FIGURE 17. 



1) Shortening and thickening of chromosomes at metaphase; 2) a lobed 

 nucleolus at prophase; 3) the formation of rings and figure 8's at somatic 

 metaphase; k) hexaploid plate; 5) the distribution of hexaploid plates in 

 roots (rye) . 



These observations, which are in agreement 

 with the observations of other authors, indicate 

 that X rays induce intracellular changes. Some 

 of these changes, which are induced by low doses 

 of X rays, are beneficial to cellular activity. 

 They increase the rate of cell division, stimulate 

 faster growth, and promote a more vigorous de- 

 velopment of the plant. Higher doses are condu- 

 cive to changes that are injurious to cellular 

 activity in that they slow down the process of 

 division, cause it toproceed improperly, and may 

 even lead to complete disintegration of the chro- 

 mosomes. This, of course, is reflected in the 

 yield, which diminishes as the dose increases. 



The increase in size of the diameters of roots, 

 depending on the dose of X rays used, has already 

 been pointed out. This increase can be caused 

 either by the number of cells or by their size. 

 In order to answer this question we made cell 

 counts and measured their sizes in a certain 

 layer of the root. Previous experience had shown 

 us that with rye it was necessary to use the sub- 

 epidermal layer of cells. For comparison we 

 took average data from cell counts for 10 roots 



(10 roots for each of the doses and for the con- 

 trol). Counts were made in each root for five 

 sections: the first, 3 to 4 sections above the 

 place where the tissues begin to differentiate into 

 the primary phloem and the axial cylinder, the 

 other four sections were taken 3 to 4 sections 

 above that, along the root. In this fashion 50 

 sections were counted for each dose and their 

 arithmetical means calculated. As can be seen 

 from the appended table, the number of cells in- 

 creases beginning with a dose of 250 r, reaches 

 its maximum at 750 r and then, at 1000 r, be- 

 gins to drop gradually. 



If a graph is constructed on the basis of 

 these data it will not differ in any respect from 

 a curve which is characteristic for physiological 

 changes induced in rye, by similar doses of X 

 rays (Fig. 18). 



When the number of cells was counted follow- 

 ing beneficial doses of X rays, the reason for 

 their increase became obvious: it was due to 

 the increase in the number of cell divisions. In 

 order to obtain precise data on this we counted 

 all the cell divisions that could be seen in a 



79 



