Sept. 1, 1925 
Thermostasy of Growth of the Date Palm 
441 
lower thermometer only from 0.36° to 2.7°, while for the correspond¬ 
ing period the air temperatures range from 21° to 38°. During the 
last days of March, with maximum air temperatures reaching 94° 
and 95°, and minima at 60° and 61°, a daily range of 33° to 35°, 
the 24-hour range of temperatures within the two trees was at most 
7.2°. 
Next we may note that the tree temperatures were slowly ascend¬ 
ing, day by day, at a rate which on the whole corresponded, with the 
rise in the daily mean temperature of the air, tending at first to 
keep above it, but as the mean air temperature increased, falling 
below it, but throughout keeping a relatively uniform distance above 
the soil temperature. 
Here it is necessary to consider more in detail the action of the 
factors (1) and (3), previously outlined, in their control of the tem¬ 
perature of the growth center. 
THE AIR TEMPERATURE 
Thermographic records show that at the Indio garden, the air 
minimum is usually reached shortly before sunrise and is maintained 
but a short time. The upward curve of the trace is a rapid one, 
often showing a gain of 20° by 9 or 10 a. m., with a maximum for the 
day 25° to 40° above the minimum; reached at 2 or 3, sometimes as 
late as 4 p. m. The afternoon decline is much slower than the 
morning rise, until after sunset, when the radiation through the dry 
desert air goes on steadily till morning. 
The regulatory influence of the daily air temperature naturally 
falls into two periods—(1) in which the air temperature is below the 
soil temperature; and (2) that in which the air temperature is above 
the soil temperature. With the preponderance of the first, the 
growth center temperature, after the lag in penetrating the protec¬ 
tion envelope, is pulled down toward the soil temperature and, if 
severe enough, is brought below it; the sap, deriving its temperature 
from the soil, being unable to wholly overcome the penetration of 
cold from without. 
This condition is shown in Table I and Figure 5, where from 3p.m. 
on February 18 to 7 a. m. on the 21st, the air temperature was 
below the soil temperature for 46 hours and above it only 18 hours. 
This is expressed by the mean temperature line C Figure 5, which 
remained below the soil temperature line until the 22d. 
The penetration, through the protecting envelope, was slow, how¬ 
ever, and the growth center minimum of 52.7° was only reached 
at from 9 to 10.30 a. m. The sap, at about the soil temperature, 
must be credited with overcoming this frost incursion and keeping 
the growth center temperature a little above the zero point of 50°. 
The thermometers show both trunk and bud temperatures below 
the soil temperature, with a slow recovering on the afternoon of the 
21st. The soil temperature, itself, under the influence of the pro¬ 
longed low air temperature was lowered from 58° to 57.5° and 
finally to 57°, only getting back to 58° on the 24th after the air 
temperature curve became higher than the soil curve for a predom¬ 
inant portion of the day. 
In successful date-producing regions the periods when the air is 
colder than the soil are very short and the main problem is with the 
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