ABSORPTION OF CARBON 



11 



was the first to employ this method some hundred years ago. In 

 recent times, it has been considerably improved by other investi- 

 gators. This procedure is expedient, as it allows one to observe 

 the plant in its natural surroundings and thus to study the absorp- 

 tion of carbon dioxide from unaltered air. 



Numerous other methods are based on the fact that in the 

 process of nutrition with carbon dioxide the plant does not con- 

 fine its activities solely to the absorption of this gas from the 

 surrounding air, but that it releases also an equal amount of oxygen. 

 "The plant improves the air" was the somewhat ingenious defini- 

 tion of the above process during the first 

 years following its discovery. 



The release of oxygen may be detected 

 by several means. The simplest procedure 

 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, for instance, direct sunlight, or a 

 strong electric light of several hundred 

 candlepower, then on the surface of the 

 plants will appear gas bubbles. When 

 these bubbles are gathered by means of 

 a funnel into a test tube or eudiometer, 

 it will not be difficult to convince oneself 

 that the gas is oxygen. For this purpose, 

 it is sufficient to introduce into the tube FlG 3 _Method of gather- 

 a glowing splinter. This experiment is ing oxygen given off by an 



aquatic plant (after Palla- 



din) . 



most easily conducted with water plants. 

 It may be vividly demonstrated with a 

 branch of Elodea canadensis with the cut end turned upwards. 

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

 from the cut intercellular spaces. In counting the number of 

 bubbles which are given off during 1 min., changes in the rate 

 of carbon dioxide absorption may be observed. This method 

 is not quite exact, since, owing to diffusion, other gases, such 

 as nitrogen and carbon dioxide, may be dissolved in water and 

 will mix with the released oxygen. The method most commonly 

 applied in recent years, therefore, is an exact quantitative de- 

 termination of the oxygen eliminated by the plant, by means of 



