878 



EXTERNAL AND INTERNAL FACTORS 



CHAP. 26 



changing the ratio of these two intensities, he obtained the family of curves 

 represented in figure 26.8 — some showing a steady increase in photosynthe- 

 sis with time, others exhibiting an equally steady decline. He interpreted 

 these time phenomena in terms of three plasmatic effects: "activation," 



Fig. 26.8. Changes of photosynthesis 

 with time in aquatic plants under constant 

 conditions. Numbers indicate increasing 

 ratios between the conditioning and the 

 illuminating intensity (after Harder 1930). 



"deactivation" and "exhaustion" (the last two being distinguished by the 

 duration of the dark period required for recovery) . 



Experiments by Gessner (1937) and Steemann-Nielsen (1942) indicated, 

 however, that only the phenomenon of Induction (Harder's "activation") 

 is of fundamental nature (it will be dealt with in chapter 33), whereas 

 "deactivation" and "exhaustion" can be avoided — at least as far as water 

 plants in light of about 40,000 lux are concerned — ^by preventing the stag- 

 nation of water, and the consequent dwindling of the carbon dioxide supply. 



o.... ^^ s 



L 



3 

 TIME, 



hr. 



Fig. 26.9. Constant photosynthesis of a light-adapted plant (L) and a 

 shade-adapted plant (S) of Elodea canadensis (after Gessner 1937). 



That stagnant water in the immediate neighborhood of assimilating plants 

 can easily be depleted of carbon dioxide (even if it contains reserves in the 

 form of carbonates and bicarbonates) was first i)roved by the calculations 

 of Romell (1927). Figure 26.9, taken from Gessner, shows the time course 



