166 



PLANT PHYSIOLOGY 



dioxide may be dissolved in water and will mix with the oxygen 

 released. The method most commonly applied in recent years 

 is an exact quantitative determination of the oxygen evolved by 

 the plant, by means of titrating a definite volume of water with 

 oxygen-absorbing substances (Winkler's method). 



The most complete idea of the process of assimilation of carbon 

 dioxide is obtained with procedures by which not only the amount 

 of carbon dioxide absorbed is determined, but that of the oxygen 



released as well. The method most commonly 

 used is the one in which the leaf of a plant is 

 placed in a eudiometer sealed with mercury and 

 filled with air of a definite carbon dioxide con- 

 tent (Fig. 49). The eudiometer is then exposed 

 to light ; and after some time, analysis is made 

 of the gas inside. By comparing the amount of 

 carbon dioxide consumed with the amount of 

 oxygen accumulated in the eudiometer, infor- 

 mation is obtained about the relative changes 

 that take place in the composition of the air 

 resulting from the activity of the plant. 



Methods of analysis of the gas contained in 

 the eudiometer have already been described in 

 the discussion of the methods for the study of 

 respiration. 



onfeter fortTe^stu"!!!; ^^ ^^^^^^ ^^^^^^ photosyiithesis has frequently 

 of photosynthesis been determined by means of the Warburg 

 {after Kostytchev). nianometric method described previously in 

 Chap. III. The change in the total volume of gas during 

 photosynthesis is caused by the fact that oxygen is considerably 

 less soluble in water than is carbon dioxide. 



A somewhat indirect, but nevertheless very important, method 

 of studying the assimilation of carbon is the so-called ''dry- weight 

 method" devised by Sachs. This determination is based on the 

 fact that as a result of the accumulation of the products of assimi- 

 lation in the leaf blade, it becomes heavier, and this increase in 

 weight may be ascertained by means of accurate analytical 

 balances. It is necessary, of course, to weigh not a fresh but a 

 dried leaf, as in a recently cut leaf the fluctuation in the water 

 content overbalances variations in the amount of dry matter. 

 Because a dry leaf does not assimilate, it is necessary to resort to 



