232 EFFECTS ON NUCLEI AND NUCLEAR DIVISION 



An exposure of not longer than 6 hours and 45 minutes to radium 

 bromide of 1,500,000 activity was sufficient to completely inhibit 

 nuclear division, and a marked tendency to double nucleoli was 

 shown in cells of roots thus exposed. In material exposed for 24 

 hours to radium of the same strength similar effects were noted, and, 

 in addition, the cytoplasm appeared disintegrated. 



In roots exposed for 8 hours to rays from radium bromide of 

 10,000 activity, and collected at 4 P. M., the nuclei possessed an 

 amoeba-like lobing that was not observed in the unexposed tissues 

 (plate 5, FIGURES i-io). The nuclei in roots exposed for 7 hours 

 and 20 minutes to the same preparation and collected at 7 : 20 P. M. 

 possessed this same lobate appearance and contained from two to 

 three nucleoli. In some instances the nucleoli appeared to be 

 dividing (plate 5, figures 9-12). 



Practically all of the mitotic figures, in whatever phase, appeared 

 distorted, or abnormal in some other way.* In almost ever3Mnstance 

 the chromosomes advanced at unequal rates toward the poles of the 

 spindle. Sometimes one or more chromosomes would appear to 

 have been carried beyond the pole, and would then frequently fail to 

 become incorporated in the daughter-nuclei (plate 5, figure 17 ; 

 plate 6, figures 2, 5). Again there would be a lagging behind 

 of some chromosomes near the equator of the spindle or at various 

 points between the daughter-nuclei (plate 5, figures 16-18; 

 PLATE 6, figures 5, 7-9). In some cells the chromosomes were 

 displaced to one side of the spindle (plate 5, figure 17 ; plate 

 6, figures 4, 7-10), while in others they were distributed with the 

 greatest irregularity all along the spindle fibers from pole to pole 

 (plate 5, figures 14, 15). Instances were numerous where one 

 or more abnormally elongate chromatin masses would extend entirely 

 across the spindle, connecting the two daughter-nuclei (plate 6, 

 figure 5), or would possess the appearance of having been stretched 

 and drawn out into a fiber at one end or in the middle (plate 6, 

 figure 7). All combinations of these irregularities were found in 

 individual nuclei. 



In figure 5 (plate 6), nine or ten chromatin masses (probably 

 not all individual chromosomes) have failed to take part in the organi- 

 zation of one of the daughter nuclei. Figure 2 (plate 6) illustrates 



*For an explanation of the conditions of exposure for the figures of plates 5 

 and 6, see page 230. 



