(rye), or the cells have a mosaic structure con- 

 sisting of diploid and tetraploid tissues (peas). 

 In order to establish the distribution of polyploid 

 cells, Atabekova* carefully studied all of the 

 successive sections of pea roots in which she 

 found polyploid cells. These cells were concen- 

 trated in a relatively small sector, the other 

 parts of the root contained diploid cells. The 

 most interesting example encountered was a 

 polyploid root obtained after the seedling was 

 irradiated by a dose of 450 r. An examination 

 of the meristem turned up diploid cells (2n = 14), 

 tetraploid cells (4n = 28), hexaploid cells (6n = 

 42), and octoploid cells (8n = 56). A thorough 

 study of 60 successive sections enabled us to 

 determine the order of these cells in the root. 

 They are distributed in vertical rows and they 

 retain this distribution as far as they can be 

 traced along the whole fixed tip of the root (Fig. 

 26). This examination shows how chimeral 

 tissues arise from diploid and polyploid cells. 

 Unfortunately, we did not succeed in obtaining 

 a polyploid pea plant by means of X rays. 



Things were even worse as far as wheat was 

 concerned. Afanas'eva* succeeded in finding 

 tetraploid cells in only one plant. 



With rye things were different; by means of 

 individual examination of roots of separate 

 plants not only was it possible to find two tetra- 

 ploid plants which arose due to the influence of 

 X rays but also to trace their descendants down 

 to the second generation. Twelve seeds were 

 obtained from one of these plants. These pro- 

 duced only four plants by the following spring. 

 Three of them grew up to form ears, and all 

 three ears were distinguished by hard, compact 

 scales which caused them to resemble the ears 

 of einkorn wheat. The pollen grains of these 

 plants as well as their elements, the two sperm 

 and the vegetative nucleus, were distinguished 

 by their extraordinarily large size. From this 

 tetraploid only four seeds were obtained, which, 

 as examination revealed, were also tetraploid. 

 We therefore had the opportunity of examining 

 three generations of tetraploid rye and tracing 



FIGURE 26. Distribution of polyploid eel 

 in a cross section of a pea root 

 (from the work of Atabekova"") . 



their morphological characteristics. The ques- 

 tions that relate to the obtaining of new forms, 

 which arise in connection with this plant, will be 

 left for another section of our book. However, 

 we may note certain interesting cytological de- 

 tails, primarily changes in the size of cells and 

 nuclei in three generations. (Table 32). 



It is interesting to note that the average root 

 diameter, which in general is enlarged in tetra- 

 ploids as compared with normal diploids, re- 

 mains the same in X and X^ , i. e. , in the gene- 

 ration that arose due to the direct influence of 

 X rays and in its offpsring, whereas a large 

 increase in diameter is observed in Xg. When 

 the diameter of cells and nuclei are compared 

 in the three generations, we see that while the 

 nuclei achieve their maximum size in the sec- 

 ond generation (Xj) the cells continue to grow. 



Table 32 



Changes in root diameters, cells, and nuclei of 

 three generations of tetraploid rye 

 (based on Breslavets' data, 1939) 



87 



