Sept. 1, 1925 
TJiermostasy of Growth of the Date Palm 
445 
Here, again, the principle of the high specific heat of water plays a 
most important part, conversely to that when air temperatures are 
below soil temperature; water is slow to cool or give up its heat, but 
is also slow to absorb it, hence serving as the great factor in stabilizing 
temperatures of the tissues of the growth center. 
A LATER SERIES OF RECORDS 
The most convincing proof that the temperature of the soil water 
is the chief governing factor in regulating the interior temperature 
of the trunk during hot weather is found in records secured in the 
spring of 1923 by the study of a large male palm known as the 
“Mosque Male” in the collection at the Indio date garden. This 
tree was a 9-year-old seedling at the time and 11 feet high to the 
top of the bud, with a trunk diameter of 2 feet at 4 feet high. Doubt¬ 
less the greater size and more mature age of this tree rendered it less 
susceptible to changes in air temperature than the smaller trees of 
the earlier tests, also the greater extent and deeper penetration of its 
roots would give a different reacton in relation to soil temperatures. 
In February a thermometer was placed in the trunk at 4 feet high, 
the bulb reaching to the center of the tree, and another in the near-by 
soil 2 feet deep. It was not expedient to place a thermometer in the 
growth center of this valuable tree. 
The records obtained showed that the trunk temperatures were 
higher than those of the soil by 2° to 4° F. in the early morning, but 
approached most closely at about 2 to 3 p. m., occasionally coin¬ 
ciding. On May 5, a very hot day, the first of the season, trunk 
temperatures from 10 a. m. to 4 p. m. were lower by 1° to 1.75° 
than the soil temperatures at 2 feet deep. On May 7 another soil 
thermometer was installed at a depth of 3 feet. 
It at once became clear that the lower trunk temperatures were 
due to soil water drawn from a greater depth than the 2-foot layer. 
Table VI, for May 5 to May 11, comprises a period of unusually high 
temperatures for the early part of May with the air of desert dry¬ 
ness, according to Indio date garden records, the relative humidity at 
2 or 3 p. m. being only 10 to 15 per cent. 
The trunk temperatures declined from about 8 a. m. until 2.30 or 
3 p. m., the period when leaf transpiration is presumably the 
greatest. Here the trunk temperature fell below that of the soil at 
2 feet, and at times approached closely to that of the soil at 3 feet, 
where the heavier feeding roots of a tree of this age are found. Evi¬ 
dently the heavy demand made upon the roots for water at the mid- 
afternoon period carried the sap up through the trunk at such a rate 
that there was comparatively little lowering of the temperature at 
which it left the soil. At wnat actual temperature this sap reached 
the growth area of the bud, 5 or 6 feet higher, must be inferred from 
the records obtained in 1918 from smaller trees. Table IV shows 
that when the soil temperature at 2 feet was 71° F., the midday tem¬ 
perature in the growth area was 3° to 5° higher. A 6° increase in sap 
temperature for the Mosque Male (from soil at* 2 feet to the growth 
area of the bud) would give this region a temperature around 80° at 
the time when the air maximum was 108° to 110°, or a protective 
temperature difference of from 28° to 30°. It must be remembered 
that the basal growing points of all incompleted leaves (of which 
there may be 6 or 8 in process of elongation at once) center in this 
