242 PHOTOSYNTHESIS 



is desired to determine this gas in the presence of hydrogen sulfide or 

 carbon dioxide ; the latter gases are not affected by the absorbent. The 

 preparation of the chromous chloride reagent is associated with consider- 

 able inconvenience, and it is doubtful whether the oxygen is completely 

 absorbed. ^^ 



Gas Samples. It is sometimes desirable or necessary to take a num- 

 ber of gas samples which cannot be analyzed at once, but must be pre- 

 served before the analysis can be made. Such samples can serve, of 

 course, not only for the oxygen-content but carbon dioxide and other gases 

 as well. It has been found absolutely essential that such containers should 

 be dry and clean, for even very small quantities of dust or grease will 

 decrease appreciably the amount of oxygen and increase the carbon 

 dioxide-content. Also the alkali of the glass dissolves slowly in water 

 and absorbs CO^- It is therefore necessary that all glass containers 

 should be thoroughly treated with a chromic acid cleaning mixture, care- 

 fully washed and dried in such a manner that no dust enters the vessel 

 (but not with alcohol and ether). 



The work of recent years on photosynthesis and respiration indicates 

 that it is far more satisfactory to arrange the experiments in such a 

 way that the plants are subjected to a stream of air rather than con- 

 fine them and determine the composition of the atmosphere at intervals. 

 The former method necessitates either the analysis of all the gas which 

 has passed over the plant for a definite period or the taking of samples 

 of the gas stream from time to time. 



g. The Determination of Oxygen in Water, 



Although in aquatic plants, just as in land plants, photosynthesis can 

 be determined far more satisfactorily by differential determination of 

 the carbon dioxide, it is nevertheless often desirable to determine the rate 

 of photosynthesis by means of changes in the oxygen-content of the 

 water. On account of the slight solubility of oxygen in water a differ- 

 ential method for oxygen is not satisfactory, i.e. in which the oxygen- 

 content of a stream of water is determined before and after it has 

 parsed over a i:)hotosynthetically active plant. ^° 



A large number of methods have been devised for the determination 

 of oxygen in water. =^^ The method suggested by Winkler has been very 

 generally used and with quite satisfactory results. The principle of the 

 method is as follows : the water containing dissolved oxygen is treated 

 with a solution of sodium hydroxide and one of manganous chloride. 

 Thereby the heavy precipitate of manganous hydroxide is formed; this 



*■ Anderson, R. P., and Rifife, J., Jour. Ind. Eng. Chew.. 8, 24 (1916). 



""Gail, F. W.. Pubs. Puget Sound Biological Station, 1922, p. 177. Fromageot, 

 Compt. rend., 177, 779, 892 (1923). Harder, Zcit. Bof.. 15. 305 (1923). 



"Cronheim, W., Zeit. angezv. Chevt., 20, 1939 (1907). Winterstein, H., Biochem. 

 Zeit., 19, 425 (1909). Report of the Metropolitan Sewerage Commission, New 

 York, 1910, p. 401. Boige and Juday, Wisconsin Survey Bulletin 22, 11, 12 (1911). 

 Soerensen and Anderson, Zeit. anal. Chem., 47, 279 (1908). 



