Sept. 1,1925 
InJiibitive Effect of Sunlight on Growth of Date Palm 
467 
fully ceased elongation at sunrise the grain plants showed steadily 
increased openings of the stomates; and where the date palms 
resumed growth under darkness supplied at midday, the grain plants 
closed their stomates. 
In the absence of observations on this point it is reasonably safe 
to assume that the stomates of the palms react toward light m the 
same manner as those of the grasses. 
While the original habitat of the date palm is only a matter of 
conjecture, in the cultivated state it is at home in the intense light 
and heat of the desert. Yet the intensity and quality of light under 
which t thrives, and especially under which it makes its best develop¬ 
ment of fruit, have heretofore received little attention. But evi¬ 
dence is not lacking that light conditions are second only in import¬ 
ance to temperature conditions for the growth of this tree. Walter 
T. Swingle (10, p. 58) anticipated this in 1904 when he wrote the 
paragraph on “ Sunshine necessary for the date palm.” 
In the writer’s study of the date palm in the northern Provinces of 
Sudan, a region of intense sunlight and dryness of the air, it was found 
that the native growers appreciated the necessity of the full exposure 
of the date-palm crown to the sun’s rays. They have a system of 
allowing several offshoots or “daughters” to grow up to full maturity 
around the “mother” tree, thus giving a much larger number of 
stems to the acre than could be tolerated in America. Yet if one of 
these becomes overshadowed by a stronger growth the fact is at once 
recognized that its vigor and fruit production are seriously impaired. 
SUMMARY 
Date palm leaves are formed from the top of the phyllophore or 
growth center deep in the interior of the terminal bud, protected from 
fight and from wide ranges of temperature. 
The leaves, in groups of from three to five, called a palus are 
pushed forward by basipetal or intercalary increment, no alteration 
in their length occurring after they reach the light and assume 
their green color. 
Normal growth, as manifested by the pushing up of the leaves 
from the growth center, is made chiefly in the time between sunset 
and sunrise, but also at a reduced rate in daylight, when direct sun¬ 
light is cut off by clouds. In full sunlight date palm leaf elongation 
entirely ceases. 
Date-palm leaf growth may be induced in darkness obtained by 
inclosing the plant in a dark chamber, at any hour of the day. Par¬ 
tial growth may be induced in a similar manner by screening the plant 
from direct s unli ght, but giving it exposure to north or reflected light. 
Growth begun in darkness was continued at practically the normal 
rate when the plant was exposed to a battery of Mazda electric lights 
giving brilliant illumination. 
When date plants after beginning growth in a dark room were 
exposed to the rays of Cooper-Hewitt lead-glass mercury vapor 
tubes, the inhibiting of growth was as prompt as in direct sunlight. 
A spectroscopic analysis of the rays irom Cooper-Hewitt lead-glass 
tubes shows that this light is confined to rays from the shorter wave 
lengths of the visible spectrum through violet, blue, green, and 
yellow to about the line D (0.578/z). The orange and red are com¬ 
pletely absent. 
