THE RESPIRATION OF PLANTS. 69 



Later, the disengagement of carbonic acid ceases, that gas being 

 instantly absorbed by the chlorophyll as soon as it is produced in res- 

 piration. The plant has now entered the adult stage. It freely ab- 

 sorbs the carbonic acid of the air, under the influence of the sun's 

 rays, and gives ofi" oxygen. The phenomenon of respiration is at this 

 period completely masked, and cannot be shown to exist except by 

 indirect processes, as we shall soon see. 



On reading the column of figures headed " Carbonaceous Matters," 

 we find that, in September, they grow rapidly less. Further, M. Co- 

 renwinder has discovered that, toward the beginning of October, the 

 leaves exhaled a little carbonic acid in the daytime. Here the phe- 

 nomenon is not of the same nature as at the beginning of vegetation : 

 the yellow leaves are dying, and lose their carbon, like all decaying or 

 dead organic matters exposed to the air. When vegetation is at an 

 end, the proportion of carbonaceous matters seems to increase, owing 

 to the rapid disappearance of nitrogenous matters. 



From the facts established in the physiological and chemical ex- 

 periments we have described we derive a very probable explanation 

 of two phenomena, which at first view would seem to be mutually in- 

 compatible, viz., exhalation of oxygen and exhalation of carbonic acid. 

 The latter of these has its seat in the nitrogenized matters, and consti- 

 tutes the respiration of plants, which is henceforth to be esteemed the 

 same as the respiration of animals. The other phenomenon has its 

 seat in the chlorophyll. It has been wrongly held to be a respiratory 

 act : it is, in fact, a true digestion of carbon. 



VI. 



From what has been said, we may unhesitatingly conclude that 

 leaves in their earliest stage simultaneously perform two physiological 

 functions : 1. They respire by means of their nitrogenous constituents ; 

 2. They assimilate carbon by means of the carbonaceous matters or- 

 ganized in their tissues, i. e., the chlorophyll. 



This act of respiration, as we have seen, becomes less apparent as 

 the plant begins to assimilate carbon, but, in reality, it goes on unin- 

 terruptedly, being only masked by the increased activity of the other 

 function. That this is the case may be proved experimentally. 



In the first place, we know that plants cannot live in an atmos- 

 phere without oxygen. When they are placed in close vessels con- 

 taining hydrogen and nitrogen, they live for a while, owing to the 

 small quantity of atmospheric air in their cells ; but yet, though out- 

 wardly they may give no sign of decay, on being taken out of the ves- 

 sels they are found to be dead. The development of the buds ceases 

 utterly in an oxygenless atmosphere. On March 29, 1872, M. Coren- 

 winder placed a chestnut-bud in a test-tube containing atmospheric 

 air, the mouth of the tube being immersed in a solution of caustic pot- 

 ash. The bud formed carbonic acid rapidly, inhaling the oxygen of 



