Essays on the Microscope / Containing a Practical Description of the Most Improved Microscopes, a General History of Insects, etc., etc.

by George Adams

When the bark and the blea are taken away, we come to the wood, which is a solid substance, on which the strength of the tree depends, and which has been considered by naturalists as being to the tree what bones are to the animal. The wood, in a general view may be considered as formed of strata, which are inclosed one within the other; these strata consist of ligneous fibres or lymphatic vessels, the cellular web or tissue, vasa propria, and what have been called the air vessels. It is more difficult to investigate the construction of the wood than that of the other parts, because the texture is in general much harder, and therefore not so easily separated, requiring very long macerations, and many subjects, before one may be found fit for examination.

If a transverse section of almost any kind of wood be examined, we shall perceive these strata very clearly and sensibly distinguished from one another. It has been generally supposed that each of these is the product of one year’s growth; though, if we cut the same wood obliquely, it will be found that each of these strata is compounded of smaller ones, which are therefore not so easy to discover as the larger. By macerating rotten pieces of trees, the wood may be divided into an immense number of leaves or strata, thinner than the finest paper.

If the foregoing strata be examined in their detached state by the microscope, we shall find them to be composed of longitudinal fibres; some pieces of rotten wood, after maceration, will divide of themselves into very fine longitudinal fibres; the existence of these is further proved by the facility with which wood may be split in the direction of these fibres. From hence we may collect, that the ligneous strata are formed of small fibres or vessels, collected together in fascicles, like the bark: in some trees they are parallel to each other, in others they are disposed more obliquely, crossing and forming an irregular kind of network. There is great probability that this reticular disposition exists in all trees, though it may be difficult to discover it in many on account of the fineness of the meshes, the hardness of the wood, and the sameness of colour in the constituent fibres.

We are here only speaking of the lymphatic vessels or ligneous fibres of the wood, which exist in it as well as in the bark, though in different states; for the ligneous fibres are always harder and less flexible than the cortical ones. Malpighi thinks they differ in another particular, namely, that a juice or fluid issues from the cortical fibres, while none is found in those of the wood. In this it would appear from the observations of Du Hamel, that he was mistaken.

A transverse section of wood generally appears formed of a number of rays proceeding from the corona to the bark, which are intersected at different distances by concentric circles, interspersed with vessels of varying magnitude: the variations in this structure afford much pleasure to the curious observer, and throw considerable light upon the nature and properties of timber; for it is by means of a variety of strainers that different juices are prepared from the same mass. Matter, considered as matter, has no share in the qualities of bodies. It is from the arrangement of it, or the recipient forms given to it, that we have so many different substances. According to the modifications that these receive, we shall find the same light, air, water, and earth, manifesting themselves in one by a deadly poison, and in another by the most salubrious food. A lemon ingrafted upon an orange stock, is capable of changing the sap of the orange into its own nature, by a different arrangement of the nutritive juices. One mass of earth will give life and vigour to the bitter aloe, to the sweet cane, the cool house-leek, and the fiery mustard, the nourishing grain, and the deadly night-shade.

The wood may be considered as composed of two parts, ligneous and parenchymatous. The former has already been treated of; the latter is that which is disposed into rays, running as it were between the ligneous fibres, and interweaving with them; it originates either with the pith or corona. There is a very great diversity in these radial insertions; in some trees there are very few, while they abound in others; in some they are very fine, in others very thick. In texture, they seem similar to the blebs of the bark, only that here they are so crowded and stretched out as to appear like parallel threads, somewhat similar to a net when drawn tight.

OF THE CORONA.

Dr. Hill gives this name to that circle which surrounds the pith, and separates it from the wood; although in his opinion it differs greatly from both, and in its composition has no resemblance to either. It is, according to him, the most important part in the whole vegetable fabric, by which the propagation and increase of the branches, buds, and shoots, are carried on.[137]

[137] Hill on the Construction of Timber, p. 55.

It has been usual to suppose the pith of vegetables to be the part in which these wonderful sources of increase reside, but this is not the case; and he asserts, that so far from being prior to the other parts, it is in reality posterior to some of them in the time of its formation.