1362 



INDUCTION PHENOMENA 



CHAP. 33 



predominating. Because of the uncertainty of interpretation of this kind 

 of measurements, we will describe them only briefly. 



Briggs (1933) kept the moss Mnium undulatum in the dark for 2 hours. After this, 

 on the average, the induction periods in hght lasted for about 30 or 50 minutes, but the 

 values scattered widely, and showed only a vague tendency to increase with increasing 

 ight intensity, decreasing temperature and extended dark incubation. 



The e.xperiments of Harder (1930) have been quoted once before in chapter 26. 

 Here, we are interested only in the part of Harder's complex time curves that can be at- 

 tributed to the long induction. Figure 33.14 shows, as an example, the gradual rise of 



14 



12 



10 



Dork 



Light 



O 8 



I- 

 < 

 cr. 

 u 

 m 6 



o 



>- 



X 



o 



< 

 cr 



12 3 

 TIME, hr. 



Fig. 33.16. Induction after 40 days dark rest (after Gessner 1937). 



the rate of oxygen evolution by four Fontinalis plants after a dark rest of 14 hours. 

 The induction periods in this case lasted from 4 to 6 hours. Later (1933) Harder found 

 that, if plants adapted to weak light ("shade plants") were brought into stronger Ught, 

 their photosynthetic efficiency declined at first, until they began to "readapt" themselves 

 to strong light (c/. lower curves in fig. 33.15). We can look upon this decline as an ex- 

 pression of "light injury" (to which shade-adapted plants are easily susceptible; cf. 

 chapter 19, pp. 529, 532 and chapter 28, p. 995), and dismiss them as having no direct 

 relation to induction, unless it is suggested that a complete theory of the changes that 

 the photosynthetic apparatus undergoes in light and darkness should include "light 

 injury" and "light adaptation," too {cf. section 4 above). In figure 26.8, we repro- 

 duced Harder's scheme of the time course of photosynthesis corresponding to different 

 ratios of conditioning and illuminating light. The uppermost curve in this figure was a 

 pure induction curve, the lowest one a pure light injury curve, while the intermediate 



