196 LIGHT, VEGETATION AND CHLOROPHYLL 



result is like a balance-sheet in which all the detailed entries 

 have been omitted and only the remainder appears. 



Measurement of the Oxygen Given Ojf 



This is very simple. A branch of elodea, in water charged 

 with carbon dioxide, is exposed to the light and the bubbles 

 of oxygen are counted as they appear. The rate of assimilation, 

 and the influence of different factors such as the temperature 

 or luminous intensity on it, can thus be estimated. 



The method, of course, has defects, the most serious of 

 which arises from the fact that all the oxygen is not necessarily 

 formed into bubbles; there may be a considerable difference, 

 depending on the temperature or other conditions of the 

 environment, between the oxygen really released and the 

 number of bubbles. Various suggestions have been made, by 

 Warburg in particular, to obviate this disadvantage. 



Measurement of the Carbon Dioxide Absorbed 



A leaf is wrapped in cellophane to shut out the external 

 air without preventing the penetration of light. The confined 

 air is renewed by suction and the air, after its contact with the 

 leaf, passes on to an alkaline solution which fixes the carbon 

 dioxide remaining in it. If the quantity of air passed is known, 

 the quantity of carbon dioxide which has come in contact 

 with the leaf is also known, and the measurement gives the 

 quantity of carbon dioxide which remains. 



Although this method is simple in principle, it is difficult 

 to apply to the study of a leaf like that of apple or beech in 

 its natural environment. With algae or aquatic plants it is 

 much easier because it consists merely in measuring the 

 carbon dioxide which has disappeared from the water or the 

 variations of the pH of the water. 



By the application of one or other of these principles 

 or several at the same time, many methods of measuring 

 the rate of assimilation have been developed and have led 

 to the discovery of the diff'erent factors which affect and 

 modifv it. 



