CHAPTER XVII 



EFFECTS OF RADIUM RAYS ON NUCLEI AND NUCLEAR 



DIVISION * 



Some of the reactions of cells to radium rays have already been 

 indicated.! Koernicke* was the first to investigate the influence of 

 the rays on nuclear division, and his results, noted on page 65, need 

 not be restated here. 



My own experiments were made with root-tips of Zca Mays (aerial 

 roots) and Allium cepa. Negative results, so far as mitosis is con- 

 cerned, were obtained with the corn, as the nuclei, in the control as 

 well as in the exposed plants, proved to be not dividing at the time 

 the material was collected and fixed. The effect of the rays on the 

 relative size of the nuclei in the corn was observed, and has been 

 described on page 229. 



Root-tips of Allium cepa, grown from bulbs in a moist chamber 

 in the dark, were exposed by placing the sealed glass tubes of radium 

 bromide close to, but not touching them. Thus, as previously ex- 

 plained, only the beta and gamma rays were effective, the alpha rays 

 being screened out by the walls of the tube. Roots for control were 

 grown under precisely similar conditions, except for the absence of 

 radium, and were collected and fixed at the same time and in the 

 same manner as those irradiated. 



The material studied was exposed, in part, as follows : 



A. For 7 hours 20 minutes to RaBrg of 10,000 x . Collected 

 at 7 : 20 P. M. 



B. For 52 hours 30 minutes to RaBrj of 10,000 x . Collected 

 at 3 : 20 P. M. 



C. For 8 hours to RaBr, of 10,000 x . Collected at 4 P. M. 

 Other conditions of exposure are mentioned later. 



Our incomplete knowledge of the periodicity of cell-division in 

 the onion leaves some doubt as to the best hour for collecting material 

 in order to secure the largest nuhiber of divisions. The subject was 



*The substance of most of this Chapter was given before the Botanical Society of 

 America at the Chicago meeting, December, 1907. 

 tPp. 181, 187, and 229. 



230 



