CHAPTER XV 



RELATION BETWEEN C0 2 -SUPPLY AND PHOTOSYNTHESIS 



Depression of general physiological activity under C0 2 — Narcotic and 

 poisonous action of excess of C0 2 on photosynthesis — Photosynthetic 

 curve for C0 2 -variation — The turning-point — Determination of the 

 photosynthetic coefficient — Effect of seasonal variation on the 

 coefficient. 



We shall study in this chapter the quantitative relation 

 between the C0 2 -content of the solution and the resulting 

 photosynthetic activity. It is obvious that there should 

 be a lower and an upper limit : no photosynthesis can, in 

 normal conditions, take place in the absence of C0 2 ; excess 

 of C0 2 , on the other hand, produces a narcotic or even a 

 poisonous effect. We have already seen that excess of C0 2 

 causes an arrest of activity in all physiological processes, 

 partly through deprivation of the oxygen so necessary in 

 all vital activities, and partly through the narcotic action 

 of the gas. Since the decomposition of C0 2 produces a 

 certain amount of oxygen, it would take a relatively larger 

 proportion of C0 2 to cause an arrest of photosynthesis. 

 An excess of C0 2 , though not immediately injurious, may 

 yet prove to be so after long-continued action. For 

 example, with a proportion of 10 mg. of C0 2 in ioo c.c. 

 there is most vigorous photosynthesis ; this activity is, 

 however, found to undergo a slow decline even to perma- 

 nent arrest. The specimen thus treated does not show 

 any revival of activity even after replacement in a weaker 

 solution of C0 2 . On the other hand, I may mention in- 

 stances where the specimen was kept in a solution of 7 mg. 

 of C0 2 per 100 c.c, yet the activity was found to remain 

 uniform for at least five days, though the cut specimen 

 was all the time confined within the plant-vessel with the 



