22 OF THE ASSIMILATION OF CARBON. 



the behavior of plants during the night. The experiments of 

 Ingenhouss were in a great degree the cause of the uncertainty 

 of opinion regarding the influence of plants in purifying the air. 

 His observation that green plants emit carbonic acid in the dark, 

 led De Saussure and Grischow to new investigations, by which 

 they ascertained that under such conditions plants do really 

 absorb oxygen and emit carbonic acid ; but that the whole volume 

 of air undergoes diminution at the same time. From the latter 

 fact it follows, that the quantity of oxygen gas absorbed is greater 

 than the volume of carbonic acid separated ; for, if both were 

 equal, no diminution could occur. These facts cannot be doubt- 

 ed, but the views based on them have been so false, that nothing, 

 except the total disregard and the utmost ignorance of the chemi- 

 cal relations of plants to the atmosphere, can account for their 

 adoption. 



It is known that nitrogen, hydrogen, and a number of other 

 gases, exercise a peculiar, and, in general, an injurious influence 

 upon living plants. Is it, then, probable, that oxygen, one of the 

 most energetic agents in nature, should remain without influence 

 on plants when one of their peculiar processes of assimilation 

 has ceased ? 



It is true that the decomposition of carbonic acid is arrested by 

 absence of light. But then, namely, at night, a true chemical 

 process commences, in consequence of the action of the oxygen 

 in the air, upon the organic substances composing the leaves, 

 blossoms, and fruit. This process is not at all connected with the 

 life of the vegetable organism, because it goes on in a dead plant 

 exactly as in a living one. 



The substances composing the leaves of different plants being 

 known, it is a matter of the greatest ease and certainty to calcu- 

 late which of them, during life, should absorb most oxygen by 

 chemical action when the influence of light is withdrawn. 



The leaves and green parts of all plants containing volatile 

 oils or volatile constituents in general, should absorb more than 

 other parts free from such substances ; for these change into resin 

 by the absorption of oxygen. Leaves, also, containing either the 

 constituents of nut-galls, or compounds in which nitrogen is 

 present, ought to absorb more oxygen than those destitute of such 



