72 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 56 



ing cells are due chiefly to the breaking of the chromosomes and inter- 

 ference with normal cell division. Young rapidly dividing cells are 

 most susceptible to X-ray damage and evidence three types of aberra- 

 tions — chromatid break, isochromatid break, and chromatid exchange. 

 In the past year, Dr. C. C. Moh and Dr. R. B. Withrow continued the 

 study of the effect of infrared, far-red, and red radiant energy on the 

 modification of chromosomal damage induced by X-rays, using root 

 tips of the horse bean, Vicia faba, as the chief experimental material. 



It has been found that infrared, from 820 to 1350 m^u., causes no 

 significant increase in X-ray damage to the chromosomes. However, 

 far-red at 710 to 820 m/t did significantly increase the frequency of 

 chromosomal aberrations induced by X-rays. The three types of chro- 

 mosomal aberrations were not effected equally, however. Chromatid 

 exchanges increased 100 percent; chromatid breaks, 34 percent; iso- 

 chromatid breaks were increased very little, if at all. These results 

 were substantiated with the pollen of the Tradescantia flower where 

 the chromosomes of the microspores showed similar results. 



When wavelengths from 680 to 820 m^u. (involving red and far-red 

 radiant energy) were used, the ability of far-red to increase the X-ray 

 damage was not secured. Thus, apparently, when red and far-red 

 are combined in proper proportions, the two regions nullify each other. 

 This could explain the negative results obtained by several workers 

 who irradiated the biological material with filters that did not sharply 

 absorb all the red. 



Red radiant energy (wavelengths from 630 to 680 m/x), when used 

 alone prior to X-irradiation, increased the yield of chromatid ex- 

 changes significantly, but chromatid and isochromatid breaks were con- 

 sistently decreased by 10 to 20 percent. This action of red radiant 

 energy on X-ray-induced chromosomal breaks is not yet clear. It is 

 suggested that the red radiant energy might accelerate the rejoining 

 process after the breakage occurs. 



PUBLICATIONS 



During the current year the following publication concerned with 

 the work of the Division of Astrophysical Research was issued by the 

 Smithsonian Institution : 



Leading operations of the Smithsonian Astrophysical Observatory, 1895 to 1955, 

 by C. G. Abbot. Smithsonian Misc. Coll., vol. 131, No. 1, Sept. 22, 1955. (Publ. 

 4222.) 



The following publications by Dr. F. L. Whipple appeared in var- 

 ious other scientific journals : 



On meteors. Proc. Astron. Soc. Pacific, vol. 67, pp. 367-386, 1955. 

 Photographic a-Capricornid meteors (with F. W. Wright and L. G. Jacchia). 

 Astron. Journ., vol. 61, p. 61, 1956. 



