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

by George Adams

It has generally been supposed, that each of those concentric circles, which are to be observed in the transverse section of almost every tree, was the product of one year, or the quantity of wood added to the tree in that space; here, however, Dr. Hill differs again from the general opinion.

From what has been said, we may deduce the following general ideas relative to the organization of trees. The most obvious and remarkable parts of a plant, or tree, are the root, the stem, the branches, the leaves, the flower, and the fruit. The component parts of these divisions are not complicated; they are simple when compared with those of an animal, and this because the offices of the vegetable are fewer than those of the animal.

The interior part may be considered as consisting of ligneous fibres, interspersed with a vast number of bladders, which are here named the cellular tissue, the vasa propria, and the sap vessels; though these are considered by some writers as mere air vessels.

The ligneous fibres are very fine tubes, proceeding nearly in a vertical direction from the top to the bottom of the tree; they are sometimes parallel to each other, sometimes they divaricate, and often leave oblong intervals or spaces. There is great reason for supposing them to be a species of lymphatic vessels. The vacant spaces between these fibres are filled up by a vesicular membrane, lying in an horizontal direction, and which is called in this chapter the cellular tissue.

The vasa propria are formed of ligneous fibres, but differ from the foregoing in their size, and in the juices which they contain. In the part properly called the wood, we meet with the sap vessels; but as in some states they seem as if they were formed of a silver-coloured spiral membrane, and are found without any juices, they have been supposed to be air vessels, and called the trachea, making up an arterial system, and supplying the place of the heart in animals.

The interior part of the tree may be further considered as divided into four principal concentric strata, the bark, the blea, the wood, and the pith; to these Dr. Hill has added the corona. Whatever part of a plant is examined, we find these and no more. The root, its ascending stalk, and descending fibre, are formed of one, and not three different substances. Thus the whole vegetable is reduced to one entire body. And what appears in the flower to be formed of altogether distinct parts, will be found to originate in these.

The bark, which is the exterior covering of the tree, is divided into two parts, a thin outer rind, and a much thicker inner one. The exterior one seems to be little more than a fine film of irregular meshes, the inner one composed of large blebs, leaving in some subjects large vacant spaces, which form its vasa propria. It is made up of several strata lying one over the other.

Next to this is the blea, which is of an uniform structure. It is an imperfect wood, waiting only for the hand of time to be brought to perfection. The duration of the blea in this middle state depends on the internal powers and strength of the tree, being so much shorter as this is more vigorous.

The wood, including the corona, comes next; it differs in density and duration both from the blea, the bark, and the wood. It is made up of strong fibres. The life of the vegetable seems to reside in it; from it all the other parts are produced. It shoots a pith inwards, and a blea and a bark outwards.

Every tree may be considered as consisting of numerous concentric strata or flakes, forming so many cones, inscribed one within the other, and whose number is almost indefinite. The most exterior contain the rudiments of the bark; the more interior, those of the wood. In the germ they are gelatinous, by degrees they become herbaceous, and in process of time assume the consistence of wood. Thus the stem, the root and the branch, may be considered as formed of a prodigious number of concentric vertical strata, each composed of different fascicles of fibres; which fibres are again formed of smaller ones. The spaces between these, and among the fibres, are filled up, interwoven with, and connected by the cellular tissue, of which the radial insertions are formed.