Sept. 1, 1925 
417 
CEREAL 
INVESV!GA v i 
* <r» 
i.y. 
Tlermostasy o f Growth of the Date Palm 
perature under which growth would be resumed in the spring. Brown 
(4) V • 65) also assumes a dormant period for this tree, and says: 
“The vegetative growth of the trees commences toward the end of 
April or the beginning of May * * 
It has been left to later observers, however, to prove that the 
date palm in favorable habitats has no resting period, but with water 
supplied to its roots, continues to push up leaves from the growth 
center throughout the year. Several complete years of observation 
show that only minimum day temperatures below 50° F., or des¬ 
tructive night temperatures of 21° or 22° will bring this upward 
growth to a temporary standstill (17). 
THE AVAILABLE DATA 
Two series of observations on the growth rate of date palm leaves 
through two full years have been kept in the United States. The 
first, a condensed report of which was published by Vinson (2If), 
was made at the Cooperative Date Garden at Tempe, Ariz., includ¬ 
ing the years 1906 and 1907, under the supervision of R. H. Forbes, 
Director of the Arizona Experiment Station. These growth measure¬ 
ments were made weekly. The second was a series of daily growth 
measurements, kept in part by the writer and completed under his 
direction, begun in October, 1916, and continued through the full 
years of 1917 and 1918 at the United States Experiment Date Gar¬ 
den at Indio, Calif. An analysis of these records is not in the prov¬ 
ince of this paper, but there is a striking agreement in the results 
obtained, considering the remoteness of the two stations, the dif¬ 
ference in the ages of the trees studied, and in the soil and water 
conditions. 
Both records agree as to two important facts in the activity of the 
date palm: (1) Except for very brief intervals under exceptional 
conditions, growth is continuous throughout the year; (2) the growth 
curve, which took no account of volume but was a record of the up¬ 
ward growth of the more active leaves of each tree, follows a course 
generally parallel to the curve of weekly mean temperatures; the 
growth is very slow when daily means are below 50° F., and the 
most rapid growth follows the highest means of 90° to 100°. 
In Figures 1 and 2 the mean weekly growth and mean weekly 
temperatures for 1917 and 1918 are plotted on the basis of zero of 
growth at a mean of 40° (unity at 42°) and a gain of 1 mm. in mean 
growth for each gain of 2° in mean temperature. 
Plotted on this scale the curve of growth rate falls below the heat 
curve until about the first or the middle of March; holds increasingly 
above it from April until midsummer when the peak of the growth 
rate is reached, and continues above it through a mild autumn to 
the end of the year. 
A decline from 65.5° to 53.5° in the latter part of November, 1917 
failed to bring the growth curve down to meet the temperature curve. 
As for going into a dormant state, as was assumed by DeCandolle, 
the average growth rate was still 10 mm. daily and with the un¬ 
seasonable rise of the temperature means to 64° at the end of De¬ 
cember, the mean growth rate for the first week of January, 1918 
(fig. 2), advanced to 14 mm. daily. The decline of the January, 
1918, temperature means from 61.5° to 49°, however, caused a 
