible for the soil being retained at a moderate temperature. 
The mean air temperature for June, July, and August, 1918, was 91°. 
The temperature of the water from the artesian wells used in irrigation 
is about 74°. With an irrigation every 10 to 14 days, the cooling 
effect of this volume of water, with the added cooling of evaporation 
would result in keeping the soil temperature several degrees below the 
mean air temperature of the hottest weather. Free (7, p. 188) at¬ 
taches much importance to the high specific heat of soil water. From 
his statements we may conclude that the retarded warming up of the 
soil due to water, a disadvantage in the spring on an eastern farm, 
