ITS 



ter they had remained sixteen hours in the air, no ef- 

 fect was produced on it. The leaves were fresh ga- 

 thered, and no decay could be observed in any part of 

 them *. Now, as the only difference in the state of 

 these leaves consisted in their removal from the 

 plant, and consequent cessation of the flow of the 

 sap, by which the usual changes on the air were pre- 

 vented, it is to that cessation arising from such re- 

 moval, and not to any other alteration in the struc- 

 ture or condition of the leaves, that this prevention 

 of change is to be ascribed. But this flow of the 

 sap is a proof of the existence of living action in 

 plants, and by its means all the vegetable secretions- 

 are carried on : whence it follows, that as the leaves 

 act upon the air, and consequently emit carbon only 

 while this flow continues, its emission is the result 

 thereof, and consequently of a living action also. 



141. By smearing over the upper surface of the 

 leaves with varnish or oil, a mechanical obstruction 

 is furnished to the emission of carbon, and the plant 

 dies (26.) just as when the circulation itself is cut off, 

 little or no change, it is believed, being then produ- 

 ced on the oxygen gas of the air. Since then the 

 leaves alone effect changes on the air, and these on- 

 ly while attached to the stem ; and since these 

 changes cease, either when the circulation of the sap 

 is suspended or cut off, or a mechanical obstruction 

 is furnished to the escape of the carhon ; we must 

 conclude .that the carbon, thus emitted by the leaves, 

 is in truth a vegetable excretion, dependent, like 



Nicholson's Journal, July 1 802, p. 159. 



