Chap, ii.] VEGETAL NUTRITION. 95 



plants, mosses, for example, living in the shade of the woods, 

 which cannot, without dying, bear an intense light. But in 

 diverse degrees, light is indispensable to the green parts of all 

 plants. At night, or in darkness, chlorophyll ceases to act, and 

 the plant simply exhales carbonic acid. 



The property of decomposing the carbonic acid of the air and 

 of absorbing carbon specially appertains to chlorophyll, just as 

 that of fbdng a great quantity of oxygen specially appertains to 

 the hsemoglobuline of the hsematia. Chlorophyll has also, like 

 hsemoglobuline, its poisons. Thus, as Boussingault has demon- 

 strated, mercury introduced into a receiver where a plant is 

 destroys the chlorophyllian property. 



It seems to result from the experiments of Dutrochet,' that a 

 part of the oxygen put at liberty by the chlorophyllian action 

 is not immediately expulsed, but penetrates previously into the 

 mechanism of the tissues. The oxygen exhaled direct is merely 

 air overplus. The rest is impelled into the aerian cavities, into 

 the globulous vessels, into the punctuated tubes, and especially 

 into the trachese. In this way it descends into the petioles of 

 the leaves, into the stem, and serves there probably the real 

 respiratory function, the oxydation of the tissues, and the 

 production of the carbonic acid disengaged by the plant. 



It is by the stomata that this expulsion and this absorption of 

 air seem especially to be accomplished ; nevertheless, the mosses 

 and the coniferae, which have no stomata, exhale carbonic acid. 



We might be astonished that the small proportion of carbonic 

 acid contained in the air suffices for the alimentation in carbon 

 of the whole vegetal kingdom, if we did not think on the 

 great density of the atmosphere, and on the considerable restitu- 

 tions that are made to the atmosphere by the vegetal kingdom 

 on the one hand, and by the entire animal kingdom on the other. 

 This last, in effect, incessantly consumes oxygen, and exhales 

 torrents of carbonic acid. Moreover, we must add to these 

 principal sources of carbonic acid all the combustions, the 

 ' Dutrochet, De VEndosmose, p. 357. 



