APPENDIX 9 



REPORT ON THE DIVISION OF RADIATION AND 

 ORGANISMS 



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

 tivities of the Division of Radiation and Organisms during the year 

 ended June 30, 1938 : 



The fundamental facts relating to plant growth and radiation have 

 been fruitfully pursued by the Division during the year. Mr. Hoover, 

 temporarily loaned for studies of radiation and photosynthesis, had 

 been recalled for continuous work with the Astrophysical Observa- 

 tory. To assist in these and other plant studies under Dr. Johnston's 

 immediate direction, Dr. R. L. Weintraub was engaged. He de- 

 veloped an improved method of measuring plant growth substances 

 which are concerned in the bending of plants toward the light. A 

 description of this method with data from a number of experiments 

 is now ready for publication under the title, "An assay method for 

 plant growth substances utilizing straight growth of the Avena 

 coleoptile." Dr. Weintraub is continuing the earlier work of Dr. 

 Johnston on the growth of the first internode in light of very low in- 

 tensities and in different spectral regions. It is interesting to note 

 that the plant responds to light intensities far below those which can 

 be detected by the most sensitive thermocouples. 



Messrs. Johnston, McAlister, Weintraub, Clark, and Fillmen have 

 been preparing a new automatic apparatus for measuring and record- 

 ing photosynthesis continuously. It is similar in nature to the de- 

 vice employed by Dr. McAlister, to which reference was made in last 

 year's report, but with the added feature of continuous automatic 

 photographic registration. This new apparatus was nearly ready for 

 use at the close of the fiscal year. It is to be used with a powerful 

 light source, comprising a 60-inch army searchlight, loaned by the 

 War Department, and a battery of mirrors at the focus of the beam 

 so placed that slender plants such as wheat, or others, may be il- 

 luminated strongly from all sides. Experiments of many kinds re- 

 lating to photosynthesis and the formation of chlorophyll are about 

 to be taken up with this efficient outfit. Another piece of apparatus 

 has been developed for the determination of chlorophyll. Very good 

 results can be obtained with concentrations as low as 0.1 mg chloro- 

 phyll per liter of solution. This equipment is being used in con- 



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