G4 OF THE SAP, 



that constant propulsion of it going on in every growing 

 plant, about which so much has been said in the pre- 

 ceding 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 presently revives, 

 for those vessels require a constant supply from the 

 root. 



This JJoxvmg of the sap has been thought to demon- 

 strate a circulation, because, there being no leaves to car- 

 ry it off by perspiration, it is evident that, if it were at 

 these periods running up the s^^p-vessels with such ve- 

 locity, 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, compar- 

 atively very small. Their secreted fluids run much 

 more abundantly. 



I conceive therefore that this Jlowing is nothing more 

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

 irritability of the vegetable body at the times above men- 

 tioned ; 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. Accordingly, lig- 

 atures 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 vegetable circulation. 

 This great facility in the sap to run is the first step to- 

 wards the revival of vegetation from the torpor of win- 

 ter ; and its exciting cause is heat, most unquestionably 

 bv the action of the latter on the vital principle, and 



