SECT. II. COMPOSITE ORGANS. 231 



But in consequence of the increase of the trunk Conver- 

 by means of the regular and gradual addition of burnum 

 an annual layer, the layers whether of wood or of mtowood ' 

 bark are necessarily of different degrees of solidity 

 in proportion to their age ; the inner layer of bark, 

 and the outer layer of wood, being the softest; and 

 the other layers increasing in their degree of soli- 

 dity till you reach the centre on the one hand, and 

 the circumference on the other, where they are re- 

 spectively the hardest, forming perfect wood or 

 highly indurated bark, which sloughs or splits into 

 chinks, and falls off in thick crusts, as in the Plane- 

 tree, Fir, and Birch. What length of time then 

 is requisite to convert the alburnum into perfect 

 wood, or the liber into indurated bark ; and by what 

 means are they so converted ? 



There is no fixed and definite period of time 

 that can be positively assigned as necessary to the 

 complete induration of the wood or bark, though 

 it seems to require a period of a good many years 

 before any particular layer is converted from the 

 state of alburnum to that of perfect wood ; and 

 perhaps no layer has received its final degree of 

 induration till such time as the tree has arrived at 

 its full growth. But this is not a subject of any 

 peculiar importance. It is a subject of some im- 

 portance, however, to trace the cause of the indu- 

 ration of the wood, concerning which there seems 

 to have existed, or to exist now, some diversity of 

 opinion. 

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