THE PH0T0SYNTHET1C CURVE 273 



now induces photosynthesis in proportion to their re- 

 spective effectiveness (p. 190). 



The relation between photosynthesis and the energy of 

 the incident radiation is ascertained by the simultaneous 

 determination of the activity by the Bubbler and of the 

 intensity of radiation by the Radiometer. Photosynthesis 

 is found to be feebly initiated even under infra-red radiation. 

 In the visible spectrum the activity at first rises gradually ; 

 at B (680 /x/x) the maximum is abruptly attained, though 

 the intensity of radiation is here lower than at A. The 

 abrupt rise of activity at B is due to absorption of these 

 particular rays by the chlorophyll. Beyond B the activity 

 undergoes a decline parallel to the decline in the intensity 

 of radiation of the different rays. Though there is an 

 absorption-band in the blue-violet there was no second 

 maximum in this region : this is because the energy ab- 

 sorbed is utilised in other physiological work. At the 

 opposite ends of the spectrum the thermal and the blue- 

 violet rays are effective in inducing tropic reaction ; 

 they are, therefore, relatively ineffective in photosynthetic 

 action. The characteristic effects in different regions of the 

 spectrum are due (1) to the energy of the rays, (2) to their 

 relative absorption, and (3) to the complementary A and D 

 reactions in the induction of photosynthesis and of photo- 

 tropism. 



The Photosynthetic Curve under Increasing 

 C0 2 -Concentration 



The curve is straight up to the turning-point at C0 2 - 

 concentration of about 8 mg. per 100 c.c. ; after this the 

 curve tends to become horizontal, but gradually and never 

 abruptly. Under very strong concentration of C0 2 the curve 

 exhibits a reversal which is due to the toxic effect of the 

 carbon dioxide. In winter the average coefficient for C0 2 

 is about 40 ; in spring the value of the coefficient is nearly 

 double that in winter (p. 120). 



