DIRECT AND INDIRECT METHODS OF DETERMINATION 215 



was taken in order to guard against the possibility of false 

 buoyancy produced by minute bubbles of gas that might 

 have remained attached to the leaves. 



After exact balance the counterpoise is left on the pan 

 of the balance and the plant is transferred to the vessel v x , 

 which is taken outside and the plant exposed to light. The 

 bubbles produced are collected in the Eudiometer e (fig. 54), 

 a gentle tap detaching any bubble that may have remained 

 attached to the cut end. The volume of the gas produced 

 is noted, as well as the temperature and the barometric 

 pressure. These data suffice for the indirect determination 

 by calculation of the amount of carbohydrate produced 

 during exposure to light. The amount of carbohydrate pro- 

 duced is then directly determined by weighing. The vessel 

 containing the plant is taken into the balance-room, and 

 the specimen transferred to the first vessel v and weighed. 

 A short period is allowed for the plant to become adjusted 

 to the constant temperature, and the additional weight now 

 required to bring the index back to zero gives the quantity 

 of carbohydrate produced. 



Determination of the oxygen-carbohydrate factor. — The 

 volume of oxygen collected during exposure to bright sky 

 light from 10 A.M. to 3 p.m. was i-io c.c, at barometric 

 pressure 762 • 5 mm., and temperature of 18 C. The tension 

 of aqueous vapour was 15-5 mm. The N.T.P. volume 



i-iox (762-5-15-5) X273 T nTAnn 



Vr\ = 7 7 I o\ =1*014 C.C. 



760 x (273 + 18) ^ 



Since the weight of 1 c.c. of oxygen is 1-44 mg., the 

 weight of evolved oxygen 



W =1-014 x 1 -44mg. =1-460 mg. . . (1) 



The actual increase of weight W x of the plant due to the 

 production of carbohydrate was found to be 



W 1 =i-3omg (2) 



By dividing (2) by (1) we obtain 



1 • 30 

 the oxygen-carbohydrate factor = ^- = o • 8906 . (3) 



