Sept. 1, 1925 
Thermostasy of Growth of the Date Palm 
451 
where the palm may survive if planted. There are limits to the lay¬ 
ing down of the flower spathes, which would be governed by the tem- 
E erature maintained in the phyllophore during the season of greatest 
eat. This would probably involve also a certain optimum intensity 
of the sun’s rays, governed both by the angle of incidence and by the 
amount of watery vapor intervening, for the relation between photo¬ 
synthesis and growth in the date palm is to a great degree a daily and 
direct one instead of the seasonal one that it is largely in exogenous 
trees. But throughout, the temperature of the sap current, under 
the insulation of protective layers of fiber, will be found as regula¬ 
tory as a thermostat. 
DUAL TEMPERATURE REQUIREMENTS OF THE DATE PALM 
Thus the date palm has two very distinct temperature require¬ 
ments. The first is effective insulation and protection of the tissues, 
while in the embryonic stage, against extremes of heat and cold. 
The second is high temperatures from both the surrounding air and 
the radiant heat of intense sunshine for the promotion of photosyn¬ 
thesis and the building up of a fruit crop with a high sugar content. 
SUMMARY 
The date palm is the most resistant to extrenjes of heat and cold 
of any member of the palm family. As a species it has survived 
without permanent injury extremes of temperature from 4° to 125° F. 
This wide range of adaptability is believed to be largely due to the 
morphological structure and physiological action outlined in the 
following paragraphs: 
The date palm, along with other members of the palm family, has its 
active growing tissue centralized in a giant terminal bud or phyllo¬ 
phore from which are in progress simultaneously the basipital or 
intercalary growth of leaves and flower spathes and the elongation of 
the trunk and its thickening to the established diameter. 
Within favorable temperature limits, and with sufficient water 
supply, the growth of the date palm is continuous throughout the 
year, the curve of mean daily leaf elongation being closely parallel to 
the curve of mean daily temperature. Growth may be continued 
when the minimum air temperature of the day is several degrees 
below the freezing point, provided the maximum temperature of the 
day is well above tne growth zero point, 50° F. 
Thermometers inserted in date palm growth centers show a stabil¬ 
izing of temperatures with a marked gradient between them and 
those of the surrounding air, but with a striking correspondence with 
the temperatures of the soil strata penetrated by the palm roots. 
The daily range of these interior temperatures is slight and rarely 
exceed 7 or 8°, but with an inverted curve, relative to the air tempera¬ 
ture curve, highest at about sunrise, lowest at from 2 to 4 p. m. 
The difference of the interior temperature from the surrounding 
air has ranged from 26° warmer on the coldest morning observed, to 
32° cooler on the hottest day. 
This protective stabilizing of the temperatures of the meristematic 
tissues of the date palm is believed to be due to two principal factors: 
(1) A 'protective envelope surrounding the growth center , of a highly 
nonconducting and insulating nature, composed of the thick, fibrous. 
