170 



INFLUENCE OF TEMPERATURE ON BIOLOGICAL SYSTEMS 



When either African violets or begonias are grown on a 24-hoiir light cycle 

 (for instance, 8 hours light and 16 hours darkness) then in the course of 

 several weeks or months they will die at 10°C, although at 23° they develop 

 normally and flower abundantly. When, at 10°, they are grown in a 32- 

 hour cycle of 10.7 hours of light and 21.3 hours of darkness, they will de- 

 velop well, although slowly, and form strong dark-green plants (fig. 7). 

 Their failure to grow at 10° in a 24-hour cycle of light-darkness is ob- 

 viously due to a lack of synchronization of their internal autonomous 

 Biinning-cycle, which at that low temperature is significantly longer than 

 24 hours, and the external 24-hour cycle. This explains also why their 



wmm m\m 



m m 



Fig. 7. Saintpaulia plants, all grown in a constant temperature of 1()°C. Upper left : 

 plant kept for 5 months on a 24-hr. light-dark cycle; lower right, plant kept on 32-hr. 

 cycle; upper right, first 2 months on 32-hr., last 3 months on 24-hr. cycle; lower lejt, 

 first on 24-hr., then on 32-hr. cycle. 



death at 10° is so slow: no toxic materials are formed, but further develop- 

 ment of the growing points has become impossible, and the previously 

 nitiated leaves remain small and chlorotic. 



It is likely that many other tropical plants which cannot grow at the 

 lower temperatures of temperate climates, even though the temperature 

 remains above the freezing point at all times, die when their Biinning cycle 

 is sufficiently slowed down by low temperatures so that they get out of 

 phase with the 24-hour day. Similarly, the actual dying of cool climate 

 plants at higher temperatures (like Baeria chrijsostoma which dies at 

 26°C nycto-temperature) might be attributable to a speed-up of their 

 24-hour cycle at the higher temperatures. 



How can we imagine that thermoperiodicity influences development es- 

 pecially if the periodic fluctuations in temperature of a 24-hour period can 



