200 CHAP. XXIII. PHOTOSYNTHETIC INCREASE OF WEIGHT 



regards loss by respiration, Sachs made a rough allowance 

 of i grm. per square metre during fifteen hours of active 

 assimilation. The translocation of carbohydrates he esti- 

 mated by finding the loss of weight in a half-leaf during ten 

 hours of continuous darkness at night. For obvious reasons 

 a different leaf had to be taken for this determination ; but 

 the loss for one leaf cannot be applied to another, since the 

 physiological characteristics of the two leaves cannot be 

 the same. Again, translocation is dependent on tempera- 

 ture ; the calculated value of the loss at night must be 

 different from the actual loss during daytime under light, 

 the temperature by day being considerably higher than by 

 night. 



The absorption of nitrogenous and other substances from 

 the soil would also produce an increase of weight not due 

 to C0 2 -assimilation. 



Indirect Estimation from the Absorption 

 of Carbon Dioxide 



Reference has already been made to the method 

 employed by Kreussler for this purpose. He experimented 

 with isolated twigs in an appropriate bell-jar through 

 which was made to pass a definite volume of air containing 

 a known percentage of carbon dioxide. ' He next deter- 

 mined how much of the carbon dioxide passed out of the 

 apparatus, and at the same time worked out how much 

 carbon dioxide was produced in the same period as a result 

 of respiration. In this way the amount of carbon dioxide 

 actually absorbed by the plant could be accurately deter- 

 mined. Since Kreussler worked with air containing a 

 relatively high percentage of carbon dioxide, and since the 

 material experimented with was illuminated by electric 

 light, his results furnish us with no information with refer- 

 ence to the amount of carbon dioxide decomposed under 

 normal conditions of assimilation. Brown (1899) employed 

 the method in a modified form. He worked with ordinary 



