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

by George Adams

1.	The Exterior		-	Juice Vessels.
2.	The Interior
3.	The Intimate
4.	The Sap Vessels.
5.	The Coronal.

Of these, the first are placed between the rind and bark; the second, in the substance of the bark; the third, in the substance of the blea; the fourth, in the substance of the wood; the fifth, in the corona.

More accurate instruments, or a more minute investigation of the parts, may probably discover new vessels in a system which appears to be entirely vascular, and brings us more thoroughly acquainted with the nature of vegetation.

OF THE RIND.

The exterior covering of all trees is a thin, dry, parched substance, which has been compared by many writers to the skin of animals, and called by names analogous thereto; thus it is called the epidermis by Duhamel, the skin by Grew, the rind by Hill.

When a tree is full of sap, this membrane may be easily detached from the part it covers; it may be separated from green branches which are not in sap, by boiling them in water; large pieces of it may also be obtained from rotten branches; the rind of the leaves of many trees is detached with singular dexterity from the other parts, by some of the mining caterpillars; artificial methods for effecting this purpose have been described in [page 160] of this work. Though the rind may at first sight be thought to be of little use, it will be found to be a principal organ in the process of vegetation. The part which covers the root has the most important offices assigned to it.

Many are of opinion that the rind is formed of dried vesiculæ; and Malpighi says, that we may see in the vascular texture of the bark of the cherry and plumb-trees an arrangement of the parts proper to form the rind, and this arrangement is occasioned by the endeavour of the vascular part to extend itself to the circumference, and the resistance it meets with from the rind; and that hence the vessels are flattened, and assume a membranaceous form.

The rind is a general covering to the young trunks of trees, to the branches, the roots, the leaves, the fruit, the flowers, &c. Upon the trunks of large trees some pieces only of the rind are to be found, having probably been broken by the increased size of the tree. The rind of some species of trees will bear being stretched much further than those of others, and remain for a considerable time uniformly spread over the bark. Du Hamel asserts, that the rind of vigorous healthy, trees remains longer whole than on those that are more languid, notwithstanding that the growth of the last is slower, and therefore makes less efforts against the rind. This circumstance is much in favour of the distinct organization of the rind, and against the opinion of those who only suppose it to consist of dried bladders.

Thin as the rind is, it is formed of many coats, adhering closely to each other, which in some species may be separated with ease, in others, with difficulty. Du Hamel says, that he has divided the rind of the birch into six distinct coats, and that he had no doubt but what the division might have been carried much further. Dr. Hill says, that unless some of these coats be obtained in a state of separation from the rest, the true construction of the rind cannot be discovered, for the connection and form of the parts are lost by the confusion in which they appear while they lie one upon another.

The following experiments may throw a little light upon this obscure subject.[130] All the rind was taken from the trunk of a cherry-tree, and the tree thus skinned exposed to the air; a part of the bark which was next to the rind dried up and exfoliated; the part next to this did the same; after two or three exfoliations, a farinaceous substance covered the superfice of the trunk, soon after which a new rind appeared. Some pieces of rind were taken from a few young branches, and the wounds were covered with a cloth that had been soaked in wax and turpentine; on these the rind appeared in a very little time, without any apparent exfoliation. From some other branches, not only the rind, but a part of the bark was also taken away, and the wounds covered as before; a slight exfoliation was observed here, which was soon followed with a new rind. The bark was taken entirely off from a vigorous cherry-tree, while it was in full sap, so that the wood appeared the whole extent of the trunk. This was protected from the rays of the sun, and from the air. A new bark and rind formed themselves upon the trunk, but they did not originate from the bark that was left on the branches and the root, but extended from different spots, which were first formed at considerable distances from each other. After a lapse of fifteen years, this new rind did not appear like the natural rind of the cherry-tree. From these experiments we learn, that the rind regenerates more readily in some cases than in others, and that it preserves and prevents in a degree the bark from becoming dry too soon, and in consequence thereof exfoliating.