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TEXTBOOK OF PLANT PHYSIOLOGY 



titrating a definite volume of water with oxygen-absorbing sub- 

 stances (Winkler's method). 



The most complete idea of the process of assimilation of car- 

 bon dioxide is obtained by using procedures in which not only 

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

 the released oxygen as well. The method most commonly used is 

 one in which the leaf of the plant is placed in a eudiometer, which 

 is closed by mercury and filled with air whose 

 carbon-dioxide content has been determined 

 beforehand (Fig. 4). The eudiometer is then 

 exposed to light. After some time, analysis is 

 made of the contained gas. By comparing the 

 amount of the CO2 eliminated with the amount 

 of oxygen accumulated in the eudiometer, a 

 complete idea is obtained as to the relative 

 changes which take place in the composition of 

 the air, resulting from the activity of the plant. 

 Originally, the analysis of gas was made in the 

 eudiometer itself, by means of adding first po- 

 tassium hydroxide, which absorbs CO2, and 

 afterwards pyrogallol, which in an alkali solution 

 readily absorbs O2. At the present time, we 

 prefer to take from the eudiometer small samples 

 of the gas and to analyze these in a special 

 Fig. 4. — Eudiometer apparatus, which allows one to determine with 

 [oayntVel^ ° f (a^ sufficient accuracy (up to one-hundredth of 1 per 

 Kostytschev). cent) the composition of as small a volume of 



gas as 1 cc. 

 A somewhat indirect, but nevertheless very important, 

 method of determining 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 prod- 

 ucts of assimilation in the leaf blade, it becomes heavier, and this 

 increase in weight may be ascertained by means of accurate ana- 

 lytical balances. It is necessary, however, to weigh not a fresh 

 leaf, but a dried one, as in a recently cut leaf the fluctuation in the 

 water content disguises the variation in the amount of dry matter. 

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

 a somewhat roundabout way to ascertain the added weight of dry 

 matter. A leaf showing as far as possible a symmetric structure 



