420 
Journal of Agricultural Research 
Vol. XXXI, No. 6 
check in the daily growth rate from 14 mm. to 4 mm., but with a 
quick rising when a temperature mean of 60° was gained, showing 
the tendency to a perennial growth when the temperature permits. 
In other words, though growth in the date palm is sustained 
throughout the year under Indio and also Tempe temperature con¬ 
ditions, growth is proportionally more rapid at the higher than at 
the lower temperatures. It will be noticed also that fluctuations of 
10° to 20° in the mean temperatures are rather closely followed by 
corresponding fluctuations in the growth rate, though there are some 
startling exceptions to this rule in the late autumn growth of both 
years. 
The conditions under which leaf elongation in the date palm is 
brought to zero are as striking as the fact of its practically all-year 
growth. While with ordinary plants growth is so nearly confined 
to “the average frostless season” that “climatic zones” are usually 
based on such seasonal lengths {12) the date palm may show incre¬ 
ment in leaf growth when minimum temperatures fall below the 
freezing point tor several days in succession. For example, for three 
consecutive days at Indio in December, 1916, with minima from 24° 
to 21° F., growth was not wholly checked when the maxima of those 
days were above 60°. On the other hand, it was observed twice at 
Indio in the winter of 1917 that with the minima above the frost 
line, but with the day’s maxima falling below 50°, growth was 
brought to zero {17). In general it may be affirmed that the growth 
rate in the date palm is more closely coordinated with the tempera¬ 
ture throughout the year than that of any other plant species yet 
studied. 
Repeated field observations made by the writer, supported by 
extended records of growth under controlled temperatures, have 
demonstrated that the minimum temperature permitting growth of 
the date palm is close to 49° or 50° F., provided this temperature 
reaches the growth center {17). This zero point of temperature for 
growth probably varies slightly with different varieties and possibly 
with the individual tree, growing under different conditions. The 
zero temperature for growth is rarely reached under the climatic 
conditions prevailing in date-producing regions, and then only fQr 
very short periods. 
It is evident that the mode of growth of the date palm is in striking 
contrast to that of ordinary exogenous fruit-bearing trees, such as 
the apple, peach, and fig. 
Gourley {8) found that the total twig growth in a New Hampshire 
apple orchard under observation in 1916 was practically made during 
35 days, from about May 25 to June 30, with a slight gain for 27 
days longer. During this time the total daily growth on the ob¬ 
served twigs, computed in sixteenths of an inch, declined from 120 
to 10, while the mean daily temperature advanced, measured by a 
smoothed curve, from about 55° to 75°; yet increase or decrease 
in daily temperature was followed more or less closely by correspond¬ 
ing changes in growth rate (fig. 3). 
The year’s twig growth of the apple orchard, chiefly made in a 
twelfth of the year’s time, was in part the expression of the food- 
building capacity of the last year’s leaf crop, and bore only an indirect 
relation to current temperatures. Increment on a single tree would 
have been distributed among a network of roots, hundreds of twigs 
