38 NUTRITION. 



Alburnum, becomes covered by new woody 

 layers, and may be discovered unaltered: thus 

 Reisel found in 1675, some capital letters in 

 the centre of a beech tree.* The nourishment 

 of the tree being entirely 'performed by the 

 young or sap wood (the alburnum) is carried on 

 when age and decay have deprived it of its heart 

 wood. Thus we see the hollow trunk of an oak 

 or willow capable of sustaining large branches 

 and putting forth foliage almost as luxuriant as 

 when in its prime. 



29. The cortical system (or Bark) of Exo- 

 genes is organized in a manner analogous to 

 that of the central, or ligneous system every 

 stem acquiring a cortical, as well as a ligneous 

 zone annually; but while each fresh woody 

 layer is deposited on, and externally to, that of 

 the year before, each layer of the bark is pro- 



* There is a singular illustration of the manner in 

 which the older portions of a stem are completely en- 

 veloped in the later deposits of woody matter, to be 

 seen in a part of the stem of the Wellington Tree, pre- 

 sented to the British Museum by Mr. Children. A 

 chain had been passed round the trunk when it was a 

 sapling, and was so entirely buried in the layers of suc- 

 ceeding years, that it was only by the violent resistance 

 the chain made to the tools of the workmen who were 

 sawing the tree, that its existence was discovered. 



