Reciprocal labelling in double-labelling experiments with H3- and C14- thymidine showed that when H3- thymidine (1μc/ml for 0.5 hr) was given first in the labelling sequence followed by a 4 hr interval before the C14- thymidine treatment, that passage of cells into and out of DNA synthesis was normal. When C14 was first in the sequence, the rate at which cells entered DNA synthesis was decreased. This was attributed to a radiation effect produced by the β-rays from the C14. The rate at which cells entered DNA synthesis was studied after 0.1, 1 and 10 rads of x-rays. A dose of 1 and 10 rads decidedly depressed the rate. Treatment of roots with 2, 20, and 200 μc/ml of tritiated water for 0.5 hr showed that the 2 higher concentrations produced an effect similar to the x-rays. This indicated that somewhere between 1 and 8 disintegrations per cell per 0.5 hr will produce a decrease in the rate at which cells enter DNA synthesis. In both the x-ray and H32O experiments the depression of the rate that cells entered DNA synthesis seemed to reach saturation at the higher doses.

An accurate determination of the parameters of the resonance excited by the interaction of the 132 eV neutrons with the Co 59 target nucleus has been made, using the fast choppers at Brookhaven National Laboratory in the United States and Chalk River Laboratory in Canada. Neutron transmission through thick and thin samples resulted in the following parameters: [parameters not transcribed]. In addition, measurements of the resonance capture γ-ray intensity gave the value for the radiation width Γγ= 0.40 ± 0.04 eV, based on the known thermal capture cross section of cobalt of 37.5 barns. The reduced and total resonance capture integrals are calculated from the above parameters to be 50.5 ± 5.5 and 67.0 ± 5.5 barns, respectively. The above results are compared with previously determined resonance parameters and also with direct measurements of the total resonance capture integral.