PROrULSION OF THE SAP, 51 



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

 Knight, " the spiral tubes are every where 

 appendages." p. 3S6. By this expresj>ion, 

 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-vessels ; 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 convevcd as it 

 is thron2:h 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 heallh of plants ; and that it 

 requires to be nicely adjusted in degree, in 



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

 to tliC leaf, seems to mc evidently to be convcytd through 

 a single kind of vessel ; for the spiral tubes will ncillKr 

 carry coloured infusions, nor in the smallest degree re- 

 tard the withering of the leaf, when the central vessels 

 arc divided." Kn'/ghf. 



