554 RHYTHMS IN PLANTS AND ANIMALS 



also dry weight production is much less. That we are here dealing 

 with the Biinning phenomenon is clearly indicated by the leaf angle, 

 which is small in the scotophil and large in the photophil phase. When 

 this angle is small at the beginning of the light exposure, light is little 

 effective (Went, 1957). 



The basic 24-hr cycle in growth of the tomato plant can be 

 demonstrated in a different way. When groups of plants are exposed 

 to 22-, 24-, 27-, and 30-hr cycles of equal periods of hght and dark- 

 ness at 20°C, these plants do not grow at the same rate. The 24-hr 

 cycle of 12 hr light and 12 hr darkness produces the heaviest and 

 fastest growing plants. In the 22- and 27-hr cycles, in which in the long 

 run the plants are exposed to exactly the same total amount of light, 

 they grew less, and in the 30-hr cycle, growth was much less. There- 

 fore, light exposure at 20°C is most effective if presented with a 24-hr 

 period. At 14°, optimal growth occurred on a 27-hr cycle and less 

 on 24-, 30-, and 33-hr cycles, indicating that at a lower temperature 

 the endogenous cycle is slightly longer. 



It seems to me that this type of experiment is very significant in 

 connection with the question of the existence of innate cycles or 

 rhythms. For we do not depend here on a demonstration that under 

 presumably constant external conditions the organism continues to 

 respond rhythmically. In such experiments the question remains open 

 whether we are perhaps still confronted with an unsuspected peri- 

 odicity in the environment. But here we determine the degree to 

 which the tomato plant adjusts itself to different rhythms, and the 

 conclusion seems warranted that maximal growth and dry matter 

 production indicates the greatest degree of adjustment of the internal 

 Biinning cycle with the externally applied light-dark periodicity. 



The necessity of light-dark cycles in the environment for normal 

 growth of tomato plants is most clearly demonstrated by their ab- 

 normal growth under continuous light. This phenomenon, first studied 

 by Arthur and Harvill (1937) and the Withrows (1949), is now 

 under investigation in the Earhart Plant Research Laboratory by both 

 Hillman (1956) and Kristoffersen (unpublished). When young to- 

 mato plants are grown in continuous light, in a constant temperature, 

 the newly developing leaves become light green, mottled, and espe- 

 cially at higher temperatures such new leaves may be completely 



