OF THE SAP, AND INSENSIBLE PERSPIRATION. 49 



This great motion, called the flowing, of the sap, 

 which is to be detected principally in the spring, and 

 slightly in the autumn, is therefore totally distinct from 

 that constant propulsion of it going on in every grow- 

 ing plant, about which so much has been said in the 

 preceding chapter, and which is proved by taking an 

 entire herb of any kind that has been gathered and 

 suffered to begin to fade, and immersing its root in 

 water. By absorption through the sap-vessels it pre- , 

 sently revives, for those vessels require a constant sup- 

 ply from the root. 



This flowing of the sap has been thought to demon- 

 strate 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, 

 comparatively very small. Their secreted fluids run 

 much more abundantly. % 



I conceive therefore that l\i\s flowing is nothing more 

 than a facility in the sap to run, owing to the peculiar 

 irritability of the vegetable 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 conveyance. Accord- 

 ingly, ligatures made at this period, which show so 

 plainly the course of the blood in an animal body, 



E 



