LECT. VII.] ANATOMY OF STEMS. 357 



way to the innermost layer of wood, until that 

 has received its final degree of induration. I know 

 of no observations which tend to prove that the 

 concentric zones are diminished in thickness as 

 they increase in solidity and density, which would 

 be the case, did they shrink or suffer any com- 

 pression ; and if it be true that no such change 

 takes place, their progressive induration can be 

 explained only by admitting that there is a con- 

 tinual deposition of new matter. The truth of 

 such an opinion is further confirmed by the change 

 of colour, which the alburnum undergoes in pass- 

 ing into the state of perfect wood. With regard 

 to the period at which the wood attains its final 

 degree of induration, I would say> with Mr. 

 Keith, that perhaps no layer has acquired this 

 state until " such time as the tree has arrived at 

 " its full growth*." 



Returning to our shoot of Horse Chesnut, if 

 we scoop out the pith from the ligneous cylinder 

 that encloses it, we perceive this is lined with a 

 thin green layer or coating; which, to the un- 

 assisted eye, appears to resemble in its structure 

 rather the cellular integument of the bark than 

 any part of the surrounding wood. This is the 

 MEDULLARY SHEATH (etui mtfdullaire) of Mirbel 

 and the French Botanists : for, although it was 



* System of physiological Botany , vol. ii. p. 230. 

 AA3 



