iySi 



FIGURE 23. Contraction of chromosomes 



in root cells of irradiated peas 



(from the work of Atabekova-') . 



in root diameter, since the size of the cells 

 remained constant. Injurious doses of X rays 

 cause the appearance of chromosomal fragmen- 

 tation, lagging at anaphase, and asymmetrical 

 binuclearity. Slowing down of the division rate 

 of cells and nuclei is closely related in peas as 

 well as in rye with a depression of development 

 and a decline in yield. From this we are enti- 

 tled to conclude that the investigation of pea 

 roots irradiated by various dosages completely 

 confirms our hypothesis concerning the relation- 

 ship between intracellular changes and yield. 



Wheat (investigation of Afanas'eva) . The 

 first object selected for the investigation was a 

 pure strain of spring wheat Triticum vulgare 

 var. Caesium. Air -dried seeds of this plant 

 were irradiated with various doses of X rays 

 (125, 250, 500, 750, 1000, 2000, 4000, 8000, 

 and 16,000 r). Exactly 24 hours after irradia- 

 tion the seeds were planted in silica sand and 

 the sprouts kept under observation. Not a single 

 dose had a lethal effect on the wheat seeds. 

 Neither were any particular differences ob- 

 served between the various dosages and the 

 control in the number of shoots that came up or 

 in their sizes. The only exception was the 

 16, 000 r dose which inhibited the development 

 of the seedlings. Before the investigation of the 

 roots of irradiated plants was undertaken, the 

 stages of resting and dividing nuclei were care- 

 fully studied. No deviations from the normal 

 behavior of the chromosomes and the nucleus 

 were observed. After that the roots of wheat 

 irradiated by each of the X ray doses were 

 examined. With the effect of 125 r, division 

 proceeds normally. With 250 r a root was 

 found, a whole sector of which consisted of 

 tetraploid cells. No changes were discovered 



when doses of 500 and 750 r were used. With 

 1000 r, the chief deviation from the control was 

 the appearance of various inclusions in the cyto- 

 plasm: they were small, in the form of chains, 

 dots, or small knots, sometimes they were 

 larger like globs of chromatin (Fig. 24). With 

 2000 r these inclusions were observed in con- 

 siderably larger numbers. They are probably 

 nuclear in origin. In addition, with this dose 

 one begins to find micronuclei (Fig. 25). The 

 same inclusions, but in larger amounts, are 

 encountered with 4000 r. In addition, fusion of 

 the ends of chromosomes in the center of the 

 [nuclear] plate is observed. In the majority of 

 cases, however, the plates were normal; only 

 the chromosomes were small and thin. Follow- 

 ing a dose of 8000 r the number of cells with 

 chromosomal inclusions is still greater, nuclei 

 begin to bud, giving rise to micronuclei; the 

 number of abnormal nuclear plates also in- 

 creases. All of these are encountered still 

 more frequently with 16, 000 r. This dose, 

 moreover, is characterized by the formation 

 of two or more protoplasts in a single cell. 



The second object selected for investigation 

 was hard wheat, Triticum durum var. mela - 

 nopus. Dry seeds of this plant were subjected 

 to the same doses of X rays as the seeds of soft 

 wheat. The first cursory cytological examina- 

 tion revealed that the number of nuclear divi- 

 sions in irradiated plants diminishes signifi- 

 cantly in comparison with the controls. This 

 was eventually confirmed by quantitative data, 

 as shown in Table 30, where M is the ratio of 

 the number of nuclei in division to the total 

 number of nuclei, expressed as a percentage of 

 the total number of nuclei. Thus, the average 

 percentage of nuclear divisions is an expression 

 of the rate of division. D is the difference and 

 [m and] ma, the probable error of the difference. 



From Table 30 we can see that the highest 

 percentage of divisions is in the control. Even 

 with a dose of 125 r the rate of division de- 

 clines, while with doses of 8000 and 16, 000 r it 

 falls to one half. This decline in rate is statis- 

 tically significant, since the difference exceeds 

 the probable error by considerably more than 

 three times. 



The counts that were then made for the 

 various stages of division provided the figures 

 for Table 31. 



The table makes it clear that the high rate of 

 nuclear division in the control is due to the 

 presence of a large number of prophases. This 

 predominance indicates that at the moment of 

 fixation an extra large number of cells has 

 begun mitosis. In irradiated plants, beginning 

 with 125 r, the percentage of prophases de- 

 creases in comparison with the control and is 

 almost equal to the percentage of all the other 

 stages. Following doses of 8000 and 16, 000 r 

 the percentage of metaphases, anaphases, and 



84 



