ASSIMILATION OF CARBON BY PLANTS 



165 



same amount of air as that drawn through the glass chamber. 

 The difference between the quantity of carbon dioxide contained 

 in the air directly entering the absorber and that of the air pre- 

 viously drawn through the chamber with the plant gives the 

 quantity of carbon dioxide consumed by the latter. The leaves 

 or branches may be placed in the chamber without being detached 

 from the plant, and thus the experiment may be performed under 

 field conditions." This method has the advantage of permit- 

 ting the study of the absorption of carbon 

 dioxide in natural surroundings or in con- 

 stant atmospheric conditions. A defect 

 of this method is the overheating of the 

 glass chamber that always occurs when 

 the experiments are carried out in direct 

 sunlight. 



A number of methods of study of carbon 

 assimilation are based on the principle 

 that in photosynthesis the absorption by 

 the plant of carbon dioxide from the 

 atmosphere is accompanied by the evolu- 

 tion of an equivalent amount of oxygen. 



There are several means of detecting the 

 liberation of oxygen. The simplest pro- 

 cedure is to use plants that are submerged 

 in water to which a certain amount of 

 carbon dioxide has been added. When 

 such plants are exposed to a sufficiently 

 bright light, e.g., direct sunlight or a powerful electric light, gas 

 bubbles will appear on the surface of the plant. When these 

 bubbles are gathered by means of a funnel into a test tube or 

 eudiometer, it is easy to prove that the gas is oxygen by introduc- 

 ing a glowing splinter, which with a high concentration of oxygen 

 bursts into flame. This experiment is most easily conducted with 

 water plants (Fig. 48). It may be demonstrated with a branch 

 of Elodea canadensis with the cut end turned upward. When 

 placed in the light, air bubbles will rise with great regularity, 

 issuing from the cut end. In counting the number of bubbles 

 that are given off during 1 min., changes in the rate of carbon 

 dioxide assimilation may be observed. This method is not very 

 exact, since by diffusion, other gases such as nitrogen and carbon 



Fig. 48. — Method of 

 gathering oxygen given off 

 by an aquatic plant {after 

 Palladin) . 



