Sept. 1, 1925 
Thermostasy of Growth of the Date Palm 
431 
FACTORS GOVERNING THE INTERIOR TEMPERATURE 
The temperature within the massive trunk of a date palm and the 
phyllophore must be governed by: 
(1) The temperature of the surrounding air; (2) the loss or gain of 
the air temperature in translation through the leaf bases and trunk 
tissues to the interior; (3) the temperature of the soil at the depth of 
the feeding roots, assuming that this governs the temperature of the 
soil moisture as it enters the rootlets and as it traverses the roots to 
enter the trunk; (4) the loss or gain in the temperature of the ascend¬ 
ing sap current in its progress through the trunk to the leaves; (5) an 
amount, difficult to estimate, of heat generated by the cell activities 
in the growth center—the heat of respiration. 
The factors (1) and (3), both indirectly the products of the air 
temperature, affect the growth center quite differently; at different 
hours of the day and during different portions of the growing season. 
Hence the influence of each should be kept quite distinct in con¬ 
sidering the results which follow. 
There must also be considered here a factor whose influence we are 
wholly unable to measure, the cooling effect of the transpiration of 
the large volume of water from the leaf surfaces. The effect of this 
in the regions of photosynthesis is undoubtedly considerable. The 
influence of the return sap current bearing the products of photo¬ 
synthesis on the general temperature of the phyllophore would be 
impossible to estimate, but it is probably negligible. 
EXPERIMENTAL DATA 
In the analysis by the writer of a series of observations on the rate 
of leaf growth of the date palm in relation to temperature some very 
significant points were brought to light. It was noted that when 
the maximum air temperatures were well above 50° F., not only was 
g rowth continued after a minimum temperature of 32° was reached, 
u£ at temperatures considerably lower; air minima of 25°, 24°, and 
even of 21° not wholly checking growth on fairly mature trees, 
though acting more severely on young seedlings. The inference 
arose that the growing center of a date palm bud must be in some 
way protected against the actual minimum of cold registered by a 
thermometer in air near it, unless the cold period is considerably 
prolonged. 
Thermograph records show that minimum temperatures in date¬ 
growing regions of California and Arizona last but a few minutes. 
Shreve (22) concludes: 
A consideration of the factors which have to do with the distribution and 
activities of the giant cactus (Carnegiea gigantea, Cereus giganteus) led me to 
believe that the greatest number of consecutive hours of freezing is the most 
important climatic datum in determining its northward range, * * *. 
While with unicellular plants, as yeast, for example, variations 
of temperature to which the multiplying cells are exposed may be 
determined by a thermometer immersed in the culture medium, with 
most plants which develop tissue systems there is no opportunity to 
insert a thermometer in the growth centers without such destruction 
