Sept. 1 ,1925 
Inhibitive Effect of Sunlight on Growth of Date Palm 
465 
W. W. Coblentz, in charge of light investigations in the United 
States Bureau of Standards, informally places the three light sources 
under consideration as follows: 
Order of intensity 
Ultra violet and violet: 
I. Sun. 
II. Cooper - Hewitt lead - glass 
mercury lamps. 
III. Incandescent lamps. 
Red and infra-red: 
I. Incandescent lamp. 
II. Sun. 
III. Cooper-Hewitt lamps. 
Summing up, on one side, the factors under which leaf growth 
occurs, and, on the other, those under which no growth occurs, the 
following result is obtained: 
Date-palm photo-activity 
Growth occurs: 
Normal growth— 
a. In total darkness, at night or in 
a closed chamber. 
b. In chamber with incandescent 
lights, giving brilliant illumi¬ 
nation. 
Partial growth— 
During cloudy days. 
d. On clear days; with plant ex¬ 
posed to indirect light. 
e. In closed cell with 1,800-watt 
illumination. 
No growth occurs: 
a. Under bright sunlight. 
b. Under Cooper - Hewitt lead- 
glass tubes. 
Analysis of light conditions 
Normal growth: 
a. In absence of all light rays. 
b. Under incandescent lamps. 
Complete spectrum but rays 
rich in red; no ultra-violet. 
Partial growth: 
c. In absence of direct sunlight; 
proportion of spectrum un¬ 
known. 
No growth: 
a. Under full solar radiation with 
intense illumination of atmos¬ 
phere low in water vapor. 
b. Under Cooper-Hewitt lead-glass 
tubes; lacking red and orange; 
containing some yellow, full 
green, blue, and violet. 
Jost (4, p* 127) concludes: 
A comparison of observations derived from all the researches which have been 
made brings out the following points: (1) Only light of wave lengths between 
770 A»A»and390 up is conducive to assimilating activity in green plants; these are 
approximately the same rays which are visible to us; (2) the assimilating effect 
of different rays is unequal, but still not in such a way that some are active whilst 
others lying beyond these are quite inactive. 
Jost’s estimate of available wave lengths, it will be noted, includes 
considerable infra-red. 
For the position of the apex of assimilation, Jost refers to “Figure 
27,” adapted from Reinke (9, pi. l,jig. 6); but in Reinke's original 
figure, while the apex is shown at about the Fraunhofer line B, efficient 
assimilation is shown down to 600 pp, which includes all of the 
orange, and considerable activity is shown down to the line D, 
which is about the upper limit oi the rays from the Cooper-Hewitt 
tubes. 
Palladin (5, p. 26 ), after discussing many intricate features of 
the problem, sums the matter up as follows: “Carbon-dioxide is 
thus seen to be decomposed most rapidly in green plants by the 
light rays between lines B and C.” 
63338—25f-5 
