278 CHAP. XXVIII. GENERAL REVIEW 



Determination of the Carbohydrate Product 



under Light 



There are inevitable sources of error in the determination 

 of the increase of weight by the half-leaf method. These 

 arise from the difference in the retention of water in the two 

 halves of the leaves after drying, from the different thickness 

 and lack of symmetry of the two halves, from the shrinkage 

 of area of the exposed half to light, and also from the trans- 

 location of soluble carbohydrate from the leaf. Nor can 

 the indirect estimation of carbohydrate from the absorp- 

 tion of carbon dioxide by the leaf be regarded as highly 

 accurate. The value of the carbohydrate-factor assumed 

 is untrustworthy, since it is not constant, but varies in 

 different plants, and even in the same plant under different 

 conditions. 



These errors are obviated in experiments with cut 

 specimens of Hydrilla. There is no loss of carbohydrate 

 due to translocation, nor is there any increase of weight 

 through accession of nitrogenous or other substances from 

 the soil. 



Simultaneous determination was made by two inde- 

 pendent methods. The first is the direct determination 

 of the increase of weight of the living plant by a sensitive 

 Torsion Balance, or by a Chemical Balance, for an exposure 

 to light of a definite duration ; the second, indirect, is the 

 measurement by a Eudiometei of the volume of the oxygen 

 evolved during the same period. The results obtained by the 

 direct and indirect methods are in the closest agreement with 

 each other (p. 217). 



The rate of production of carbohydrate under sunlight 

 per square metre of photosynthetic surface of the leaf of 

 Hydrilla was found to be 2 • 8 grms. per hour. 



The methods of determination of the rate of photo- 

 synthesis have been rendered so sensitive that records 

 may be obtained from which it is possible to estimate the 



