On the Flciiithuj of Forest- Trees. 2G5 



bark, will be seen a thin transparent membrane, called the epi- 

 dermis ; under this, a green pulpy vascular matter, which in the 

 root is colourless; and then ihe true bark or liber, consisting of 

 tubes and cells, somewhat resembling the sap-wood beneath. 

 The wood consists of two parts, the sap-wood and the heart; these 

 are of the same nature, only that the sap-wood, or outer part, is 

 whiter and less firm in texture, and is that part through which 

 the greater portion of the sap is transmitted to the branches and 

 leaves. Straight lines of apparently a more compact substance 

 radiate from the centre to the bark ; these are collectively called 

 the silver grain, which differs considerably in the different kinds 

 of trees, and presents to the curious in timber, a means, by which 

 one kind much resembling another, may be readily distinguished 

 in the dry state. If a small branch or twig of the same tree be 

 divided in the same manner, the like appearance will be presented, 

 except that there will be no heart-wood, and the centre will be 

 occupied by a soft substance called the pith, which, though ori- 

 ginally visible in the stem of the sapling tree, disappears from that 

 part as the tree advances in age. 



If a very thin transparent slice be taken off the twig and placed 

 under a powerful microscope, the pores, or interstices of the cells 

 and vessels, both in the wood and bark above described, will be 

 distinctly visible. Before speaking of leaves, it may be proper to 

 premise that the atmosphere contains about 1 • 1000 part of its 

 weight of carbonic acid gas, consisting of carbon united with 

 oxygen in the proportion of 6 parts of the former to 16 parts of 

 the latter. This gas arises from the combustion and putrefaction 

 of vegetable and animal substances, and the breathing of animals ; 

 and if it were allowed to accumulate, the air would be rendered 

 unwholesome and unfit to support life. It is one of the most im- 

 portant offices of the leaves to separate the carbon from the 

 oxygen, by which the latter is set at liberty, and passes again into 

 the air, while the carbon, uniting with the elements of water in 

 the leaves, forms the true sap which produces the wood, or at 

 least about 95 parts in 100 of that substance. The other 5 parts, 

 which constitute the ashes when wood is burnt, consist of earthy 

 and alkaline salts, which are taken up from the earth by the roots 

 in a state of solution. Thus the leaves purify the air, while they 

 abstract from it a large portion of the substance of the tree. It 

 appears, therefore, that the leaf is the most essential of the organs 

 of vegetation, if indeed "any preference in point of utility can be 

 claimed, where all are dependent upon each other. This curious 

 organ, as was noticed before, is composed of the tubes with their 

 accompanying cells^ which ascend from the trunk and branch out 

 in every direction, constituting what are commonly, but impro- 

 perly, called the nerves of the leaf. The spaces between these 



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