256 PHYSIOLOGY OF TREES. 



vessels and organs already there : and as to the 

 second, it is well known, that the vital speck or 

 rudiment of the chick is formed in, and proceeds 

 from the oviary of the hen, and (Uke the spawn 

 of fishes and reptiles) only requires the vivifying- 

 influence of the male to excite it into life and 

 perfect form during incubation. 



But there are other difficulties attending this 

 supposed generative property of the sap. As 

 accretions take place over the whole plant, and 

 as the leaves are the supposed laboratories of 

 this organisable matter, means must be invented 

 for the conveyance of it from them to the lower 

 parts and roots of the plant. For this purpose 

 descending vessels are necessary, i. e. ducts 

 which allow the prepared sap to sink somehow 

 or other into the inferior parts of the system. 

 Hence the trunk of a tree must be conceived to 

 consist of a most complicated tissue of vessels, to 

 permit the counter currents of the simple and 

 matured sap. Indeed, so occult are the pro- 

 cesses, that it cannot be explained without 

 recourse being had to almost all the powers of 



