l68 CHAP. XX. EFFICIENCY OF DIFFERENT COLOURS 



small piece of thin mirror m attached at the fulcrum, care 

 being taken to place a small weight w below the point of 

 suspension so as to make the system more stable by lowering 

 the centre of gravity. 



The cut specimen immersed in water undergoes a periodic 

 variation of buoyancy under the photosynthetic action of 

 light, in consequence of which the arm of the balance from 



Fig. 46. Record of Photosynthetic Activity under Different 

 Coloured Lights by the Method of Flotation 



Hydrilla plant h suspended from one arm of balance ; p, pan 

 suspended from the second arm ; the up and down movement 

 of the Hydrilla plant under light completes an electric circuit 

 by pressure of the left arm of the balance on two tinsels carried 

 by rods on stand c ; record is made on the revolving 

 drum d by following the movement of the spot of light 

 reflected from the mirror m with the writer w (see text) . 



which the plant is suspended moves up and down. This 

 is due to the appearance of a bubble of oxygen at the cut 

 end of the stem, which grows larger and larger, the rate of 

 enlargement depending on the photosynthetic activity. 

 Imagine a thin-walled balloon (with attached car) suddenly 

 filled with gas : the balloon rises, and as the superincum- 

 bent pressure decreases the expanding gas bursts the en- 

 velope, the car being then precipitated down. The twig of 

 the Hydrilla plant is the car, and the bubble of oxygen is the 



