444 
Journal of Agricultural Research 
Vol. XXXI, No. s 
interior temperature at 52.7°. This is an example of the importance 
in growth-temperature studies of knowing the actual temperature 
maintained within the growth-center, although such knowledge is 
usually difficult to obtain. 
Another conclusive proof that the lower curves of the interior 
temperatures are in large measure due to the cooling influence of 
the ascending sap current lies in the records of April 21 to 27 (Table 
IV and fig. 6) where on the 23d, 26th, and 27th particularly, the 
midday temperature shown by the lower trunk thermometer was 
3° to 5° lower than that of the upper one and on the latter date 
actually coincided with the soil temperature. While the retarded 
effect of the cool night air has an undoubted influence in bringing 
about the lowered temperature of the interior at midday, the records 
of February 19 show that the interior of the palm at the growth 
center is much more sensitive, and more quickly reacts to external 
cold than does the trunk interior near the ground. Affected only 
by the cooling of the night air, then, the upper thermometer, after 
allowing time for the lag, should show a lower midday temperature 
than the thermometer 2 feet from the ground. This has occurred 
only a few times; once on February 19 under the influence of the 
sudden drop to 25°, which was more than sufficient to overcome the 
influence of the soil water at 58°. 
The influence that could keep the lower trunk center at a lower 
temperature than the upper one at midday would be that of the 
ascending sap current, which would be most active during the period 
of greatest transpiration. Although the growth-center temperature 
is no doubt lowered by the same influence, the ascending sap reaches 
it later, after having absorbed more heat from the surrounding tissues. 
Though no observations have been made on the hourly transpiration 
rate of the date palm, the studies of Briggs and Shantz at Akron, 
Colo. (3)j show that for a variety of plants, but especially wheat and 
rye, the maximum of the day’s transpiration occurs later than mid¬ 
day, often between 2 and 3 p. m. It will be noted from the Tables 
I to VI of interior temperatures of the date palm that the minimum 
recorded by both the upper and the lower thermometers is usually 
at a point about midway between 12 noon and 6 p. m. 
April and May, being unusually cool, did not afford the usual 
examples of high temperatures, but April 29 and 30 and May 1, 2, 
and 3 gave maximum air temperatures of from 95° to 99.5° F. with 
daily means from 78.5° to 82.5° (Table V). Yet the minimum 
interior temperature of the Zaheedy tree by the lower thermometer 
at 3 p. m. (the upper one was broken) was 71.6° on the 29th and 
72.5° on the 30th, with the soil >t 70° and 71°. During the first 
three days of May, with air maidma reaching 98° and 99.5°, this 
interior temperature was only 2.5° to 2.7° above that of the soil. 
On May 2, with the maximum air temperature 99.5°, the interior of 
the Zaheedy touched a minimum of 73.4°, a protective cooling of 
26.1°. At the same time the growth center of the Maktoom tree 
gave a mi nimum of 76.1°, giving these active cells a cooling effect 
of 23.4° below the air maximum. For this entire period the tem¬ 
perature curve for the lower Zaheedy thermometer was below the 
daily mean air temperature. With two exceptions, the growth 
center temperature of the Maktoom was also below it. This gives 
another proof of the cooling influence of the ascending sap current. 
