F. W. WENT 



171 



be replaced by light fluctuations? For this we must try to find a cyclic 

 process in the development of a plant, which could be influenced by ex- 

 ternal cycles. Such processes actually have been known for some consid- 

 erable time. It has been common knowledge among cytologists that at 

 certain hours of the day (or especially of the night) the number of mitoses 

 is much greater than at other times. This has recently been re-investigated 

 by Biinning (3) who found that in stem tips most cell divisions occur dur- 

 ing a few night hours (see fig. 8) . For roots no such periodicity in mitoses 

 was found. Biinning was unable to correlate this with the photoperiodic 

 behavior of these plants {Perilla is a short-day plant, Spinacia a long-day) . 



50-1 



40- 



30- 



■Tradescantla 

 stem apex 



20- 



8:00 



I ' ' ' I ' ' ' I ' — ' ' I ' — ' ' I 

 I6:00 24:00 



Fig. 8. Number of mitoses at different times of day and night, either in roots of 

 Vicia jaba (no periodicity) or in apical meristems of Tradescantia, Spinacia or Perilla 

 From Biinning 1952. 



That is to say, the mitosis maximum occurred at the same time for both 

 long- and short-day plants. 



It is obvious that in the growing i)oint of the stem tip of a plant there 

 must be a perfect regulation of cell divisions to produce the exactly spaced 

 organs. This could conceivably be a diffusing substance or hormone. But 

 no such hormones have been found. Besides, we do not know of any radia- 

 tion which induces mitosis. But the mitoses in the stem tip are perfectly 

 synchronized with a certain night hour. Therefore the assumption is ob- 

 vious that the cells in the stem tip have a certain period over which they 

 can divide, which period becomes synchronized through a 24-hour cycle in 

 temperature, or light, or l)oth. If we assume, for instance, that 48 hours 

 after one division the cell is again able to divide, a plant kept on a 36-hour 

 cycle would have perhaps some of its cells divide after 36 hours, the others 



