THE DEPENDENCE OF CARBON DIOXIDE ASSIMILA- 

 TION IN A HIGHER PLANT ON WAVE 

 LENGTH OF RADIATION 



By W. H. hoover 



Division of Radiation and Oi-ganisms, Smithsonian Institution 



(With Three Plates) 



Growth in dry weight in autotrophic plants requires the absorption 

 and assimilation of carbon dioxide from the air, and this is the basic 

 physiochemical reaction of all life. 



The problem of the effect of different wave lengths of light on the 

 carbon dioxide assimilation by a plant has been the subject of investi- 

 gation for many years. In most of the earlier investigations broad 

 spectral regions were used, and not much attention was given to the 

 distribution of energy in the various spectral regions. Thus, very 

 little information may be obtained from these earlier investigations. 



Among the first investigators to realize the importance of deter- 

 mining the distribution of energy in the spectral regions were Kniep 

 and Minder (1909). They used sunlight during the middle of the day 

 with glass filters to isolate the red and blue regions of the spectrum 

 and a liquid filter for the green region. They came to the conclusion 

 that red and blue light of the same intensity produced about the same 

 rate of photosynthesis. Warburg and Negelein (1923), working with 

 Chlorella suspended in solution and using a quartz mercury arc with 

 line filters, reported that red light was the most efficient in producing 

 photosynthesis, and blue light the least. In their experiment they 

 assumed complete absorption of the light. 



On the basis of equal energy absorption, Burns (1933) and 

 Gabrielsen (1935), working with higher plants, both draw the con- 

 clusion that red light is the most efficient in producing photosynthesis, 

 green next, and blue light the least. They both used rather broad 

 spectral regions and obtained values for only a few regions of the 

 spectrum. It is also not clear from their data that they obtained a 

 true measure of the absorbed energy. 



The purpose of the present investigation is to determine the rate 

 of photosynthesis as a function of the wave length of light on the 

 basis of equal values of incident radiation. 



Smithsonian Miscellaneous Collections, Vol. 95, No. 21 



I 



