J. S. KIRBY-SMITH, C. W. SHEPPARD AND DORIS L. CRAIG 



pollen irradiations are shown in Table I together with aberration frequencies 

 for X-ray and gamma ray treated samples of the same material for com- 

 parison. Data for the neutron-irradiated inflorescences, both hydrated and 



Table I. — Effect of fast neutrons, medium-energy X-ray, and ^^Co gamma rays on 



Tradescantia pollen 



desiccated, as well as for an additional independent experiment in which 

 normal buds were used, are summarized in Table II. These data show no 

 differences in aberration frequencies for the wet and dry material and thus 

 indicate that variations in the degree of hydration normally encountered 

 in the flower buds do not affect their neutron sensitivity. 



Table II. — Effect of fast neutrons on hydrated and nonhydrated Tradescantia inflorescences 



The RBE of fast neutrons with respect to cobalt gamma rays and 60-kV 

 X-rays is presented in Table III. The results of previous work^ on the 

 relative effectiveness of X-, gamma and beta rays are also included in this 

 table. Owing to the variation in biological effectiveness over the X-ray 

 region, the effectiveness of 1-MeV gamma rays in producing the biological 

 effects of chromosome breakage is assumed to be unity. This convention, 

 which differs from previous usage in which X-rays are made the reference 



263 



