62 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1956 



A method for the measurement of atmospheric ozone using the absorption of 



ozone in the visible spectrum, by Oliver R. Wulf and James E. Zimmerman. 



Smithsonian Misc. Coll., vol. 123, No. 3, October 27, 1954. 

 Concerning Smithsonian pyrheliometry, by C. G. Abbot, L. B. Aldrich, and A. G. 



Froiland. Smithsonian Misc. Coll., vol. 123, No. 5, November 2, 1954. 

 Silver-disk pyrheliometry simplified, by Norman B. Foster and T. H. MacDonald. 



Month. Weather Rev., vol. 83, No. 2, February 1955. 

 Washington, D. C, precipitation of 1954 and 1955, by C. G. Abbot. Smithsonian 



Misc. Coll., vol. 128, No. 2, March 1, 1955. 

 Sixty-year weather forecasts, by C. G. Abbot Smithsonian Misc. Coll., vol. 128, 



No. 3, April 28, 1955. 

 Periodic solar variation, by C. G. Abbot. Smithsonian Misc. Coll., vol. 128, No. 4. 



June 14, 1955. 



DIVISION OF RADIATION AND ORGANISMS 



(Prepared by R. B. WITHROW, Chief of the DiviBion) 



The work of the Division of Radiation and Organisms was con- 

 cerned with three general areas of biological research during the year : 

 (1) the photocontrol of the formative and developmental processes in 

 plant growth; (2) the inhibition and potentiation of X-ray damage by 

 red and far-red visible and near infrared energy; and (3) the mecha- 

 nism of action of the plant hormone, auxin, in the control of growth. 



1. — Sunlight exercises its effect on plants chiefly through two groups 

 of photochemical or light reactions. The first is a high-energy process 

 and provides the plant's food supply, converting the energy of sunlight 

 into chemical energy through the synthesis of simple carbon com- 

 pounds. The second group, which is one of the areas of the Division's 

 research, involves the photocontrol of the formative growth processes, 

 which require relatively little energy but are just as essential to the 

 normal functioning of plant life as is carbohydrate synthesis. These 

 latter photochemical reactions control the development of stems, leaves, 

 and reproductive structures, and are most actively promoted by light 

 from the red end of the visible spectrum. This formative action of 

 the red can be blocked by irradiation with energy from the far-red im- 

 mediately adjacent to the red. 



Previously the laboratory had established that the region of maxi- 

 mum effectiveness in promoting the normal formative processes in 

 seedling development is at 660±:5 m/t, in the red, the results substanti- 

 ating those of other laboratories investigating seed germination and 

 flower-bud initiation. This year the photoreversal reaction involving 

 the blocking of the formative processes has been found to have two 

 maxima in the far-red at 710 and 730 m/x, with a minimum of 480 m/x. 

 Several rather poorly defined secondaiy maxima occur at 520 to 550 

 m/x and 650 m/i,. Since the maximum efficiency of the far-red energy 

 occurs about 1.5 hours after the end of the red irradiation period 

 rather than immediately following, the results are indicative that the 



