556 



RHYTHMS IN PLANTS AND ANIMALS 



To tie this tomato behavior up with photoperiodism as affecting 

 flowering we should point out that both operate under a 24-hr cycle 

 and that morphological development of the tomato in the absence of 

 cycles in the environment is slowed down. The growing point is slower 

 in forming nodes, and it becomes much smaller when not subjected 

 to a 24-hr cycle of either temperature or light. Therefore the cycle of 

 cell division requires also an external cyclic change to continue nor- 

 mally. 



The requirement of adequate cycles in the environment also is 

 clearly indicated in the case of peas. When these are grown in con- 

 tinuous light at a constant temperature, they start to grow very fast, 

 at least as long as the food reserves in the cotyledons last, and then 

 their growth rate drops rapidly to a very low value. This drop in 

 growth rate can be prevented either by a dark period or by a tempera- 

 ture fluctuation. The latter two conditions are apparently equivalent 

 again; by changing both at the same time there is no further change in 

 response, and therefore ultimately both must control the same growth 

 mechanism. This is also indicated by the coefficient of variability^ in 

 these peas (see Table I) . It is quite obvious that a 24-hr cycle regulates 

 growth, which it may well do through a regulating effect on mitosis. 



Biinning (1952) has found a very clear-cut 24-hr cycle in mitosis 

 in the growing points of a number of plants (see Fig. 2). We know 

 that neither light nor temperature as such causes mitosis, but cell 

 divisions can become synchronized by changes in light or temperature. 

 This has been found in a number of organisms (bacteria, Para- 



1 Mean value of a set of measurements divided by the standard deviation of 

 the mean. 



