REPORT OF THE SECRETAET 93 



BADIATION BFFECT8 



The initial phase of the study of the spectral sensitivity of the oat 

 mesocotyl has now been completed. The general finding, which is 

 expected to be published shortly, is that this organ shoy>s its maxi- 

 mum light sensitivity in the red region of the spectrum and de- 

 creased response at shorter or longer wave lengths. This is espe- 

 cially interesting since it is very different from the spectral sensi- 

 tivity of the contiguous organ of the oat seedling, the coleoptile, as 

 demonstrated in growth and phototropism. The diversity of be- 

 havior raises several problems with respect to the mechanism of 

 the light effect which are now being investigated. One of these 

 concerns the nature of the photoreceptive pigment involved. It has 

 been possible to demonstrate the presence, in dark-grown oat seed- 

 lings, of a pigment which appears to have the requisite absorption 

 spectrum. Its spectral properties correspond with those recorded in 

 the literature for protochlorophyll. However, because of the incom- 

 plete and contradictory nature of the data in the literature, it seems 

 desirable to undertake an extensive investigation of the whole proto- 

 chlorophj'll problem. 



A further result of the study is that the magnitude of the light 

 effect is proportional to the logarithm of the light intensity. This 

 fact suggests the possibility that more than one photochemical re- 

 action is involved. It is hoped to pursue this problem also. 



Experiments on the stimulation effects of ultraviolet radiation 

 on the multiplication of cells of the green alga Stichococcus hacil- 

 lans Naeg. have been continued during the past year. Four sec- 

 cessive exposures of the algal cells were made to stimulative amounts 

 of each of the wave lengths 2352, 2483, and 2652 A. After each ex- 

 posure the growth rate (expressed as number of cells) increased un- 

 til at the conclusion of the fourth exposure it was 4 to 4.8 times that 

 of the control cultures. Cells irradiated with the optimum stimu- 

 lative exposure of 2967 A. increased at a rate of 1.5 to 1.6 times the 

 control in the first exposure; but after the second exposure the rate 

 of muliplication of cells was similar to that of the controls. The 

 stimulated cells diminished in length with each successive exposure. 

 They increased slightly in width after the first two exposures, then 

 decreased with the next two exposures so that after the fourth and 

 final exposure, the cells were less wide than those of the controls. 

 Numerous disintegrated cells were present in the cultures that had 

 been exposed three and four times when they were examined 2 to 3 

 months after the final exposure, whereas the cells exposed only twice 

 appeared to be a darker green and more healthy than the controls. 

 The sum of the three optimum dosages given to the algae was twice 

 that of the lethal quantity. 



