CHAPTER XXIV 



SIMULTANEOUS DETERMINATION OF CARBOHYDRATE- 

 FORMATION BY TWO INDEPENDENT METHODS 



Direct and indirect estimation of photosynthetic production of carbo- 

 hydrate — Indirect methods based on absorption of carbon dioxide and 

 on evolution of oxygen — Oxygen-carbohydrate factor — -Indirect deter- 

 mination of increase of weight from volume of evolved oxygen — 

 Direct determination by weighing the plant before and after exposure 

 to light — Simultaneous determination of carbohydrate by direct and 

 indirect methods — The Eudiometer and the Balance — -Modification 

 of the carbohydrate-factor by external conditions- — -Effect of strong 

 light and of semi-darkness. 



In the last chapter the method of direct estimation of 

 the photosynthetic product by weighing was explained 

 and shown to give consistent results. The experimental 

 methods and conditions were shown to be such as to 

 eliminate many possible sources of error. The question 

 now arises whether it is at all possible to devise an inde- 

 pendent method for determining the increase of weight. 

 Success in this would enable us to compare the results 

 obtained by the two methods, the mutual agreement of 

 which would offer a conclusive test of the reliability of each. 

 Such a method is to be found in the accurate determination 

 of the volume of oxygen evolved during photosynthesis. 



It will also be shown that, in normal specimens of Hydrilla 

 growing in a tank exposed to northern light from the sky 

 and not to direct sunlight, the ratio of the increase of weight 

 under light to the weight of oxygen evolved during the same 

 exposure is remarkably constant. The increase of weight 

 in normal plants may, therefore, be estimated by multiply- 

 ing the weight of oxygen by the constant oxygen-carbo- 

 hydrate factor. 



The method of determining the increase of weight 



