F. W. WENT 169 



Another phenomenon was associated with growth under constant condi- 

 tions. The variabiHty of the peas (both Zelka and Unica varieties) was 

 much greater, indicating that part of the regulatory mechanisms control- 

 ling growth are not operative at constant temperature or in constant light. 

 It was found that fluctuations in temperature and light are equivalent in 

 their effect on reducing variability, and that a 7° or 13°C temperature 

 or an 8- or 16-hour photoperiod light fluctuation are all about equally ef- 

 fective (see table 1). Therefore it seems that the significant aspect is not 

 the intensity of the fluctuating factors, but the fluctuation itself. 



In addition to the requirement of fluctuating temperature, expressed in 

 the term 'thermo-periodicity' which is apparently important in a large 

 number of plants, there is another requirement of this periodicity, namely, 

 it must have a 24-hour period. When the cycle is either shortened or 

 lengthened, the fluctuation in light is unable to prevent damage due to 

 continuous illumination. Obviously we are dealing here with the 24-hour 



Table 1. Coefficients of variability of unica pea plants grown for 3 



* In parentheses the number of groups from which the coefficients of variability 

 were averaged. 



autonomous cycle which Biinning has shown to be present in so many 

 plant processes and which he considers to be the basis of photoperiodism. 

 In the tomato a number of processes were found which have to have a 

 24-hour cycle to make the plants develop normally. For instance, when 

 such tomato plants, instead of receiving 8 hours light and 16 darkness in 

 each 24-hour period, are given 4 hours light and 8 hours darkness over 

 every 12-hour period, then growth of these plants is considerably reduced, 

 even though they receive exactly the same total amount of light. It will 

 be seen that in such plants at the beginning of each light period the leaves 

 are in so-called night position and therefore photosynthesis during the 

 4 hours would have to occur largely in the scotophil-phase of Biinning. 



Biinning in earlier work (2) had found that this autonomous rhythm 

 of the nyctinastic movements of bean plants had a slight temperature-de- 

 pendence although more recently one of his students (8) could not con- 

 firm this temperature-dependence. On the basis of a slight increase in 

 cycle length with a decrease in temperature it is possible to explain the 

 behavior of a number of tropical plants under cold growing temperatures. 



