228 PHOTOSYNTHESIS 



methods. Most of these are of a quaUtative nature rather than methods 

 of quantitative measurement. However, by every one of these methods 

 some knowledge has been gained if it has been only to serve as a check 

 on some previous observation. Nevertheless, only the last two can be 

 considered as at all possessing the qualities of exactness and in the 

 strictest sense this applies only to the last, the analytical gas method. 



In order to demonstrate the formation of oxygen Boussingault - in- 

 troduced the use of phosphorus, the presence of oxygen being demon- 

 strated by formation of a fog of P2O5, the luminescence of the phosphorus 

 in the dark and the reduction of the total volume of the gas. The method 

 is of limited value on account of the poisonous character of the phosphorus 

 and of the acids formed. 



a. The Use of the Leucobases 



When certain dyestuffs in solution are reduced, the colorless leucobases 

 are formed. These leucobases in turn react very readily with oxygen 

 reforming the colored compound and thus are very sensitive reagents 

 for this gas. One of the best dyes for this purpose is indigo or indigo- 

 carmine. This fact has been used for the detection of the evolution of 

 oxygen by algae by Beijerinck ^ and has found extensive use in bac- 

 teriology for anaerobic cultures. A good method of preparing the in- 

 dicator is to shake a concentrated solution of sodium bisulphite with 

 zinc dust for about five minutes, keeping the mixture cool all the time. 

 Neutralize exactly with a very thin paste of Ca(OH)2. This is allowed 

 to settle, and the clear solution is used to decolorize a solution of indigo- 

 carmine, adding the reducing solution drop by drop. The blue dye 

 solution turns a light yellow; naturally air must be kept away from 

 this solution as much as possible and it is best kept in a vessel which 

 the solution fills entirely. If a few filaments of an alga are placed in 

 the solution and the whole exposed to light, the blue color will appear 

 in the solution immediately surrounding the plants. It is essential that 

 the water used for making up the solutions has stood in the air so as 

 to have dissolved some carbon dioxide and also that not too much 

 Ca(OH)2 is added. The method is applicable for demonstration rather 

 than for purposes of investigation. Unless the indicator is very care- 

 fully made it will turn blue in the light even in the absence of oxygen. 

 Furthermore, it is now quite well established that in complete absence 

 of oxygen photosynthesis does not take place even in intense light. It is 

 therefore not desirable to produce strictly anaerobic conditions in experi- 

 ments on photosynthesis. If an indicator for such conditions is desired 

 the method given by Fuhrmann * for the preparation of an indigo solu- 

 tion is very serviceable. 



^Boussingault, Ann. Set. nat. V., 10, 331 (1869). 



'Beijerinck, Bot. Zeitung, 48, 741 (1890). Linsbauer, L. and K., "Vorschule 

 der Pflanzenphysiologie," Vienna (1906). 



* Fuhrmann, F.. in Abderhalden, E., "Handbuch der Biochemischen Arbeits- 

 methoden," Vol. III-2, p. 1241, Berlin, 1910. 



