Wept. 1, 1925 
Minimum Temperature -for Growth of Date Palm 
409 
Summation of hours 50° F. or below of the thermograph record in Figure 8 
Hours 
Mar. 1, 4 a. m. to midnight_ 20 
Mar. 2, 12 midnight to noon_ 12 
Mar. 2, 8 p. m. to midnight_ 4' 
Mar. 3, midnight to midnight_ 24163 hours con- 
Mar. 4, midnight to midnight_ 24 tinuous. 
Mar. 5, midnight to 11 a. m_ 11 
Total_ 95 
This makes a time lapse of 103 hours, of which 95 hours were 
below 50° F. and only 8 hours, on March 2, above this temperature. 
The depressing effect of this period reduced the daily leaf growth 
from 20, 18, and 14 mm., respectively, on the 1st of March to 2, 1, 
and 1 on the 5th and (after a characteristic lag) to 1 mm. each on 
the 7th; but it is to be noted that zero of growth is not completely 
reached even with this 95-hour exposure to a temperature below 50°. 
It must be remembered that these were seedling trees of unknown 
parentage, with the possibility of a lower zero point than those ob¬ 
served at Indio, and the fact of their failure to come to a completely 
dormant stage still leaves the absolute zero point for the species yet 
to be established. With the return to higher temperatures recovery 
was prompt, and the previous growth rate was surpassed on the 11th 
and 12 th. 
These widely dissociated and independent records of date-palm 
growth at Indio and San Antonio, all indicating that the lowest tem¬ 
perature limit for growth lies somewhere at from 48° to 50°, become 
so cumulative in value that only a laboratory test of the tempera¬ 
ture at which growth will cease under prolonged exposure can add 
to their conclusiveness. 
So long as maximum temperatures of 48° to 50° F. are accompanied 
on the same days by minimum temperatures from 5° to 18° lower, 
as has been the case on the crucial days at both Indio and San 
Antonio, it is not safe to affirm positively that the point of cessation 
of growth in the date leaves has not been in some measure influenced 
by the lower temperatures. 
The fact that under field conditions growth has been made with 
similar or lower minima but with higher maxima makes the pre¬ 
sumption a strong one that the zero point of growth is reached at a 
definite maximum temperature of the day, which becomes the 
minimum for cell activity. When laboratory conditions permit of 
exposing a plant to a very narrow range of temperature, the point 
at which growth ceases after a considerable exposure may be accepted 
without further question as the physiological minimum or zero point 
for growth of that plant. 
Reference should here be made to the article by A. E. Vinson, 4 
who as early as 1914 made use of the 50° temperature point in 
studying date-palm growth, but did not, as is shown below, assume 
this as the actual zero point. As the Tempe Cooperative Date 
Garden growth measurements Were made only weekly, they could 
not be coordinated with the air temperatures closely enough to secure 
a real zero point determination. 
4 Vinson, A. E. the effect of climatic conditions on the rate of growth of date palms. 
Gaz. 57: 324-327, illus. 1914. 
63338—25f-2 
Bot. 
