556 
Journal of Agricultural Research 
Vol. XXXI, No. 6 
to different light periods. For these tests Cosmos sul'phureus Cav., 
a typical short-day plant, has been used. When exposed to a day¬ 
light period of 12 hours or less, this species flowers readily, while it 
tends to remain in the vegetative stage for an indefinite period when 
exposed to a daylight period in excess of 13 hours (pi. 1 and pi. 2, B). 
Flowering normally begins at the apex and rather rapidly extends 
downward along the axis, numerous axillary flowering branches being 
developed. 
The first experiment was made to ascertain the results when the 
lower portion of the axis is exposed to a short daily light period while 
the upper portion is exposed to a long one. The test was begun in 
the greenhouse in winter. Seedlings were grown with a daily illumi¬ 
nation period of 16 hours till 3J^ months old, electric light being used 
to prolong the light period after sunset. From the beginning of the 
experiment on January 7, the control plants were exposed to only 
the natural daylight, which was of about 10 hours’ duration, and they 
began flowering February 9. Around the upper portion of the axis 
of the test plant a box with open front and top was so arranged that 
it could be raised as the plant grew taller. A 100-watt electric light 
placed within the box and 1 foot above the apex of the plant was 
turned on daily from sunset till midnight, thus giving about 16 hours 
of illumination daily. The front of the box about the upper part of 
the plant was kept covered with black cloth while the electric light 
was on so as to prevent the light reaching the lower part of the test 
plant and the controls. The upper portion of the plant inclosed in 
the box showed the usual response to prolonged daily illumination. It 
remained in the vegetative state and continued to increase in stature. 
The lower portion of the stem, exposed only to the short daylight 
period of winter, soon developed flowering branches at the nodes, 
the first blossom opening March 5. The appearance of the test plant 
and a control on March 9 is shown in Plate 2, A. Both the upper and 
lower portions of the axis have shown the characteristic responses 
to their respective light periods and the normal position of the flower¬ 
ing portion of the stem has been reversed. 
The treatment of the test plant was continued till May, when the 
days had become sufficiently long so that the use of the electric light 
could be discontinued. The plant was then transplanted outdoors. 
Although the entire above-ground part of the plant was exposed to 
the long days of summer, the lower portion continued to flower 
throughout the summer, while the upper portion continued its vege¬ 
tative growth, as shown in Plate 2, B. By the middle of August the 
plant had reached a height of 11 feet. By October 1, in response to 
the decrease in length of day, the entire upper portion of the plant 
passed into the flowering condition, flower buds developing rapidly 
on all branches. 
In this experiment and those subsequently described, as well as 
in the one mentioned in the opening paragraph, it would seem that 
any possibility of effects due to differences in water supply, plant 
nutrients, or other soil factors has been eliminated. It will be ob¬ 
served that in the earlier test the two branches growing from the 
same node, but differently illuminated, obtained at all times their 
supply of the soil solution through the same mother stem. In the 
present case the soil solution reaching the upper portion of the 
