204 CHAP. XXIII. PHOTOSYNTHETIC INCREASE OF WEIGHT 



specimen is first weighed in the dark, and then at different 

 intervals after exposure to light of moderate or strong 

 intensity. The difference gives the increase of weight due 

 to photosynthesis. The complete investigation is thus 

 carried out on an identical plant with constant physiological 

 characteristics. 



2. Increase of weight due solely to C0 2 - assimilation. — 

 The sprig of Hydrilla is cut off from all connection with the 

 soil from which it might otherwise derive nitrogenous or 

 other substances. The increase of weight must, therefore, 

 be entirely due to C0 2 -assimilation. 



3. Natural conditions of the experiment. — The experiment 

 is carried out with the plant placed in a large vessel of water, 

 so that it is not under restraint ; the conditions of the ex- 

 periment are thus as natural as it is possible to make them. 

 The temperature of the plant is the same as that of the large 

 mass of water in which it is immersed. There is thus 

 no indefinite rise of temperature in the plant due to local 

 heating under strong light, as is the case in the leaves of aerial 

 plants placed within a bell-jar and subjected to strong light. 



4. Absence of transpiration and shrinkage. — The plant 

 being immersed under water, there is no transpiration with 

 a resulting shrinkage of the area of the leaves. 



5. No diminution of weight by translocation of carbo- 

 hydrates. — In the half-leaf method, an amount of carbo- 

 hydrate is lost by translocation from the leaves to the body 

 of the plant. In the new method no isolated leaf is used, but 

 the sprig as a whole bearing the leaves. The increase of 

 the weight of the sprig gives the total quantity of carbo- 

 hydrate formed, however it may be distributed. 



6. Allowance for, or elimination of, loss due to respiration. 

 — Two different methods of experiment have already been 

 described on which allowance for loss by respiration can 

 be made. The effect of respiration is, however, eliminated 

 by the particular method of the experiment described in 

 the next chapter. 



7. Direct determination of increase of weight. — The in- 



