CHAPTER XXIII 



DETERMINATION OF THE INCREASE OF WEIGHT DUE TO 



PHOTOSYNTHESIS 



Errors in determination of the photosynthetic increase of weight by 

 the half-leaf method — Indirect determination from absorption of 

 carbon dioxide — Indefinite value of the carbohydrate-factor — Neces- 

 sary conditions for accurate results — Direct determination of the 

 increase of weight of the living plant due to photosynthesis — Theory 

 of the method— Advantages of the new method— Torsion Balance 

 for determining increase of weight — Determination by Chemical 

 Balance; — Determination of photosynthesis under diffused light and 

 under sunlight — Maximum rate of formation of carbohydrate per 

 unit area. 



Under the action of light, organic substances, mainly 

 carbohydrates, are formed in the leaf as the photosynthetic 

 product. No direct method of measuring the increase of 

 weight under the action of light has hitherto been found 

 possible. It has generally been determined from the intake 

 of C0 2 by the assimilating leaf, the amount of carbohydrate 

 being estimated by multiplying the quantity of absorbed 

 C0 2 by the carbohydrate-factor. This factor, as will be 

 presently seen, is not a constant, but varies in different 

 plants. The C0 2 -absorption is, moreover, not the most 

 reliable method of estimating photosynthesis. 



There is the other possibility of the indirect estimation of 

 the carbohydrates from the evolution of oxygen : but, so 

 far as I am aware, no attempt has been made in this 

 direction, probably on account of the inaccuracies in the 

 older bubbling-method. These have now been completely 

 eliminated, and I will presently show how great is the 

 reliability of the oxygen-method. 



It is desirable at this stage to consider the advantages 

 and the disadvantages of the different methods of which 



