54 MR. KNIGHT'S EXPETtTMRNTS. 



plant, being absorbed by the root and become sap, are 

 curried up into the leaves by these vessels, called by him 

 central vessels, from their situation near the pith. A 

 particular set otthem, appropriated to each leaf, branches 

 off, a few inches below the leaf to which they belong, 

 from the main channels that pass along the alburnum, 

 and extend from the fibres of the root to the extremity 

 of each annual shoot of the plant. As they approach the 

 leaf to which they are destined, the central vessels be- 

 come more numerous, or subdivided. " To these ves- 

 sels," says Mr. Knight, " the spiral tubes are every 

 "where appendages." /i. 336. 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, O'- else 

 that he was speaking of other tubes with a spiral coat, 

 companions of the sap-vessels ; but the plate which ac- 

 companies his dissertation, and the perspicuous mode in 

 which he treats the subject 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 con- 

 sideration 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 order to suit 



* « 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 retard the withering of the 

 leaf, when the central vessels are divided." Knight. 



