APPENDIX 8 



KEPOET ON THE DIVISION OF KADIATION AND 

 ORGANISMS 



Sir: I have the honor to submit the following report on the 

 activities of the Division of Eadiation and Organisms during the 

 year ended June 30, 1937 : 



Notable successes have been attained during the year in the studies 

 of photosynthesis, of phototropism, and of special reactions of ultra- 

 violet rays in the economy of various plant forms. 



W. H. Hoover published the results of several years' study of 

 photosynthesis in wheat. This basic study was made with wheat 

 grown in glass tubes of measured temperature, humidity, and carbon 

 dioxide content, under nearly monochromatic selected spectral rays 

 of measured intensity.^ Various radiation sources were employed, 

 sometimes the Mazda electric light, sometimes the mercury arc, some- 

 times the sun. The results are of high accuracy. They give to a 

 probable error of only 2 percent in most spectral regions the de- 

 pendence on wave length of the assimilation of carbon dioxide by 

 wheat. The accompanying figure shows that photosynthesis in 

 wheat, starting from zero at the end of the visible red, reaches a 

 high maximum in the red at 6500 A, diminishes through the yellow 

 and green, reaches a subordinate maximum in the blue at 4400 A, 

 and then fades away in the violet. 



Mr. Hoover's work was accomplished by a chemical method of 

 estimating the air content of carbon dioxide. During the year Dr. 

 McAlister has further perfected a spectral absorption method of 

 extraordinary sensitiveness and extreme rapidity for measuring car- 

 bon dioxide concentration in air. The apparatus has been standard- 

 ized by him and has become a tool which bids fair to be of immense 

 value for the detection and measurement, not only of carbon dioxide, 

 but carbon monoxide, and other organic chemical compounds of ex- 

 treme interest in plant physiology, human metabolism, mine explora- 

 tions, and perhaps in other industrial fields. In connection with this 

 apparatus, L. B. Clark has developed an extremely sensitive and 

 rugged thermocouple, the evacuated housing of which is sealed by a 

 bubble window of microscopically thin glass. These beautiful devices 

 together add greatly to the practical success of the spectral absorption 



1 Smithsonian Misc. Coll., vol. 95, no. 21, pp. 1-13, figs. 1-4, pis. 1-3, 1937. 



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