THE PERIODIC ASPECT 



557 



meciiim), and the main requirement is that cell division had been 

 inhibited by some means prior to the light or temperature change. 

 Such inhibition can then be overcome by the change in the environ- 

 ment. And a periodic change in the environment might then result in a 



50-1 



UJ 40- 



cn 



o 



I- 



30- 



o 



UJ 



O 



UJ 



a: 



20- 



10- 



adescantia 

 stem apex 



■ ••. ^Perilla 



\ '"C stem apex 



Root tip Vicia--^ 



' I ' 



' I ' 



i6:oo 



T T 



T 



T 



-I — r 



T — I r 



T — I — r 



8:00 



24:00 

 TIME OF DAY 



8:oo 



Fig. 2. Frequency of mitoses (ordinate) for different tissues as a func- 

 tion of time of day (abscissa) for a number of different plants. (Data 

 from Biinning, 1952.) 



periodic sequence in mitosis. We can even imagine that the synchro- 

 nization in development between all cells in the growing point, which 

 is necessary to insure the proper sequence of node formation, is 

 actually produced by this cycle in the environment. This is all the 

 more attractive as a working hypothesis, since thus far no concrete 

 evidence has been found for the existence or activity of hormones 

 which control these processes within the growing point of the higher 

 plants. 



That in the photocontrol of vegetative growth we are dealing with 



