208 CHAP. XXIII. PHOTOSYNTHETIC INCREASE OF WEIGHT 



It is thus seen that the production of carbohydrate under 

 light can be directly measured by a sensitive balance, and 

 that the weight of the plant increases with the intensity and 

 duration of the light to which the plant has been exposed. 



Determination of Increase of Weight by a Chemical 



Balance 



As the increase of weight under prolonged photosynthesis 

 is sufficiently large, the increase of weight was next deter- 

 mined by a Chemical Balance with which it is not difficult 

 to measure an increase of weight of a hundredth of a milli- 

 gram, especially as the weight of the sprig is as small as 

 0-025 gnn. or so. Details of the method of measurement 

 will be fully given in the next chapter. We shall anticipate 

 some of the results and obtain a general idea of the increase 

 of weight produced during photosynthesis. 



Photosynthesis under diffuse light from the sky. — The 

 sprig of Hydrilla had 32 leaves ; the increase of weight after 

 exposure to light from the sky for five hours from 10 a.m. to 

 3 p.m. was found to be 1 -2 mg. (December 24, 1922). 



Rate of increase of weight, 



under sky light . . 0-24 mg. per hour . (1) 



Another experiment was carried out on the same day, 

 with this difference, that the plant was placed in sunlight. 

 The area of the assimilating surface was in this case the same 

 as before. 



(a) For one and a-half hours between 9 and 10.30 a.m. 

 the increase of weight observed was 1 • 3 mg. 



Rate of increase of weight, 



under sunlight (forenoon) . o • 86 mg. per hour . (2) 



(b) The increase of weight was next determined for the 

 next hour and a-half between 10.30 a.m. and 12 noon. The 



