sept. i, 1925 Inhibitive Effect of Sunlight on Growth of Date Palm 
461 
one-tenth inch vertical spaces. Graph I of Figure 3 shows above 
the base line the beginning of growth at 7.20 p. m. of May 21, with 
a rather even gain until 8 a. m. May 22, followed by the horizontal 
line of no growth to 15 minutes after the dark cell was closed. Active 
growth was recorded from this time till 5 p. m., five minutes after 
the dark cell was removed and the tree exposed to full afternoon 
light, when growth ceased. 
From this the horizontal line till 7.25 p. m. indicates that the normal 
daylight dormant condition as to elongation was resumed in a very 
few minutes after the plant was exposed to the normal sunlight and 
continued until the usual night elongation commenced, which was 13 
minutes before sunset. In Graph II this night growth is transcribed 
from 7.25 p. m. May 22 till 8.20 a. m. May 23, from which point a 
horizontal line of no growth continues till 10.15 a. m., 35 minutes 
after the dark cell was closed at 9.40 a. m. Apparently when the 
sunlight was shut out the growth did not start quite so promptly as on 
the previous day. Here the lack in sensitiveness in the recording 
mechanism is to be regretted, as a precise reaction time can be only 
Fio. 3.—Effects of artificial darkness and incandescent light on date-palm growth at the Government Date 
Garden, Indio, Calif., May, 1918 
approximated. On this day the dark cell was opened at 1.37 p. m. by 
removing the panel on the north side. With the top and three sides 
remaining in position the tree was still deprived of direct sunlight, re¬ 
ceiving only the reflected north light, much of which was absorbed by 
the black interior walls, giving about the effect of a dull cloudy day. 
Instead of the growth being wholly checked, as when the dark cell was 
entirely removed and the tree exposed to full s unlig ht on the previous 
day, growth continued as shown by the graph at a less rapid rate than 
in complete darkness. Therefore exposure to full direct sunlight 
must be assumed as one of the conditions for growth cessation. 
It has been suggested that in securing the resumption of growth 
within the dark ceil at midday other factors than the complete dark¬ 
ness may have been involved, as a marked rise in temperature and an 
increase in the relative humidity. The reply would be that growth 
action was recorded before there was time for any very marked 
change in either temperature or humidity. Moreover the continu¬ 
ation of growth with the north side of the dark sell removed was made 
under conditions where but little gain in temperature could have 
resulted, and no increase in humidity, leaving the continuance of 
G rowth under partial lighting to be ascribed wholly to the absence of 
irect sunlight. 
