Sept. 1, 1925 
Thermostasy of Growth of the Date Palm 
443 
becomes a positive advantage under the extreme heat of a desert 
summer. The seasonal range of soil temperature at the Indio garden 
at the depth of 2 feet is a narrow one, extending in 1918 from 57° in 
January to 85.5° in August, an amplitude of only 28.5°. For com¬ 
parison, soil temperature studies by Bouyoucos at the Michigan 
Agricultural Experiment Station in 1913 (I, f, 63, 68) may be quoted, 
showing in loam soil at 18 inches deep a minimum temperature of 
31.3° in February, and a maximum of 76.9° in July, giving a yearly 
amplitude of 45.6°. The Indio date garden soil temperature at 2 feet 
declined from 58° on February 18 to 57° on February 21, then slowly 
advanced to 67° by March 31 and to 71.5° on May 4 (the time 
covered in Tables I to V). 
INFLUENCE OF THE ASCENDING SAP CURRENTS 
In the records from February 18 to 21, Table I, also from March 9 
to 15, and again from March 19 to 21, the position of the daily air 
mean below the soil temperature, and the close correspondence of the 
tree temperature with that of the soil point unquestionably to the sap 
current as a controlling temperature factor. 
Most significant is the record of the morning of February 19 
(Table I and fig. 5), when there was a sharp frost, with an air tem¬ 
perature of 26.6° F. beside the tree (standard minimum in Weather 
Bureau shelter, a few hundred yards distant, 25°). At 6.30 a. m. the 
tree record was 55.4°, which slowly declined to 52.7° between 9 a. m. 
and 10.30 a. m., then ascended in the afternoon to 55.4°. 
The thermograph trace kept near the tree shows that the air tem¬ 
perature was below the freezing point for 5 hours, from 2.15 to 6.45 
a. m., so we must conclude that the freezing temperature produced its 
greatest depressing effect on the interior tissues of the bud only- 
after a lag of from 7 to 8 hours. The lower thermometer in the trunk 
did not reach its minimum of 56.3° until 2.30 p. m., showing here, as 
do the subsequent records, that the trunk tissues are less sensitive to 
the air temperatures than those of the bud. 
With the thermometer in the bud zone showing a drop to only 
5.3° below the soil temperature of 58°, and the lower or trunk ther¬ 
mometer indicating only 2° below the soil temperature, it is evident 
that the ascending sap current must have been the controlling factor, 
protected from air temperature by the insulating outer layers. 
Although the movement of the ascending sap current due to trans¬ 
piration is reduced to its minimum at this part of the 24 hours, the 
large volume of water in the trunk and bud has a specific heat about 
five times greater than that of the solid soil particles. This offers 
to the outside cooling influence a resistance which is of the utmost 
importance to the tender tissues at this critical period. 
During this frost period of February 19 both tree thermometers 
registered temperatures safely above 50°, which temperature the 
writer has found to be the minimum point for growth of the date palm 
leaves (17). 
A sharp decline in the growth rate of the leaves of this tree for 
both February 19 and February 20 indicates that the zero point 
of this rather sensitive variety had been closely approached; yet 
the growth of 1.5 2 mm. on February 20 shows that the actual zero 
point, as applied to growing tissue, had not been reached with the 
LIBRARY 
CEREAL INVESTIGATIONS. 
