THE MITOTIC CYCLE 



tissue cultures. The calculations of Laughlin^* on the intermitotic 

 period in the onion root meristem, which are derived by different 

 reasoning, suggest values of this interval between 194 and 33 minutes 

 over a temperature range of io-30°C. In the bean root, Gray and 

 ScHOLES^^ find that the average intermitotic period in the most rapidly 



110 



100 



80 



60 



I SO 



2 t 6 8 10 12 n Iff 18 



Percentage of cells in mllos/s 



Figure 30 The relationship between the percentage of cells 



seen in division in a tissue and the average intermitotic period, 



plotted for three values of the duration of mitosis. 



dividing part of the meristem is i6"8 hours (i,ooo minutes). At 

 i9-20°G. the proportion of cells in division in the onion meristem is 

 about three times that in the bean. 



Mitotic rhythms 



The Crick formula assumes that the rate of entry of cells into mitosis 

 is uniform. It does not apply if there is any tendency towards syn- 

 chronous nuclear division, such as is found in the meiosis of flowering 

 plants where the post-diplotene stages in all pollen mother cells 

 proceed in step during the course of a morning. Mitotic divisions in 

 post-embryonic stages of animals and plants do not keep pace in this 

 manner, although as we have already described, Laughlin^* found 

 that the waves of entry of cells into mitosis in the onion root were 



90 



I 



