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 VESSELS WHICH ARE CONTAINED BETWEEN THE RIND AND THE BARK.

These are called by Du Hamel the cellular coat, enveloppe cellulaire; by Hill, the exterior vessels, and the vasa propria exteriora.

It has been already observed, that in trees the juice vessels, or vasa propria, do not form those constituent parts of the wood of which the timber consists, but that it is from the nature of these recipient vessels that it derives its virtues, qualities, and specific properties.[133] A tree may grow, live, and give shade without them; but on those its peculiar character and decided virtues depend; these are greatest where the vasa propria are largest or most numerous; and where we do not find these, we scarce find any thing that will affect the taste or the smell. There are different ranges of these vessels between the several parts, each of which has its allotted place, its peculiar form, its different structure, and its separate use. Many trees have them in all their parts, others only in some of them, while others do not exhibit any.

[133] Hill’s Construction of Timber, p. 73.

On taking off the rind, we find a substance of a deep green colour, succulent and herbaceous, formed of a prodigious number of filaments interwoven together in various directions; it is more abundant in some trees than in others, particularly in the elder, and more succulent in summer than in winter; it is then also less adherent to the rind. Dr. Hill thinks the best time of separating the rind, in order to view this part, is in a living branch, at the time of its swelling for the spring, or for the midsummer shoot, but much easier by the means of maceration.[134]

[134] Hill’s Construction of Timber, p. 75.

When the rind is perfectly separated, it leaves the vasa propria of this class behind it; they scarce adhere to the inner bark, and but little to the rind; they are disposed in packets, and do not run straight down the branch, but interweaving with each other, form a kind of net. These packets may be separated easily from the bark; when a thin transverse section of one of them is examined, it is found to be composed of twelve or fifteen distinct vessels with hard rinds. Dr. Hill says, that with a great deal of patience, a vast number of objects, and a good microscope, we may see by what means these vessels adhere to the bark; for we shall find upon the sides small oval depressions which fit thereto, and that are probably a kind of glands, that separate from the general store of sap, with which the bark is filled, the juices peculiar to these vessels.