PHOTOSYNTHETIC ACTIVITY UNDER DIFFERENT COLOURS 169 



balloon. The Hydrilla, suspended on one arm of the Torsion 

 Balance, floats up till the cut end of the plant approaches 

 the surface of the water, when the bubble bursts ; the twig 

 now sinks, losing its adventitious buoyancy. The arm of 

 the balance thus moves up and down, corresponding to the • 

 alternate flotation and sinking of the plant. Each periodic 

 movement of the arm of the balance represents the photo- 

 synthetic production of a given quantity of oxygen. The 

 next step is to obtain an automatic record of the evolution of 

 the gas. This is secured by a device such that the periodic 

 movement of the balance causes the completion of an electric 

 circuit which includes the electro-magnetic writer previously 

 described. Below the left arm of the Torsion Balance are 

 adjusted two pieces of tinsel separated by a distance of 

 2 mm. The ascensional movement of the plant under the 

 action of light causes up-movement of the right arm of the 

 balance and down-movement of the left arm ; this latter 

 presses one tinsel against the other, thus completing the 

 electric circuit, with the result that a dot is made on the 

 recording surface. 



This method of recording photosynthetic activity may 

 be rendered as sensitive as desired, first by increasing the 

 sensitiveness of the Torsion Balance, and secondly by 

 diminishing the depth of immersion of the cut end below 

 the surface of the water. When this depth is reduced, the 

 frequency of successive flotations becomes very rapid. 



Photosynthetic Activity under Different Colours 



The frequency of oscillation of the floater thus enables 

 us to measure the photosynthetic activity under different 

 conditions. Experiments on this method were undertaken 

 to determine the effect of different coloured lights, in which 

 sunlight was reflected by a heliostat into the experimental 

 room, different coloured lights being obtained by the inter- 

 position of suitable screens. Two coloured glasses, red and 

 blue, were spectroscopically selected, each of which was 



