DNA Synthesis 



141 



These observations are interpreted to mean that DNA is 

 not synthesized during cell division but only during the inter- 

 phase, and that there is a lag between the end of synthesis and 

 the beginning of visible prophase of about 6 hours. 



The relationship between DNA synthesis and the later 

 behaviour of the cells can be discovered by making use of 

 either of two facts. The first is that differentiating cells 

 elongate, while meristematic ones are rarely more than 50 /x 

 long. By taking squashes from the final 3 mm. of the tip of 

 roots which had been given ^^P for 20 and 40 hours, a cell 



100 



80- 



o, 60- 



X X— 



7 



/o 

 ARG 



40- 



20- 



Q-t^nm — r 

 O lO 



20 30 40 

 TIME HRS. 



60 



Fig. 2. Percentage of cells showing autoradiograph in relation 



to time of ^^p treatment. Solid line: non-dividing meristematic 



cells. Broken line: dividing cells. 



population is obtained which includes elongating as well as 

 meristematic cells. The ratio of positive to negative nuclei 

 from such a squash, in relation- to cell length in microns, 

 is shown in Fig. 5. It is clear that the proportion of positive 

 cells falls sharply with increasing cell length. The maximum 

 value is higher for the 40-hour root than for the 20-hour, and 

 some of the elongated cells show autoradiograph after 40 

 hours. 



The second fact which may be used in this connection is the 

 distribution of mitotic cells in the length of the root. The 



