EFFECTS ON NUCLEI AND NUCLEAR DIVISION 23 1 



first investigated by Lewis, ^ who found tliat, in normal light, the 

 greatest percentage of dividing nuclei occurred at midnight, while in 

 roots grown in darkness the lowest percentage was at midnight and 

 the highest at 4 P. M. 



Kellicott^ reported the occurrence of two maxima and two minima 

 in the rate of cell-division during 24 hours. A " primary maximum " 

 was detected shortly before midnight (11 P. M), and a "primary 

 minimum " about 7 A. M. " Secondary maxima " occurred at about 

 I P. M., and "secondary minima " at about 3 P. M. No corre- 

 spondence was observed between the rate of nuclear division and 

 slight variations in temperature. On the basis of this fact the slightly 

 higher temperature in the vicinity of the radium-tube has been dis- 

 regarded as a factor in the following experiments. 



In Kellicott's experiments the roots were grown in moist sand or 

 pine sawdust, while in those of Lewis they were grown in moist air. 

 Since the latter method was adopted in the radium experiments, it is 

 probable that the roots exposed as in B and C above, if not in those 

 of ^, were collected at suitable hours for favorable results. At any 

 rate division figures in all phases were very numerous in roots ex- 

 posed in all three ways. 



It has not seemed necessary to give here normal division figures 

 for comparison, as this process in the onion departs little from the 

 typical karyokinesis of the higher plants, and its individual peculiari- 

 ties are well known through the work of Schaffner,^* Nemec,'' Mer- 

 riman,^ and Gregoire.^ Miss Merriman's observations indicate that 

 the number of chromosomes in Allium cepa, commonly reported 

 as 16, is not constant, and may vary from 10 to 30 or more. 

 In one instance she figures as many as 38 in one nucleus. In the 

 material used for the radium experiments the number, as shown in 

 the figures (plates 5 and 6), was clearly more than 16, and as it 

 appeared to vary in the normal, unexposed roots, any attempt to 

 detect a variation in number as a result of exposure to radium rays 

 was impractical. 



All the tissues exposed to the rays blackened more rapidly than 

 those unexposed when placed in the Flemming solution for killing 

 and fixing. 



* Subsequent cell-studies, as is well known, have not confirmed the occurrence of 

 centrosomes in Allium cepa (or in any other higher plant) as reported by Schaffner. 

 It is hardly necessary to add that no traces of such bodies were found in my material. 



