sept. 1,1925 Inhibitive Effect of Sunlight on Growth of Date Palm 463 
mercury vapor electric lights working on a 220-volt direct current 
circuit. These have*50-inch lead glass tubes of “Type P,” and are 
rated by the manufacturers as consuming a current oi 385 watts each. 
Two seedling Thoory palms in 8-inch pots, having leaves in active 
growth, were selected and prepared for growth measurement by 
placing in the pot small standards carrying a horizontal gauge piece 
close to the midrib of the leaf to be recorded. Glass-headed steel 
ribbon pins were inserted for markers in the leaf rib in contact with 
the gauge. 
A very slight degree of growth could thus be detected by the 
light being visible between the pin and the gauge, and this space 
was readily measured by the gentle insertion of one or more thick¬ 
nesses of paper. Their total thickness was measured with a microm¬ 
eter caliper gauged to 0.001 inch, but easily read to the half space. 
As in the artificial light tests at Indio, Calif., it was arranged to 
record first a period of active growth in complete darkness, then a 
period of exposure to the light, followed by a second period in dark¬ 
ness. 
In preparation for the test the plants were placed in position 
between the mercury vapor tubes and well below them so that the 
reflectors would give them the full light. The room was completely 
darkened from 4.30 to 8 p. m., October 2. Then the 3H-hours’ 
growth in darkness was measured, the pins put down to the gauge 
and the mercury lights switched on. After an exposure from 8 
p. m. until 12 midnight the growth indicated was only the thick¬ 
ness of one sheet of paper (0.002 inch) on palm No. 1, and of two 
sheets (0.005 inch) on palm No. 2, which may have been a continua¬ 
tion of the darkness growth before the check. Left again in darkness 
till 8.30 a. m. of October 3, the gauge showed that the growth must 
have been quickly resumed, as 0.0245 inch of growth was recorded 
for each plant. The exposure was repeated the night of October 3 
under the same conditions, but with a slight modification in time, 
the results confirming the first night’s record. Table II shows the 
growth in thousandths of an inch for both nights. 
Table II.— Growth of two date palms during the nights of October 2 and 8 
Oct. 2, 4.30 to 8 p. m.. 
Oct. 2, 8 to 12 m__ 
Oct. 2 to 3,12 m. to 8.30 a. m... 
Oct. 3, 4 to 8 p. m.. 
Oct. 3, 8 to 11 p. m.. 
Oct. 3 to 4 ,11 p. m. to 8.30 a. m 
Date and hours 
Growth 
made 
by tree 
No. 1 
Growth 
made 
by tree 
No. 2 
Inch . 
0.0130 
.0020 
.0245 
Inch. 
•0.0080 
*>.0050 
•.0245 
.0250 
•.0250 
.0025 *.0025 
.0340 •.0500 
• In darkness. 
' Under light. 
The interpretation of these records, showing only a minute amount 
of elongation during the lighted period, seems clearly to be that after 
the early evening growth in darkness the exposure to the mercury 
vapor rays checked the growth wholly after a lapse of a few minutes. 
This reaction period is similar to that found to occur between dark¬ 
ness and sunlight action in the Indio tests. 
