202 CHAP. XXIII. PHOTOSYNTHETIC INCREASE OF WEIGHT 



5. The loss of weight due to translocation of carbo- 

 hydrate, as also the gain of weight due to inorganic sub- 

 stances derived from the soil, should be prevented. 



6. The loss due to respiration should be determined or its 

 effect eliminated. 



7. The increase of weight should be ascertained directly 

 in the living condition of the plant. 



8. The fact that the increase of weight is due to photo- 

 synthesis should be demonstrated by proving that it is 

 dependent on the intensity and duration of light. 



9. The experimental investigation should be completed 

 in as short a time as possible. 



The ideal conditions detailed above are fulfilled in the 

 following investigations carried out with the water-plant 

 Hydrilla. They were undertaken on two independent 

 methods— (1) the direct determination of the increase of 

 weight on the living plant ; and (2) the indirect determina- 

 tion of the carbohydrate from the amount of oxygen evolved 

 during the same period. It is thus possible to test the 

 reliability of each of the two methods by comparing the 

 respective results. 



Determination of the Increase of Weight of 

 the Living Plant due to Photosynthesis 



Before describing the method for the determination of 

 increase of weight, a few words may be said of the choice of 

 experimental specimens. As the experiments are to be 

 carried out with cut specimens, it is desirable to select very 

 sturdy plants in which the activity is vigorous and persistent. 

 Weak specimens are liable to exhibit increasing depression, 

 culminating with the death of the plant. Middle-aged 

 specimens prove to be more sturdy than very young 

 ones. The physiologically vigorous specimens can be easily 

 picked out after some experience : such cut specimens con- 

 tinue to exhibit vigorous photosynthesis even for days in 

 succession. 



