38 EVOLUTION AND GROWTH. 



resents almost exactly the volume of oxygen which it would be 

 necessary to add, in order that the oxygen collected should represent 

 the whole of that which entered into the constitution of the carbonic 

 acid decomposed. It is probable that the excess of azote which ap- 

 peared in all these experiments was present in principal part in the 

 air contained and condensed within the interstices of the plants, or 

 held in solution in the water which bathed their roots. It would be 

 difficult to assign it any other origin ; such, for instance, as that 

 from changes in the azotized principles of the plants that were the 

 subject of experiment. In his first experiment, in fact, iM. de iSaus- 

 sure fixes the weight of the dry matter of the seven periwinkle 

 plants at 41.6 grains. Now, from numerous determinations of azote 

 which I have had occasion to make in regard to plants of very dif- 

 ferent ages and species, I think I can say that these periwinkles, 

 taken as dry, did not contain more than .385 of azote; this, in ref- 

 erence to the weight assumed by M. de Saussure, would be 1.042 

 grs. or 20.8 cubic inches of azote ; and the volume of azote disen- 

 gaged in this first experiment was 54.6 cubic inches. It is proj)er 

 further to observe, that the state of health which the plants pre- 

 sented on the conclusion of the experiment does not allow us to sup- 

 pose a total decomposition of the azotized matters which entered 

 into their constitution. These various considerations lead us to in- 

 fer that the excess of azote collected must have been displaced by 

 oxygen. We are, therefore, at liberty to presume, froni the experi- 

 ments now referred to, that the volume of oxygen produced probably 

 represents the volume of carbonic acid decomposed. 



The necessity of oxygen gas in the decompounding action which 

 plants exposed to the light exert so energetically upon carbonic acid, 

 loads us to study particularly tiie phenomena which oxygen exhibits 

 in connection with growing plants. When a number of freshly 

 gathered and healthy leaves are placed during the night iinder a bell- 

 glass of atmospheric air, they condense a portion of the oxygen ; 

 the volume of the air diminishes, and there is a quantity of free car- 

 bonic acid formed, generally less than the volume of oxygen which 

 Kis disappeared. If the leaves which have absorbed this oxygen 

 during their stay in the dark, be now exposed to the sun's light, they 

 restore it nearly in equal quantity, so that, all corrections made, the 

 atmosphere of the bell-glass returns to its original composition and 

 volume. 



Leaves in general have the same elTect when they are placed alter- 

 nately in the dark and in the light; there is, however, a very obvious 

 dilTerence in the intensity with which the phenomenon is produced, 

 according to the nature of the leaves. The quantity of carbonic 

 acid formed during the night is by so much the less, as the leavea 

 are more fieshy, thicker, and therefore more watery. The green 

 matter of lleshy leaved plants, of the cactus opunlia, to quote a par- 

 ticular instance, does not j)roduce any sensible quantity of carbonic 

 acid in the dark : but these leaves condense oxygen, and exhale il 

 again like those which are less lleshy, when they are brought into 

 he sun, after having been kept for st^nie time in the dark. 



