168 



INFLUENCE OF TEMPERATURE ON BIOLOGICAL SYSTEMS 



present Mr. T. Kristofferson is investigating this process in greater detail 

 and, like Hillman, he has found that by fluctuating the light, the injury 

 symptoms will disappear. At the same time, injury can be prevented by 

 keeping the plants in continuous light, but subjecting them to a 24-hour 

 cycle of temperature fluctuation. This in itself is most interesting, because 

 here we find an equivalence of temperature and light in preventing the 

 injury just described. When peas are grown in constant conditions no in- 

 jury symptoms appear, but as Highkin has found (5), in the course of 



I2l 



— I— 



10 



—r- 



15 



— I — 

 20 



Daus 



Fig. 6. Growth rate as a function of age of Zelka pea plants, when these are grown 

 in continuous light diliotoperiod, pp = 24 hr.) or in hght-dark alternation (pp = 16 

 hr.), either in a constant temperature of 17° or 23°C or in alternating temperature 

 (16hr. at 10°, 8 hr. at 23°). 



several generations the ability of these plants to grow is seriously im- 

 paired. When pea plants are grown for only one generation it will be found 

 that plants in which either light or temperature is fluctuating with a 24- 

 hour cycle continue to grow vigorously, whereas the growth of plants kept 

 in continuous light and constant temperature decreases in the course of 

 four weeks. 



This phenomenon is shown in figure 6. Whereas in the early stages of 

 growth the peas grew slowly, especially at the low temperatures, soon the 

 rate increased. But in constant temperature and light, the growth rate 

 reached a maxinunn rate, after which it decreased more or less rapidly. At 

 fluctuating temperatures this drop in rate did not occur or came much later. 



