ESTIMATION OF THE CARBOHYDRATE PRODUCT I99 



detailed accounts are given below. It should be borne in 

 mind that, under ordinary conditions, the dry substance 

 formed is not merely carbohydrate but proteid also. But 

 no allowance has hitherto been made for the possible 

 formation of proteids, nor any attempt made to reduce it. 



Estimation of the Carbohydrate Product 

 by the Half-leaf Method 



Sachs, in his classical work on the subject (1884), 

 attempted to measure the amount of the photosynthetic 

 product by the relative increase of the dry weight of the leaf 

 after exposure to light. He selected large leaves, e.g. of 

 Helianthus growing in the open, and cut off one longitudinal 

 half of the leaf early in the morning, the other half being 

 allowed to remain attached to the plant and exposed to 

 sunlight for a definite period. Equal areas of the exposed 

 and unexposed halves of the leaves were dried, powdered 

 and weighed ; the weight of the sun-exposed half was found 

 to be the greater, and the difference of weight was taken as the 

 amount of carbohydrate produced under the action of light 

 for the given period. In Helianthus the rate of increase in 

 weight was found to be 0-914 grm. per hour for a square 

 metre of leaf-surface. This value can be taken only as an 

 approximation, for there are various inevitable errors in the 

 half-leaf method. These arise (1) from possible difference 

 in the retention of water in the two halves of the leaf 

 after drying ; (2) from the different thickness and lack of 

 symmetry in the two halves ; and (3) from the shrinkage of 

 area of the exposed leaf caused by insolation and by greater 

 transpiration. Brown and Morris, from their experiments 

 on the subject, came to the conclusion that the error intro- 

 duced by the half-leaf method may even amount to 100 per 

 cent. 



There are other complications in the half-leaf method 

 due to loss by respiration, and to the translocation of soluble 

 carbohydrates from the leaf to the body of the plant. As 



