64 OP THE SAP, AND INSENSIBLE PERSPIRATION. 



This flowing of the sap has been thought 

 to demonstrate a circulation, because, there 

 being no leaves to carry it off by perspiration, 

 it is evident that, if it were at these periods 

 running up the sap-vessels with such velocity, 

 it must run down again by other channels. 

 As soon as the leaves expand, its motion is 

 no longer to be detected. The effusion of 

 sap from plants, when cut or wounded, is, 

 during the greater part of the year, compa- 

 ratively very small. Their secreted fluids run 

 much more abundantly. 



I conceive therefore that this flowing is no- 

 thing more than a facility in the sap to run, 

 owing to the peculiar irritability of the ve- 

 getable body at the times above mentioned; 

 and that it runs only when a wound is made, 

 being naturally at rest till the leaves open, 

 and admit of its proper and regular convey- 

 ance. Accordingly, ligatures made at this 

 period, which show so plainly the course of 

 the blood in an animal body, have never 

 been found to throw any light upon the vege- 

 table circulation. This great facility in the 

 sap to run is the first step towards the revi- 

 val of vegetation from the torpor of winter ; 



