466 
Journal of Agricultural Research 
Vol. XXXI, No. 5 
In the date-palm experiment leaf growth is continued under 
brilliant illumination from incandescent lamps rich in red rays, 
while it is inhibited under Cooper-Hewitt tubes which emit no red 
rays, but are rich in rays of the shorter wave lengths. 
That this is simply a rest period for this particular function, while 
other vital activities of the tree are in full progress is self-evident. 
DAYLIGHT ACTIVITY OF THE DATE PALM 
The next inquiry should be, What normal activities of the palm 
in sunlight are inhibited in darkness? According to general princi¬ 
ples of plant physiology they would be: First, photosynthesis or the 
assimilation of carbon-dioxide; second, the greater proportion of 
the day’s transpiration. From the records of leaf elongation in 
darkness it appears that in the case of palms such action is diamet¬ 
rically opposed to the daylight activities. The one begins where 
the other ceases. Whether tne leaf elongation during the darkness 
or absence of direct sunlight is the result of cell multiplication with 
cell elongation, or of cell elongation only, is a question difficult of 
determination, but probably the whole cell constructive work of 
the phillophore is performed during this period. 
Tne intense transpiring activity of date palms during heated days 
has been referred to in another paper by the writer (7) and this is 
accompanied by photosynthesis on a corresponding scale. There is 
evidently a close relation between the day’s assimilation and the 
growth of the following night; which is proved by the close correla¬ 
tion between the daily growth curves and temperature curves. But 
this general relation would not explain the almost immediate growth 
reaction in response to artificial darkness produced in the middle of 
a bright day; nor the quick checking of growth when sunlight is 
restored. Ii the governing action of the stomates on transpiration 
is conceded, then dv supposing that the stomates close in darkness 
or partial shade, the checking of transpiration must follow with 
increased turgesceuce of the cells in the meristematic region. 
In this connection there may be much significance in the recently 
published studies of Gray and Peirce (S). on the reaction of the guard 
cells and the opening or closing of the stomates of barley, oats, rye, 
and wheat on exposure to brigpit sunlight or in its absence. These 
authors found that under the conditions at Stanford University, 
Calif., the stomates began to open on bright days soon after sunrise, 
reaching the fullest expansion from about 11.30 a. m. to 2.30 p. m., 
and gradually closed as the sun declined. When only portions of 
the day were bright, the curve of stomatal opening corresponded 
to these bright portions. On wholly dark days the stomates remained 
closed. They also found that if two pots of plants in similar con¬ 
dition, both showing the stomates partially open, were taken for 
experimentation, the plant in the pot subjected to darkness (with 
other conditions unchanged) soon closed its stomates. But the 
plant left in continued light (with other conditions unchanged) soon 
expanded its stomates completely. Reversal of the exposure of 
these two pots resulted in a reversal of the reactions of the plants. 
Now, the conditions as to light and darkness were practically 
similar to those under which the Indio, Calif., date palms were 
observed. Where the date palms began growth the cereal plants 
of Gray and Peirce closed their stomates; where the date palms 
