EFFECT OF DIRECTIVE ANGLE OF LIGHT 



47 



to a thin glass rod ; the upper end of the rod carried an 

 index moving against a circular scale. The leaf, with its 

 cut end upwards, was placed vertically in a rectangular 

 trough of water containing the normal proportion of C0 2 , 

 photosynthesis being measured by counting the number of 

 bubbles given out at the cut end for a given length of time. 

 Sunlight reflected by a mirror was thrown perpendicularly 

 on to the surface of the trough ; since the leaf was parallel to 

 the surface of the trough, the directive angle was 90 . The 

 leaf was then rotated alternately through + 45 and — 45 

 and the evolution of oxygen in these two positions measured, 

 the mean being taken as the value at 45 . The object of 

 the alternate rotation was to make allowance for any 

 asymmetry in the setting of the index. It was also necessary 

 to make allowance for the gradually diminishing evolution 

 of oxygen-bubbles on account of the deposit of the mucilage 

 at the cut end. This was secured by taking observations 

 in the following order : (1) at 90 , (2) at 45 , and (3) once 

 more at 90 . The mean of (1) and (3) gave the average 

 value at 90 . 



Table IX. — Showing the Effect of Directive Angle on 



Photosynthesis 



Sine of 45 



Hence photosynthetic activity is approximately pro- 

 portional to the sine of the directive angle. 



