THE PHOTOSYNTHETIC CURVE 27I 



infinitesimal traces of HN0 3 showed that one part of this 

 substance dissolved in two billion parts of water induced 

 an increase in the rate of C0 2 -assimilation of nearly 200 

 per cent. Minute traces of extract of thyroid gland, of 

 iodine, and of formaldehyde, showed similar enhancement 

 of photosynthetic activity. 



The effect of a minute quantity of formaldehyde in 

 enhancing activity is of special significance in regard to the 

 possible formation of formaldehyde as the first product of 

 photosynthesis. This substance is toxic only as a strong 

 dose ; before there could be any great accumulation of this 

 substance in the cells it would have become polymerised 

 into carbohydrate. 



The Photosynthetic Curve under Increasing 

 Intensity of Light 



Photosynthesis is feeble under low intensity of light ; 

 the activity then increases at a uniform rate, the curve being 

 straight with a uniform slope. The turning-point occurs at 

 or about 1200 lux, after which the curve tends to become 

 horizontal. Under very strong intensity of light the curve 

 exhibits a reversal, notably in summer. 



In the median range of the curve, i.e. from about 200 to 

 1200 lux, the increase of photosynthetic activity is propor- 

 tional to the intensity of light. The coefficient is found by 

 dividing the increment activity by increment of light. In 

 spring the average coefficient of Hydrilla for light is 25. In 

 winter the coefficient is about half that in spring. The 

 minimum intensity for photosynthesis in spring is about 

 100 lux, whereas in winter it is about 500 lux or more. The 

 activity for intensity of light L is found from the following 



formula : 



A L =Aj + K(L-/) 



The limiting maximum activity is found to be relative 

 and not absolute. It increases with the intensity of light, 



