250 



PHOTOSYNTHESIS 



For this purpose various methods have been used. These have in- 

 cluded the absorption of the carbon dioxide by barium hydroxide solutions 

 in Pettenkofer tubes, Meyer's bulb tubes and the Reiset apparatus. The 

 change in strength of the barium hydroxide solution due to the formation 

 of barium carbonate is then determined by means of titration. If this 

 method is used it is essential to add some barium chloride to the barium 

 hydroxide solution when it is prepared. The latter solution is usually pre- 

 pared as a tenth normal solution with one gram of barium chloride added 



Fig. 16.— Container for excised leaf used in experiments on photosynthesis and 

 respiration. The petiole of the leaf is in a nutrient solution the level of which 

 can be adjusted. The air-stream enters at the top, leaves at the lower edge of 

 the frame, and is dried over P2O5 before passing to the absorption tubes. 



for each liter. Barium carbonate is slightly soluble in barium hydroxide 



solution and from the work of Vesterburg *^ and of Weisenberger *^ it is 



evident that the addition of barium chloride suppresses the hydrolysis of 



barium carbonate. 



The carbon dioxide can, of course, also be absorbed in soda-lime or in 



Liebig absorption bulbs and determined gravimetrically, though this 



method is probably not as accurate as the volumetric one. 



*' Vesterburg, Zeit. phy. Chcm.. 70, 550 (1910). 

 ^"Weisenberger, ibid., 88, 257 (1914). 



