NUTRITION. 39 



duced on the inner side of that previously 

 formed. The younger and more flexible por- 

 tion is called the Liber, and is deposited on the 

 alburnum of the wood ; the older layers are 

 pushed outwards, and are the cortical layers, or 

 true bark : they represent in the bark, what the 

 heart wood is in the central portion, but with 

 this great difference, that the woody layers 

 being deposited beyond each other in the order 

 of their formation, remain perfectly entire ; 

 while the layers of bark, acquiring fresh zones 

 from within, undergo considerable distension 

 thus, although the number of cortical layers 

 equals those of the wood, their fate is very dif- 

 ferent: those of the bark, distended by the 

 growth of the tree after the first year, always 

 present more or less flexuous fibres, and this 

 tendency augments with age, while on the con- 

 trary the fibres of the wood continue straight 

 and rigid. The woody layers remain in the 

 state of alburnum till they have acquired their 

 proper hardness, the layers of bark on the 

 contrary, soon lose their freshness, and never 

 attain the same degree of solidity. The first, 

 placed beyond the reach of atmospheric influ- 

 ence, preserve the appearance of life ; the lat- 

 ter, exposed to the action of the air and light, 



