Chemical Processes in Plants. 321 



results, and the reasons for this variability were discovered 

 somewhat later by Ingenhouss, who showed that improve- 

 ments in the properties of vitiated air were due entirely 

 to the green parts of plants, and that the action took 

 place only in sunlight. The exact reasons for the im- 

 provement of vitiated air by plants were not clear to either 

 of the above-mentioned investigators. The subsequent re- 

 searches of Senebier, de Saussure, and Boussingault have 

 demonstrated, however, that plants take up the carbon 

 dioxide of the air and evolve oxygen. It was found that 

 the ratio of the carbon dioxide absorbed to the oxygen 



evolved (-77^) was approximately 1 (it is actually a little 

 \ U 2 / 



less). In other words, the carbon dioxide appears to 

 undergo some reduction process in the green part of 

 plants in the presence of sunlight. 



The evolution of oxygen by a green plant can be 

 readily demonstrated in the following manner. A water 

 plant, such as Elodcea, is allowed to grow in water and is 

 covered by a filtering funnel, over the tube of which is 

 inserted a test-tube filled with water. On exposure to 

 light, bubbles will be observed to be evolved, which will 

 pass up the funnel and collect in the test-tube. This gas 

 will be found to consist of oxygen. 



The fact that carbon dioxide is taken up from the air 

 can be demonstrated by passing air over a leaf exposed to 

 sunlight in a chamber and analyzing it before and after 

 its passage. 



Two other important observations in connection with 

 the carbon dioxide assimilation by plants must be recorded ; 

 firstly, as shown by Saussure, the absorption of carbon 

 dioxide is accompanied by the formation of starch, and 

 secondly, as demonstrated by Engelmann, the presence of 

 chlorophyll is necessary. 



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