212 CHAP. XXIV. CARBOHYDRATE-FORMATION 



from the volume of oxygen evolved has the advantage 

 of greater simplicity and accuracy over the method of 

 intake of C0 2 . By determining the relation between the 

 gain in weight and the volume of oxygen evolved, the 

 unknown factor of the loss of oxygen in respiration is 

 eliminated : for the loss is the same on both sides of the 

 equation. 



Simultaneous Determination of Carbohydrate by 

 Direct and Indirect Methods 



Direct Method of weighing. — The sensitiveness of the 

 chemical balance used was such that it was easy to weigh 

 accurately to o-oi mg. The balance was placed in an 

 underground vault free from vibration, in which the tem- 

 perature remained practically constant, as shown by the 

 records of the thermograph kept in the room. The maxi- 

 mum variation of less than i° C. occurred at noon ; the 

 temperature was, however, the same in the forenoon and 

 in the afternoon, these being the times at which deter- 

 minations were made of the difference in weight of the 

 plant before and after exposure to light. The balance 

 in the dark room was lighted by an electric bulb placed 

 outside, the light entering through a small window covered 

 with blue glass, blue light being relatively ineffective in 

 photosynthesis. The object of placing the source of light 

 outside was to guard against variation of temperature 

 from the heat given out by the incandescent lamp. The 

 light was only put on for the short period required for 

 weighing. The general mode of experimental procedure 

 is as follows. The cut sprig of Hydrilla, immersed in water 

 containing 7 mg. of C0 2 in 100 c.c. of water, is weighed 

 before and after exposure to light, the evolved oxygen 

 being collected in a Eudiometer. The increase of weight 

 due to the production of carbohydrate is directly found 

 by weighing. 



Indirect Method. — The production of carbohydrate can be 

 indirectly estimated from the weight of the oxygen collected 



