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

by George Adams

OF THE PITH.

The pith is found in the center of every young shoot of a tree; it is large in some, less in others, but present in all. It is placed close within the corona.

It seems to be nothing more than a congeries of the cellular tissue; it is generally found near the center of the tree, inclosed as it were within a tube; in general, the cells of the pith are larger than those of the cellular tissue, with which, according to Du Hamel, it communicates. For the rays which extend from the pith to the bark are, in his opinion, produced from it. Thus, though it may differ in name from the parenchymatous parts of the bark, and the radial insertions in the wood, yet it is of the same nature and texture, and is continuous with them; so that, according to this idea, the skin, the parenchyma, the insertions, and the pith, are all one piece of work, filled up in divers manners with the vessels.

The bark and the wood grow thicker every year, while the pith, on the contrary, grows more slender, so that in a branch of one year it is of a larger size than it is in the same branch when two years old, and so on. In very young branches, while in an herbaceous state, the pith forms the greatest part of its substance; but when the fibres are stronger, the pith becomes less succulent, and surrounded with a tube of wood; when the branch has arrived to a certain age, it is so compressed as to be almost annihilated. In examining different branches that proceed from others in their first state, a small communication between the pith of the one and the other will be found; but this communication is generally entirely closed up in the second or third year.[138] The cells of which the pith is formed are at first entirely one connected body; but as the plant grows up, it is often so broken and ruptured, as to remain no longer a continuous substance.

[138] Du Hamel Physique des Arbres, tom. 1, p. 38.

This, as well as many other particulars in the history of the pith, corroborates the opinion of Dr. Hill,[139] who thinks it is formed for the purpose of moistening the clusters of the corona, and regulating its extension; it has been supposed coeval with, or primordial to all the other parts, but he thinks it is postnate, and comes after them in the order of time, as well as in its uses; that exhaled air gives origin to its blebs, and the thickness of the juices cloathing the bubble, gives it form and substance. The first season is the time of its greatest use, and it immediately after begins to decay.

[139] Hill’s Construction of Timber, p. 66.

The pith has in general been represented as much more complex than it really is. It consists of a range of bladders lying one over the other. The membrane is simple, the outline single; but as it is very difficult to procure it in this simple state, it is often seen and represented under a variety of irregular, though pleasing forms, which are occasioned by the intersections of the outlines of the blebs, as seen one over another.

A cluster in any part of the corona, protruding itself onward and outward in the growing season,[140] carries a part of the circle out with it. The cluster itself is a perfect piece of the wood and blea, and the bark which follows it out in its progress perfectly clothes it; thus is the first protrusion of the shoot made, but all this while there is no pith. The continuation of growth is made by the extension of all the parts obliquely upwards; in the course of this extension they hollow themselves into a kind of cylinder, of the form of the future branch, and by this disposition a small vacancy is made in their center. This enlarges as they increase, and as it enlarges it becomes filled with the exudation of those little bladders which remain and constitute the pith, fed from the inner coat of the pith, which already begins to form itself into a new corona. Grew seemed to think, that in some instances the pith was of posterior growth to the other parts, and derived its origin from the bark; and that the insertions of the bark running in between the rays of the wood meet in the center, and constitute the pith.

[140] Hill’s Construction of Timber, p. 99.