PROPULSION OF THE SAP. 51 



or subdivided. " To these vessels/' says Mr. 

 Knight, " the spiral tubes are every where 

 appendages." p. 536. By this expression, 

 and by a passage in the following page*, 337, 

 this writer might seem to consider the spiral 

 line, which forms the coats of these vessels, 

 as itself a pervious tube, or else that he was 

 speaking of other tubes with a spiral coat, 

 companions of the sap-ve3sels ; but the plate 

 which accompanies his dissertation, and the 

 perspicuous mode in which he treats the sub- 

 ject throughout, prevent our mistaking him 

 on the last point. In order to conceive how 

 the sap can be so powerfully conveyed as it 

 is through the vessels in which it flows, from 

 the root of a tall tree to its highest branches, 

 we must take into consideration the action of 

 heat. We all know that this is necessary to 

 the growth and health of plants ; and that it 

 requires to be nicely adjusted in degree, in 



* te The whole of the fluid, which passes from the wood 

 to the leaf, seems to me evidently to be conveyed through 

 a single kind of vessel ; for the spiral tubes will neither 

 carry coloured infusions, nor in the smallest degree re- 

 tard the withering of the leaf, when the central vessels 

 are divided.' Knight. 



e2 



