78 



THE BARK OF TREES. 



the tree is sufficiently mature, incisions are made from the top to the bottom of the 

 stem, so that the cork may be more quickly removed. The sheets are then placed 

 upon the ground to flatten, and are at length cut up into convenient lengths for packing. 

 The tree will permit this process to be renewed during seven or eight successive years. 

 The cause of the exfoliation has not well been determined. It certainly does not 

 depend upon the growth of the tree, as though the increased size of the stem caused 

 the bark to rupture and thence to fall off; but it is said that a layer of tabular cells 

 are formed within it which cuts off its communication with the internal structure of 

 the stem, and thence it dies. No doubt can exist as to the fact of the constant destruc- 

 tion of the old bark and the formation of new structures, and it appears to arise either 

 from the death of the external layers only, or from the formation of cork on the inner- 

 most layer of the bark, which causes an arrest of the circulation, and at length the 

 death of the more external parts. 



It is said that the bark of exogens is much more extensible than that of endogens ; 

 and that, as a consequence, the stems of the former exceed in diameter those of the 

 latter. But the fact just mentioned seems to prove that in fact the cellular part of the 

 bark of exogens possesses but little extensibility ; for, when the enlargement of the 

 trunk has proceeded but even to a moderate extent, the bark cracks off from a lack of 

 this power of extension. It is far more probable that the increasing size of the zone 

 of bark is less due to the extensibility of the old bark than to the formation of new cells 

 year by year as the stem enlarges, and in a layer at all times proportioned to the increasing 

 size of the stem in fact, that the old coat becomes too small, and rends, and a new one 

 is supplied of larger dimensions. It is quite clear that the external layers, after rupture, 

 either peel off, or the width of the rents increases as the tree grows larger. 



The Epiphlceum consists of several layers of thin flattened cells, usually without 

 colour. 



Thirdly, the Mesophlaum is a thin layer of green cells lining the epiphlseum, and, in 

 the cork tree, exfoliating with it. Its cells lie in a direction different from that of the 

 cells of the epiphlaeum, and sometimes contain cellular secretions. 



The vascular part of the bark is called the liber, from its offering a smooth enduring 

 structure, which was formerly used as paper (liber a book). It consists of several 



layers of small interlaced bundles of woody fibre, 

 connected together by loose cellular tissue. In some 

 trees, as the lace bark tree (Lagetta lintearia, Fig. 

 132), it resembles a textile fabric, and may be ob- 

 tained from the tree in sheets of large size. The 

 woody fibre of the liber has always the peculiarity 

 of being very strong, and of lying in small bundles, 

 and has been used as cordage by most nations. It 

 is still employed in Russia in the manufacture of 

 mats, and in many parts of the world for whip- 

 lashes. It is not equally smooth on both aspects ; 

 since, on its outer side, it has cellular connexion 

 with the mesophlaeuin, but on its inner surface it 

 is opposed to the smooth wood, or is covered by 

 the semi-fluid cambium. Its mesh-work character 



Fig. 132. Bark of the Lace Tree of 

 Jamaica, composed of fine and 

 loosely arranged bundles of woody 

 fibre. Natural size. 



permits the medullary rays to pass through it, and to keep up a circulation with the 

 cellular part of the bark. It is not subsequently converted into the wood of the tree, 



