PHYSIOLOGY OF TREES. 249 



layer is formed of the radkles which descend 

 from the superior buds between the wood and the 

 bark, which uniting- with the converging; rays in 

 their passage down, compose the new layer. 

 Differing from all these, another idea has been 

 entertained, which supposes the living principle 

 to be a distinct organ of itself, always situated 

 between the wood and the bark, and annually 

 divisible into layers of bark and wood for ever. 



Now, as one or other of these opinions are held 

 by many men of the first authority and respecta- 

 bility, as well in science as practice, it is worth 

 while to examine these opinions, in order to show 

 how far they are consistent or inconsistent with 

 the laws of nature, or agreeable to practical facts 

 and observations. 



With respect to the doctrine that all the accre- 

 tions of a plant are formed out of the matured 

 juice, it will be necessary to consider first the 

 nature of the sap itself. 



The sap of vegetables has been very justly 

 compared to the blood of animals. It is not an 

 organ, but a constituent only, which pervades, 



