14 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 97 



In the experiments reported in the present paper, a method for 

 mixing artificial lights was used, but one quite different from that 

 used by the above-mentioned investigators. A beginning was made 

 by using two light sources, one rich in red, the other rich in blue 

 light. By locating the plants on small rotating tables at different 

 distances from these light sources, practically any intensity ratio of 

 the two could be obtained. With this general scheme the number of 

 lights could be increased, thus making it possible to study the effects 

 of any given mixture of restricted wave-length regions on the growth 

 of plants. With each added light, however, the interpretation of 

 data becomes more difficult. By using this method it is difficult to 

 grow many duplicate individuals at one time, especially if they grow 

 large. This objection may be met in part by repeating an experiment 

 often enough to obtain more reliable statistical date. 



The first two experiments with the Mazda light vs. the Mazda light 

 filtered through a CUSO4 solution were mostly exploratory in nature. 

 There is some indication that the greatest dry weight produced is 

 associated with wave-length distribution and not entirely correlated 

 with intensity of radiation. Although the data of experiment 3 are 

 meager, a considerable difference in growth was obtained between 

 plants receiving different amounts of red (wave lengths longer than 

 6040 A) and blue (wave lengths shorter than 6040 A). The dry 

 weight increase for the plants receiving red-blue light in the ratio 

 72 : 28 was about 40 percent over those receiving white light and 

 those receiving a mixture in the proportion 96 : 4, although the total 

 intensities of these two cultures were over 40 percent greater. 



An attempt was made to change the type of red and blue light by 

 the use of neon and mercury grids. In these experiments (nos. 4 and 

 5) it was found that the intensity of radiation was too low for good 

 growth. This made it impossible to draw any definite conclusion 

 regarding the proportion of red to blue that gave best growth. Yellow- 

 ing or lack of greenness was associated with those light mixtures 

 predominant in red. 



In order to obtain lights of higher intensities, one rich in red, the 

 other rich in blue, the water- jacketed projection Mazda lamp used in 

 experiments i and 2 and the 400-watt high-pressure mercury lamp 

 used in experiment 5 were employed. With this combination of lights 

 very good growth was obtained under 100 percent artificial conditions. 

 Because of this good growth and the increased number of plants per 

 treatment, more weight can be attached to the data from experiments 

 6, 7, and 8, than to the earlier ones. Where light and not carbon 



