DETERMINATION OF THE INCREASE OF WEIGHT 203 



Theory of the method. — Let us consider the case of 

 Hydrilla growing in water containing carbon dioxide in 

 solution. On exposure to light, carbohydrates are formed 

 inside the cells, which therefore become heavier than before. 

 The volume of water displaced by the immersed plant 

 remains the same, and the increase of weight produced 

 should, therefore, be capable of measurement by means 

 of a delicate balance. Imagine a closed and weighted 

 hollow bulb sunk under water and attached by a fine thread 

 to one pan of a delicate balance with exact counter-weight 

 in the other pan, the condition of equilibrium being assured 

 from the position of the index at zero. Suppose in some 

 way heavy particles are formed in the interior of the closed 

 bulb ; as the volume of water displaced by the bulb remains 

 constant, the displaced index can only be brought back 

 to zero by placing an additional weight in the second pan 

 of the balance exactly equal to the increased weight pro- 

 duced in the bulb. Now the cells of the plant correspond 

 to the closed bulb ; the carbohydrates are the heavy sub- 

 stances formed in the interior under the action of light, 

 the weight of which may thus be determined directly. 

 There may be some misgiving that the volume of the plant 

 might undergo a change by growth ; against this it is to be 

 said (1) that the hypothetical growth of the plant for the 

 short time of the experiment must be slight ; (2) that growth 

 is lowered or even arrested under the action of strong light ; 

 and (3) that the experiments to be described were carried 

 out not with growing but with fully-grown specimens. 



In this new method of investigation the experimental 

 conditions fulfil all the requirements previously enumerated. 



1. Complete investigation with a single plant. — The sprig 

 of Hydrilla for the experiment is about 10 cm. in length ; a 

 small piece of platinum wire is attached to the upper end 

 and the twig suspended upside down from one arm of a 

 sensitive balance. The plant is completely immersed in 

 oxygenated distilled water, previously boiled, which has 

 a definite content of about 7 mg. of C0 2 per 100 c.c. The 



