40 PROPULSION OF THE SAP. 



page*, 337, this writer might seem toconsider 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 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 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 order to suit the constitutions of 

 <lifFerent tribes of plants destined for different parts of 

 the globe. It cannot but act as a stimulus to the living 

 principle, and is one of the most powerful agents of 

 Nature upon the vegetable as well as animal constitu- 

 tion. Besides this, however, various mechanical causes 

 may be supposed to have their effect; as the frequently 

 spiral or screw-like form of the vessels, in some of 

 which, when separated from the plant, Malpighi tells 

 us he once saw a very beautiful undulating motion that 

 appeared spontaneous. This indeed has not been seen 

 by any other person, nor can it be supposed that parts 



* " The whole of the fluid, which passes from the wood to the leaf, 

 seems to me evidently to he 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 ves- 

 sels are divided." Knight. 



