BIOCHEMICAL MECHANISM OF CELL DIVISION DISTURBANCE 227 



Table II. Mitotic index in the point of growth on the removal of locally 

 irradiated leaves at different periods of time after irradiation with a dose 



of 10 hr {from 3.000 rr//.s-) 



normally grown leaves (where the cell division is inhibited) decreased 

 the mitotic index in the meristem tissue of roots. But this effect was 

 far more pronounced when extracts from irradiated leaves were used. 

 It confirmed our suggestion that substances inhibiting mitosis are 

 normally present in a fully grown plant leaf but that their formation 

 is sharply accelerated by irradiation. 



We investigated the radiosensitivity of this activation process, the 



Table III. Mitotic index in the roots of Vicia faba fed on extracts from 

 bean leaves for 24 hr {average from 5 experiments). 



experiment was repeated at various dose rates, from 10 r to 25,000 r. 

 The inhibiting effect on mitosis of such extracts is shown in Fig. 3. 



We can see that 100 r irradiation already causes a certain increase of 

 antimitotic substances in the exposed leaf. The amount continues to 

 increase up to 500 r but after that our method failed to reveal a further 

 increase until a dose of 25,000 r. The appearance of antimitotics in 

 fully grown leaves is not only characteristic for Vicia faba. Experiments 



