100 



FORMATION OF NEW LAYERS. BARK. 



from the fluid portion of the cambium as they descend ; and at 

 last they partly unite themselves with the vessels, &c. of the 

 new woody layer, and, in smaller proportion, with the tissue of 

 the bark. 



137. In some kinds of trees, the bark contains a great deal 

 of cellular tissue, and is therefore thick and spongy ; this is the 

 case with those that furnish cork, which may be regarded as a 

 sort of external pith. The inner layers, however, to which the 

 name of liber is given, are usually thin and delicate in their 

 texture, and have been applied to various useful purposes. One 

 of theseis indicated by the meaning of the term liber in Latin, 

 which signifies a book ; and thus in that language, a book, and 

 the inner bark of a tree, had the same name. The fact was 

 that, before the invention of paper, the inner bark was one of the 

 substances used by the Romans for the same purposes, as the 

 leaves of the papyrus (from which the term paper is derived) 

 were employed in Egypt. It is the liber of other trees, which 

 is used by the islanders of the Polynesian Archipelago, for cloth, 

 mats, sails, &c. A very beautiful kind of liber, is that obtained 

 from the Vegetable-Lace tree (as it is called) of Jamaica; when 

 its layers are unfolded, it has the appearance of a delicate lace. 



138. From what has been stated, as to the successive form- 

 ation of new layers within, and the gradual loss of those on the 

 exterior, it is evident that each layer of bark will in its turn be 



brought to the surface and be thrown 

 off. On the other hand, each layer 

 of wood is gradually being imbedded 

 more deeply. Hence it follows that, 

 if any substance be placed in the 

 newest layer of wood, it will gra- 

 dually be covered by others ; so that, 

 if the tree be cut down at any future 

 time, the number of years that have 

 FiG.so. VBUTICALSBCTIONOFATRBE, passed since it was imbedded, may 



THE LATERAL BRANCHES Of WHICH 11 i ji i 



HAD CONSECUTIVELY DIED, OR BEKN CUT be known, by counting the number 



WHEN THREE YEARS OLD. o f layers on its exterior. On the 



contrary, if the substance be not driven into the wood, but 



