28 OF THE ASSIMILATION OF CARBON. 



ganism, 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 calculate which of them, du- 

 ring life, should absorb most oxygen by chemical 

 action, when the influence of light is withdrawn. 



The leaves and green parts of all plants, contain- 

 ing volatile oils or volatile constituents in general, 

 which change into resin by the absorption of oxy- 

 gen, should absorb more than other parts which 

 are free from such substances. Those leaves, also, 

 which contain either the constituents of nut-galls, 

 or compounds, in which nitrogen is present, ought 

 to absorb more oxygen than those which do not 

 contain such matters. The correctness of these 

 inferences has been distinctly proved by the obser- 

 vations of De Saussure ; for, whilst the tasteless 

 leaves of the Agave americana absorb only 0.3 of 

 their volume of oxygen, in the dark, during 24 

 hours, the leaves of the Pinus Abies, which contain 

 volatile and resinous oils, absorb 1 times, those of 

 the Quercus Robur containing tannic acid 1 4 times, 

 and the balmy leaves of the Populus alba 2 1 times 

 that quantity. This chemical action is shown, very 

 plainly, also in the leaves of the Cotyledon calycinwn, 

 the Cacalia ficoides and others ; for they are sour 

 like sorrel in the morning, tasteless at noon, and 

 bitter in the evening. The formation of acids is 

 effected during the night, by a true process of oxi- 



