464 
Journal of Agricultural Research 
Vol. XXXI, No. 5 
The rays of the mercury vapor lights appear to have checked the 
leaf elongation as perfectly as did bright sunlight under Indio condi¬ 
tions, a result not attained with much more intense illumination by 
incandescent bulbs. 
The next step is to compare the spectrum of the incandescent lights 
and the spectrum of the mercury vapor lights with the solar spectrum. 
Rays of the solar spectrum, which are lacking in the spectrum of the 
incandescent lights but present in the spectrum of the mercury vapor 
lights, must be the important rays in the physiological activity of 
t he date palm, and presumably in that of the other palms which make 
their leaf elongation chiefly at night. 
The subject of the effect of light rays of various colors on the 
growth of plants has occupied a great deal of attention for more 
than ha If a century, and many writers have contributed to its litera¬ 
ture. Growing plants in light passed through glass plates of differ¬ 
ent colors or through colored solutions has shared the field with tests 
of growth under different portions of the prismatic spectrum. Elec¬ 
tric-light culture has also received its full share of attention. The 
whole subject of the influence of light rays from different regions of 
the spectrum upon the physiological action of plants is too complex 
for more than brief consideration. 
The writer is not aware, however, of any comparison of growth 
reaction having been made between light from ordinary incandescent 
bulbs and light from special illuminants, which either afford a close 
approximation to pure white light or exclude certain portions of the 
spectrum, as is done by the Cooper-Hewitt types of lamps. 
The Smithsonian Tables 361 and 366 give the following wave 
lengths, in microns, for the standard colors (2): 
Violet_0.44 
Blue_0.46-0.48 
Green_ 0.60-0.52-0.54 
Yellow_0.56-0.58 
Orange_ 0. 60-0. 62-0. 64 
Red_... 0.66-0.68-0.70 
The same volume, Table 368, gives the visible spectrum as ranging 
from 0.644 /x to 0.405 /x and the ultra-violet from 0.384 /x to 0.280 /x. 
R. D. Mailey, of the engineering department of the Cooper-Hewitt 
Electric Co., Hoboken, N. J., wrote as follows, in a letter dated 
October 9, 1918: “The lamps which you are using are made of lead 
glass, and you will have to keep this in mind, remembering that lead 
glass does not pass appreciable amounts of light below wave lengths 
of 3,800 ” Gx/xh 
According to the Smithsonian Table 368 (2) the ultra-violet rays 
begin with wave lengths of 0.384 n, which would leave practically no 
ultra-violet rays in the radiation from these tubes. 
Mr. Mailey did not state the limit of wave length of these tubes 
in the direction of the red, but fortunately >R. A. Steinberge, of 
the Bureau of Plant Industry, United States Department of Agri¬ 
culture, had made a spectroscopic analysis of the rays from the 
identical tubes from which the results under discussion were ob¬ 
tained. These notes he kindly placed at my disposal. Five lines 
were observed with a hand spectroscope; a bright line in the violet 
at about 0.405, and one in the blue, a faint line in the blue-green, 
a strong line in the green, and one in the yellow at about 0.578 /x. 
Nothing was visible in the higher wave lengths. 
