46 CHAP. VI. RELATION BETWEEN LIGHT AND PHOTOSYNTHESIS 



logically equivalent to about 1750 lux, 0-04 S on a bright 

 day in March was both physically and physiologically the 

 same as 1700 lux. The intensity of sunlight on a bright day 

 in March is therefore three times as great as that on a 

 dull day in January. 



I now go on to describe experiments on the important 

 problem of the relation between the quantity of incident light 

 and the activity of photosynthesis as determined by the 

 total volume of oxygen evolved. In order to solve this 

 problem we have to find the effect of (1) the duration, 

 (2) the intensity, and (3) the directive angle, of the incident 

 light ; by this last is meant the angle between the surface 

 of the organ and the incident ray. 



Effect of Duration, Intensity and Directive 

 Angle of Light on Photosynthesis 



It has been shown that — 



(1) The amount of photosynthesis under uniform in- 

 tensity of light increases with the duration of the exposure. 

 If N bubbles of oxygen are given out in a given period, 2N 

 bubbles are produced in twice the time. 



(2) Photosynthesis increases (within limits) with the 

 intensity of light. It is shown in Table VII. that while 

 under the intensity of 500 lux the quantity of oxygen 

 evolved was 128 c.mm. per hour, under twice the inten- 

 sity or 1000 lux the gas evolved per hour became doubled 

 to 257 c.mm. A similar relation also holds good under 

 sunlight : at the intensity of 0-06 S the gas evolved was 

 167 c.mm. per hour ; at • 12 S it became 337-5. 



It remains to ascertain what is the relation of photo- 

 synthetic activity to the angle of the incident light. 

 Special difficulties arose in the measurement of the directive 

 angle ; in Hydrilla the small leaves of the plant are dis- 

 posed in various directions, and it was therefore impossible 

 to use this plant for this purpose. A leaf with a flat surface 

 is required such as that of Vallisneria. The leaf was tied 



