Table 29 



Average diameters of nuclei in microns 

 (based on the data of Breslavets and Afanas'yeva, 1937) 



Quantitative changes in plants, just as the 

 qualitative changes presented above, indicate 

 that these changes parallel changes in yield. 

 When soaked seeds are irradiated by X-ray 

 doses up to and including 1000 r, the growth and 

 development of plants is stimulated, thus in- 

 creasing the number of ears and seeds and 

 raising the quality of the latter as far as their 

 weight is concerned. Cytological examinations 

 indicate that with these dosages first the diame- 

 ters of the roots increase and, secondly, the 

 number of cells increases due to a more rapid 

 rate of division. Beginning with 2000 r, X rays 

 have a less beneficial effect on the yield; doses 

 of 4000 and 8000 r noticeably depress the devel- 

 opment of the plants. We note a corresponding 

 decrease in the diameters of the roots and a 

 slowing down of the rate of division. 



On the basis of these observations and meas- 

 urements, we formulated in 1933 the following 

 working hypothesis: Certain doses of X rays 

 induce beneficial changes, which include the 

 appearance of binuclear and polyploid cells . 

 These beneficial intracellular changes are 

 related to increases in yield. Higher doses of 

 X rays induce injurious cellular changes: the 

 appearance of asymmetrical binucleate condi - 

 tions, destruction of the chromatin, and reduc - 

 tion of the rate of division. These changes in 

 the cells produce a decrease in the yield . 



In the following year we enlarged our experi- 

 ments by the introduction of seeds obtained from 

 irradiated plants. We have already pointed out 

 in a previous section the after effect of X radia- 

 tion, shown by an increase in yield when benefi- 

 cial doses are used. This [after effect] has a 

 similar effect on intracellular changes. We 

 used seeds of plants that had received a dose of 

 750 r in the previous year, since this dose 

 exerted the most beneficial influence on the 

 plants. We did not find any abnormalities in the 

 root cells of these plants: the resting and divid- 

 ing nuclei behaved normally. But in contrast to 

 normal cells the number of cellular divisions 

 was significantly greater. We consider that 

 this acceleration in the rate of division stimu - 

 lated the growth and development of the plants 

 and thaF these, in turn, affected the yield, which 

 increased by about 30% above normal. 



Peas (investigation of Atabekova ). Cytological 

 investigation of peas can be regarded as a check 

 on our hypothesis concerning the relationship 

 between intracellular changes and yield. It com- 

 pletely confirmed our hypothesis. These experi- 

 ments also were begun by a study of normal 

 roots of unirradiated peas, the cells of which 

 are distinguished by a rather large nucleus and 

 (as a rule) by a single nucleolus. The process 

 of division of cells and nuclei proceeds quite 

 normally. At metaphase, just as in rye, we can 

 observe 14 chromosomes which have clearly 

 defined characteristics. When the behavior of 

 resting and dividing nuclei was determined, the 

 roots of plants that had received a dose of 250 r 

 as seedlings were examined. This dose was of 

 interest to us because small experiments gave 

 increased yields of seeds. Cytological investi- 

 gation showed that the division of the nucleus, 

 almost without exception, proceeded normally. 

 In the root cells of only one plant were chromo- 

 somes observed to lag and form the character- 

 istic bridges. The most typical and constant 

 characteristic of this dose was the appearance 

 of symmetrical binuclearity. By this expres- 

 sion we mean the existence of two nuclei of 

 equal size in the cell itself. Symmetrical bi- 

 nuclearity in rye is accompanied by an increase 

 in yield. The same is true of peas. (Fig. 21). 



When dry pea seeds were exposed to the 

 action of X rays, the most interesting results 

 were obtained with doses of 1000, 8000, and 

 36, 000 r. When a dose of 1000 r was used, 

 binucleate cells were found in 3 roots and tetra- 

 ploid cells in one out of 30 roots examined. 

 These phenomena were observed in plants which 

 gave increased yields. However, even with this 

 dose occasionally injurious deviations were 

 observed: disintegration of chromosomes, 

 numerical aberrations, and lagging chromo- 

 somes at anaphase. 



When a dose of 8000 r is used, the process 

 of nuclear division is disrupted abruptly; anom- 

 alies appear in all phases of karyokinesis. The 

 number of binucleate, trinucleate, and poly- 

 nucleate cells rises sharply. The binucleate 

 cells are usually asymmetrical; occasionally 

 the diameter of one nucleus is several times 

 greater than that of the other (Fig. 22). 



82 



