844 METHODS OF KINETIC MEASUREMENTS CHAP. 25 



It was mentioned on page 673 that, in working with solutions, the neces- 

 sity of estimating R is usually avoided by using a blank cell, whose reflec- 

 tion is assumed to be equal to that of the solution cell. Many authors have 

 hoped to get around the necessity of measuring Ra for leaves or thalli in a 

 similar way, by using as "blanks" plant tissues deprived of pigments. 

 This idea has been carried out in different ways: Reinke (1886) used algal 

 thalli from which the pigment had been extracted by alcohol; Linsbauer 

 (1901), Brown and Escombe (1905), Seybold (1932, 19331-2) and Meyer 

 (1939) compared the transmission by green parts with that by white parts 

 of variegated leaves; Wurmser (1921) determined the transmission of 

 thalli before and after bleaching by prolonged illumination. However, the 

 interpretation of results obtained in this way presents considerable dif- 

 ficulties. It has already been said (page 673) that equation (22.2b) is 

 only an approximation, although a satisfactory one, even in the work with 

 transparent media. Weigert (1911) thought that it could also be used, as 

 such, for leaves, and applied it to the data of Brown and Escombe; but 

 his calculation led to absurdly low values of A, and its fallacy has been 

 pointed out by Willstatter and StoU (1918) and Warburg (1925). 



In all precision experiments on light absorption by plants, measure- 

 ments of the three quantities I, T and R cannot be avoided. The determi- 

 nation of T and R can be carried out either by means of integrating devices 

 that collect the reflected and the transmitted light, or by differential "gonio- 

 photometric" methods, i. <?., by determining scattering as a function of 

 the angle between the incident and the scattered beam. 



3. Measurements of Oxygen Evolution 



Oxygen produced in photosynthesis can be identified and measured by 

 different chemical or physicochemical methods, either in the liquid phase 

 containing the aquatic plants, or in the gas phase. Because of the low 

 solubility of oxygen in water, methods of the first kind (e. g., the potentio- 

 metric determination of the oxygen concentration in solution) are suitable 

 only for the measurement of small effects, e. g., for the observation of the 

 photosynthetic activity in the first minutes of illumination {cf. chapter 33). 



We cannot deal here with the analytical technique of the determination 

 of oxygen. White phosphorus, organic oxygen absorbers (such as pyrogallol 

 or the leuco dyes), copper, sodium hyposulfite and chromous chloride are 

 among the reagents used for this purpose. For solutions, Winkler's 

 method is perhaps the most popular one; it utilizes oxygen (through the 

 intermediary of the system manganous chloride-manganic chloride) for the 

 liberation of an equivalent quantity of chlorine, which can easily be de- 



