THE VEGETABLE CELL. 83 



of gas are set free : ia red ; in red and orange 24, 75 ; yellow 

 and green 43, 75 ; green and blue 4, 10 ; blue 1, ; indigo 0. The 

 light here acts according to the intensity of its illuminating power ; 

 the chemical and heating rays of the spectrum are without effect. 



Ohserv. The amount of oxygen given oJQf is determined by the amount 

 of caii)onic acid furnished to the plant ; the volume of the gas given off 

 from the plant also corresponds to the carbonic acid taken up by it, but 

 the gas exhaled does not consist of oxygen alone, a more or less consi- 

 derable quantity of nitrogen being intermingled with it Draper Q, c. 

 180) obtained the following results : — > 





Finns Tmda. 





Experiment. 

 1 

 2 

 3 



Oxygen. 

 16, 16 

 27, 16 

 22,33 



Foob annua. 



Nitrogen. 

 8. 34 

 13, 84 

 21, 67 



1 



2 



90, 



77,90 



10, 

 22, 10 



When the experiments are made by exposing plants to the sim xinder 

 spring-water, a part of the nitrogen is doubtless derived from the water, 

 as well as another part from the ah' contamed in the air-cavities of the 

 plant; but these circumstances do not explain the exhalation of nitrogen 

 completely, for according to Draper's experiments, it takes place when 

 plants totally deprived of air by the air-pump are experimented on in 

 water containing no nitrogen, and the quantity of nitrogen exhaled in- 

 creases in proportion to the amount of oxygen during the experiment, 

 while the reverse must occur if this intermixture depended on a diffusion 

 taking place between the oxygen exhaled by the plant and the nitrogen 

 contained in the water and in the plant. Draper draws from his experi- 

 ments the conclusion that the exhalation of nitrogen, is a constant pheno- 

 menon, connected necessarily with the exhalation of oxygen, and conjec- 

 tures that it is even the primary pi^ocess which fix^st sets in operation the 

 decomposition of the carbonic acid, that it is to be ascribed to a decompo- 

 sition of a nitrogenous substance in the leaf, which exercises the function 

 of a ferment in the decomposition of the carbonic acid. 



Boussingault (^^Economie Murahf' i. 5) drew the opposite conchision 

 from the results of Saussure's experiments, since in particular experi- 

 ments the exhalation of nitrogen was so considera«Me, that the nitroge- 

 nous contents of the plant did not suffice for it; he therefore thought 

 we could scarcely assume otherwise than that the nitrogen was derived 

 from the air contained in the water and the plant. Under these circum- 

 stances, a trial of these conditions by accurate experiment is greatly 

 required. 



The reason that the quantity of oxygen gas given off by the plant is 

 uneqtial to the carbonic acid taken up, is doubtless that a portion of the 

 oxygen gas set free in the green parenchyma of the plant, enters into 

 combiaation with oxidable substances contained in it. Many phenomena 



a 2 



