BARK. 1^ 



action of external influences. In certain trees, the cuticle is covered 

 with wax or resin. The most remarkable example of this kind 

 which can be quoted, is that of the wax-tree (ceroxilon andicola) 

 which grows abundantly upon the slopes of the Andes. This tree, 

 (a palm,) which attains a height of between 130 and 164 English 

 feet, is covered over the whole surface of its trunk with a mixture 

 of wax and resin.* In gramineous plants, the epidermis is almost 

 entirely formed of silica. The bark of the birch-tree is covered 

 with a pellicle of an unctuous nature, capable under the agency of 

 nitric acid of yielding a peculiar suberic (the) acid.f 



After the epidermis, in going from the circumference towards the 

 centre, a layer of cellular tissue appears, which is designated by 

 many physiologists under the name of the herbaceous envelope. In 

 the cork oak, the cork represents the tissue by which the liber or 

 true bark is covered, an organ formed of a vascular tissue, which 

 with care can be separated into numerous very thin flakes or layers, 

 which have been aptly compared to the leaves of a book. 



The origin of the liber, or bark, is found in the most central part 

 of the trunk ; it is the result of the exudation of the woody parts, as 

 Duhamel, with the same wonderful sagacity which characterizes all 

 his works, has proved. Having cut away a portion of the bark of 

 a tree in full vigor, and taken care to preserve the incision from 

 contact with the air, he perceived that from the surface of the wood 

 laid bare, and the edges of the bark adhering to it, a viscous mat- 

 ter exudes, which accumulates, acquires consistency, and ends by 

 becoming cellular, thus regenerating the liber which had been taken 

 away. Grew called this viscous secretion cambium, a title which it 

 still retains. It is now generally admitted that cambium proceeds 

 from the descending sap. 



The liber is a very important organ in vegetables ; we know 

 for instance that it is necessary for the success of a graft that its 

 liber penetrate or be penetrated by that of the tree on which it is 

 grafted. 



The woody layers are situated under the liber. Those which are 

 at the greatest distance from the axis of the trunk, although they 

 present the fibrous structure, and the principal characteristics of the 

 woody tissue, still differ from it in being less hard and less tena- 

 cious ; this zone, which at the first glance is easily distinguished 

 from the wood properly so called, is the alburnum, the soft or false 

 wood. Its fibres are much looser, and its color paler than that of 

 the wood beneath it, the difference of shade being particularly ap- 

 parent in the dye and deeply colored woods. 



The alburnum becomes harder and tougher with age, and passes 

 into the woody tissue, the duramen or hard wood, properly so called. 

 The wood begins where the alburnum terminates, and reaches to 

 the centre, to the pith or medullary canal. 



In dicotyledonous trees, a certain quantity of wood is formed 



•_ Boussingault, sur le Palmier a cire. Annales ^ Climie et de FhysiquA 3* iitie^ 

 the observations of M. Chevreul. 



t 59, p. 19. 



t ftOBt 



