42 



PHYSIOLOGY OF NUTRITION. 



which is open at the top. The capacity of the whole apparatus is 

 about 115-120 c.c., of> which about 75 c.c belong to the bulb. We 

 take the free outer opening of the tube a as the zero for gradu- 

 ation, but the actual divisions only begin below the bulb, the 

 diameter of the tube, as remains to be noticed, being here about 



14 or 15 mm. The graduation may 

 conveniently be carried to T % c.c., 

 a detail which is not represented in 

 the illustration. 



In conducting volumetric obser- 

 vations on the gas exchange of 

 assimilating leaves, it is necessary 

 to cut away the leaf-stalk, leaving 

 only a short piece to which is 

 fastened a very thin iron wire d, 

 the wire being run through the 

 stump of the leaf-stalk, and wound 

 several times round it. The leaf 

 thus fixed on the wire is passed up 

 into the bulb, an operation which 

 is successfully effected by gently 

 bending the edges of the leaf back- 

 wards, introducing it into the gradu- 

 ated portion of the tube, and push- 

 ing it up with a wooden rod. The 

 apparatus is now fixed with its 

 lower end dipping into mercury,* 

 and we at once introduce into it 

 over the mercury 0'3 c.c. of water, 

 by means of a pipette bent round at 

 its lower end and drawn out to a 

 fine point. A piece of rubber 

 tubing is slipped over the end of 

 the tube a, which has so far been 

 open. This is put into connection 

 with a suction flask containing 

 water. When we remove air from the flask by suction, the 



* The mercury employed must be very pure. The purification of mercury 

 which has been several times used may be effected as follows : We treat the 

 metal in a thick- walled bottle with an equal volume of water, add to it a little 

 Nitric acid, and shake well for a quarter of an hour or half an hour. We 



FIG. 16. Apparatus for determining 

 the quantity of Carbon dioxide which 

 assimilating plants decompose. 



