12 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 95 



for wave lengths longer than 6800 A are probably in error by more 

 than 5 percent, since they are materially affected by the assumption 

 that photosynthesis ceases at 7500 A. 



Some measurements made by Dr. McAlister, in this laboratory, on 

 the amount of light transmitted by a wheat leaf, combined with data 

 by Shull (1929) on the reflecting power of green leaves, show that 

 less photosynthesis may be expected in the green region, owing to 

 the fact that the leaves transmit and reflect a larger portion of the 

 incident radiation in this region. Although it is not possible as yet 

 to interpret the results of my experiments in terms of equal absorbed 

 energy, a correction for the reflecting power and transmission of the 

 leaf would probably raise the photosynthetic activity of absorbed green 

 radiation to be equal to or greater than that for the blue region. 

 There may probably still be a maximum in the red. 



The long wave-length limit of photosynthesis appears to be between 

 7200 A and 7500 A. Using the strong line of mercury, some slight 

 amount of photosynthesis was still found at 3650 A in the ultraviolet. 



SUMMARY 



The rate of photosynthesis on the basis of equal incident energy 

 was determined as a function of the wave length of light for a wheat 

 plant. The entire visible spectrum is effective in producing photo- 

 synthesis. The wave-length limits, although not accurately determined, 

 appear to be between 7200 A and 7500 A on the red end, and less 

 than 3650 A on the blue end of the spectrum. A principal maximum 

 occurs at 6550 A in the red, and a secondary one at 4400 A in the blue. 

 Increased reflection and transmission of radiation in the green region 

 by plant leaves diminish the effectiveness of incident green rays to 

 promote photosynthesis. 



LITERATURE CITED 



Burns, G. R. 



1933. Photosynthesis in various portions of the spectrum. Plant Physiol., 

 vol. 8, pp. 247-262. 

 Gabrielsen, E. K. 



1935- Die Kohlensaureassimilation der Laubblatter in verschiedenen Spek- 

 tralgebieten. Planta, vol. 23, pp. 474-478. 

 Hoover, W. H., Johnston, E. S., and Brackett, F. S. 



1933. Carbon dioxide assimilation in a higher plant. Smithsonian Misc. 

 Coll., vol. 87, no. 16, pp. i-Kj. 

 Kniep, H., and Minder, F. 



1 909. Uber den Einfluss verschiedenfarbigen Lichtes auf die Kohlen- 

 saureassimilation. Zeitschr. Bot., vol. i, pp. 619-650. 



