364 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1936 



continuously operating device measured the change of carbon-dioxide 

 concentration in the slow air stream which bathed the plants. The 

 effects observed depended on the color of light employed. Three 

 series of experiments were made. In one, Mazda lamps with Chris- 

 tiansen filters were the light sources. In a second series, the discon- 

 tinuous line spectrum of the mercury arc combined with glass filters 

 gave monochromatic sources. In the third, sunlight itself, passing 

 through a large-sized Christiansen filter some 60 feet from the plant, 



3500 4500 5500 6500 7500 



WAVE LENGTH. 

 FiODRB 5. — Wave-length effect of light on carbon dioxide assimilation of wheat plants. 

 Aj, the corrected form of the curve obtained with the large Christiansen filters ; B,, the 

 corrected form of the curve obtained with the small Christiansen filters. Points 

 marked X, the results obtained with the line filters and quartz mercury arc. 



furnished floodlights of nearly monochromatic rays. The results, 

 illustrated graphically in figure 5, were in close accord from all three 

 series. 



Thus it is seen that red rays are most promotive, blue rays second, 

 green and yellow rays useful, and the infrared and the ultraviolet 

 contribute nothing to the assimilation of carbon dioxide in wheat. 

 Experiments with other plants are proposed. 



The response of English ivy to light intensity has already been 

 mentioned. In this response the leaves arrange themselves in a 

 mosaic pattern with a maximum of leaf surface exposed to light. 



