the conclusion from graphs of their development 

 that doses of 1000 r were most conducive toward 

 an increase in yield. But by the time the seeds 

 of the irradiated plants ripen, doses of 250, 

 500, and 750 r outstrip the 1000 r dose, which 

 by then is somewhat depressing; it is with this 

 dose that signs of decline begin to show. How- 

 ever, even this dose cannot be excluded from 

 determination of the effect of X rays on other 

 plants, for there are times, depending on the 

 problem we want to solve, when it becomes 

 important to obtain a greater initial develop- 

 ment. 



The advantages of X irradiation are not lim- 

 ited to an increase in yield of the irradiated 

 plants; the advantages can be transmitted to the 

 offspring of the plants. These data were ob- 

 tained by us in the fall of 1932 as a result of 

 planting seeds which were obtained from plants 

 irradiated in 1931. For sowing material we 

 selected seeds of the plants which gave the 

 greatest yields. For controls we used offspring 

 of the control plants of 1932, from which we 

 similarly selected the best plants, hence the 

 best examples of the control plants of 1932. 

 During the growing season phenol ogical observa- 

 tions of the irradiated and control plants were 

 conducted, and no appreciable difference in the 

 length of the various phases of development was 

 observed. The ripened plants were subjected 

 to the same detailed analysis as in 1932, i. e. , 

 the number of ears and the number and weight 

 of grains were determined. The data thus ob- 

 tained are compared in Table 7. 



An examination of this table indicates an 

 obvious secondary action which shows up in 

 increased yields with certain doses of X rays. 

 Although we see that the 1933 yield is less than 

 that of 1932, nevertheless the increase is evi- 

 dent in the total weight of the seeds as well as 

 in their number. In order to facilitate a com- 

 parison between the effect of direct irradiation 

 and its secondary action, we have drawn up 



Table 8, where the results are expressed in 

 percentages, taking the index of the control 

 plants as 100. 



From the data of this table it is possible to 

 arrive at the very definite conclusion that a 

 dose of 750 r not only increases the weight of 

 seeds by 166% as a result of direct X radiation, 

 but that it retains an increase of nearly 30% in 

 the next year's yield, as a secondary action. 

 Doses of 250 r, which produce such sharp in- 

 creases (21%) over the controls during the year 

 of irradiation, here give strongly negative 

 indications. We shall return to this question 

 later (see Chapter 3). Doses of 1000 and 2000 r 

 reveal in their secondary action a gradual de- 

 crease in yield which takes place more slowly 

 than under the direct effect of X rays. The 

 secondary action is distinguished by the fact 

 that high doses (4000 and 8000 r) which depress 

 the development of the plant in the first year 

 (as a result of direct irradiation) give a positive 

 effect eventually. A probable explanation of 

 this phenomenon is that those plants upon which 

 the rays had the most depressing effect died. 



Peas ( investigations of Breslavets and 

 Atabekova). 2 This plant was selected as an 

 object for investigation chiefly because it is 

 well known in the literature for its sensitivity 

 to X rays, and, in addition, is useful for genetic 

 research because it is self-pollinating. 



Taking into account the sensitivity of peas 

 to X rays, we used lower doses than for rye, 

 i. e. , we began with 50 r. Pea seeds irradiated 

 under air-dry conditions were planted in vege- 

 tating pots on the day after irradiation. When 

 the irradiated and control plants formed pods, 

 they were photographed. 350 r should be 



This work, started jointly, was transferred the 

 following year to Atabekova alone. 



Table 7 



Average data per plant 

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



21 



