[ 224 j 



bark stripped off in spring, and which have been cut 

 in the autumn or winter following. The reason of 

 the superiority of this timber is, that the concrete 

 sap is expended in the spring in the sprouting of the 

 leaf; and the circulation being destroyed, it is not 

 formed anew ; and the wood having its pores free 

 from saccharine matter, is less liable to undergo fer- 

 mentation from the action of moisture and air. 



In perennial trees a new alburnum, and conse- 

 quently a new system of vessels, is annually produc- 

 ed, and the nutriment for the next year deposited in 

 them ; so that the new buds, like the plume of the 

 seed, are supplied with a reservoir of matter essential 

 to their first development. 



The old alburnum is gradually converted into 

 heart-wood, and being constantly pressed upon by 

 the expansive force of the new fibres, becomes harder, 

 denser, and at length loses altogether its vascular 

 structure ; and in a certain time obeys the common 

 laws of dead matter, decays, decomposes, and is con- 

 verted into aeriform and carbonic elements ; into those 

 principles from which it was originally formed. 



The decay of the heart- wood seems to constitute 

 the great limit to the age and size of trees. And in 

 young branches from old trees, it is much more liable 

 to decompose than in similar branches from seedlings. 

 This is likewise the case with grafts. The graft is 

 only nourished by the sap of the tree to which it is 

 transferred ; its properties are not changed by it ; the 

 leaves, blossoms, and fruits are of the same kind as if 

 it had vegetated upon its parent stock. The only ad- 



