98 



LIGHT, VEGETATION AND CHLOROPHYLL 



At the ends of the spectrum, especially towards the violet, 

 the fall of power per unit wave-length in the solar spectrum 

 combines with the decline of efficacy to reduce the partici- 

 pation of these radiations in photosynthesis. 



I— 

 I 



\ Solar radiation on the 



ground 



\ 

 I 



? 



I 

 I 

 I 

 I 





^ .'-^v 



^ 



Photosynthetic efficacy 



0*3 4 



0-5 06 



Visible 



0'8^ 



Fig. 1, 14. Curves showing the relative efficacy of radiations 



for photosyntheses as a function of wave-length, according 



to Hoover, 1937. Dotted curve: composition of the solar 



radiation on the ground 



Duration of the Illumination and Interruptions 



When a green plant is brought out of the darkness into the 

 fight, photosynthesis begins immediately with absorption of 

 carbon dioxide and liberation of oxygen. If the illumination 

 is kept constant from the beginning of the experiment, the 

 speed of photosynthesis does not remain constant but may 

 steadily increase or decrease. By a process of adaptation, it 

 increases if the experimental ifiumination is lower than that 



