SECRETARY'S REPORT 95 



nism. A similarity of response between exposure to X-rays and 

 gamma rays has been established, as well as a similarity of re- 

 sponse by monocotyledonous and dicotyledonous leaf tissue. A 

 maximal effect on chlorophyll synthesis lag phase is obtained with 

 dosages of 14 kiloroentgens. Experiments in which fresh sucrose 

 solution is supplied to the tissues subsequent to the X-irradiation indi- 

 cated that X-rays do not affect chlorophyll synthesis by means of their 

 action on carbohydrate substrates. Kesults of exposure to red or 

 blue broad band radiation indicate that red is effective to a greater 

 degree than blue. These data imply that there is not a reversal of 

 ionizing radiation effects, but rather a photomorphogenic response 

 superimposed on the damaging etTects of X-irradiation. 



Action spectra of the light-induced and light-inhibited germina- 

 tion of seeds of Arabidopsis thaliana have been completed. Except 

 for a slight shift of about 10 m/x toward longer wavelengths of the 

 reversal action spectrum, the action spectra correspond to those de- 

 termined previously for the bean hypoctyl hook. The action spec- 

 trum for germination has a broad maximum at 660 m/* with decreased 

 sensitivity extending through 400 m/x. On the long wavelength side, 

 there is a sharp drop to the isobestic point at 690 m/i. The action 

 spectrum of reversal has a sharp maximum at 720 m/x and a sccondai-y 

 peak at 740 m/x, with little or no activity beyond 780 m/x. 



The effects of polarized light on the phototropic and light-growth 

 responses of Phycomyces sporangiophores immersed in paraffin oil 

 have been investigated in order to determine whether the photo- 

 receptors in Phycomyces are oriented or nonoriented. The effects 

 obtained in paraffin oil are not definitive and the data could fit either 

 possibility. In order to resolve this question, measurements must be 

 made in an unmersion fluid which has an index of refraction between 

 1.6 and 1.7. Numerous such media have been tested, but all have 

 proved toxic to the sporangiophores. 



Three new members were added to the research staff of the Divi- 

 sion: Dr. Lars Loercher, plant physiologist; Dr. Richard L. Lat- 

 terell, cy togeneticist ; and John L. Edwards, plant physiologist. 

 Dr. Loercher and Mr. Edwards came to the Division from the 

 Agricultural and Mechanical College of Texas, where they were 

 working on photomorphogenic responses as affected by photomimetic 

 substances. Dr. Latterell comes from the Brookliaven National 

 Laboratory, where he was engaged in genetic studies. 



The installation of the radioisotope laboratory and counting room 

 was completed, and these are in current use. The greenhouse is in 

 operation, and the control rooms are nearing completion and should 

 be ready for use in the research program by the fall of 1960. 



