The Evolution of Oxygen. 99 



Then the total oxygen produced by the assimilatory 



process in the given time is o + o' 

 and the total carbon dioxide absorbed in assimilation 



is c + c' 

 And the ti-ue assimilatory coefficient is 



+ ' O 



c + c'~ C "■ 



2. The second method employed by Bonniei- and Mangin is 

 based on Bernard's observation (1878) that by the use of chloroform 

 the assimilation may be suppressed and respiration alone takes 

 place. By comparison of the gaseous exchanges taking place in 

 two similar quantities of leaves exposed to light under the same 

 conditions, but in which one was anaesthetised with ether, and the 

 other not, the gaseous exchange due to assimilation may be 

 estimated. 



3. Bonnier and Mangin's third method is based on the 

 suppression of assimilation byjwioval of all carbon dioxide from the 

 neighbourhood of the leaves. Two similar vessels contain equal 

 weights of similar leafy tissue ; one of the vessels contains 

 concentrated barium hydroxide solution, the other an equal volume 

 of pure water. In the former, not only is the carbon dioxide 

 of the atmosphere lemoved and assimilation prevented, but the 

 carbon dioxide evolved in respiration is absorbed by the baryta. So 

 that, as in the second method, the difference between the oxygen 

 content and carbon dioxide content of the two vessels at the end 

 of the experiment, gives the true values for oxygen and carbon 

 dioxide e\olved and absorbed respectively in assimilation. 



4. The fourth method depends on the measurement of the 

 gaseous exchanges in branches of the same plant which are 

 unequally green. Thus a yellow_branch of Euonymus ja^onicus on 

 exposure to light evolved 2-89 units of carbon dioxide and absorbetT' 

 2-1 1 of oxygen, while in the same time a green branch evolved 2-27 

 of oxygen and absorbed 0-54 of carbon dioxide. 



The four different methods gave concordant results. The 

 results obtained by Bonnier and Mangin for a number of species at 

 different times of the year are shown in Table XXV. It will be 

 observed that the true assimilatory coefficient is always greater than 

 unity, whereas the respiratory coefficient is below unity. The 

 consequence of this is that the apparent assimilatory coefficient, 

 which neglects the respiration, is always lower than the real 



