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 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.

[130] Du Hamel Physique des Arbres, tom. 1, p. 12.

Aided by the microscope, a number of luminous points may be discovered in the rind;[131] these are so many minute holes for other purposes of transpiration. In the cane these holes are visible to the naked eye. A few oval holes may also be perceived in it; these are, however, no more than a separation of the parts, occasioned by the extension of the vasa interiora.

[131] Du Hamel Physique des Arbres, tom. 1, p. 9.

Dr. Grew supposed the rind to be formed of small vesicles, or bladders, clustered together, and intermixed with ligneous fibres or vessels, which run through the length of the rind; these are conjoined by other transverse ones, but that as the rind dries, the bladders or blebs shrink up and disappear. This account does not differ much from that of Dr. Hill, who says, that the rind is formed of a series of longitudinal vessels, and a filmy substance, between them, which, when viewed in a transverse section, form small circles, the sides of which are supported and made up of these longitudinal fibres; that the transverse vessels are only a deception, occasioned by the spaces between them and part of the film. The mode of obtaining an accurate view of the organization of this part, by conveying coloured liquors into the several vessels thereof, has been already described in [page 160] of these Essays; by these means, together with the microscope, we find that the vessels are everywhere pierced with small dots or openings: of the use of these, the following conjectures have been formed.[132]

[132] Hill’s Construction of Timber, &c. p. 37.

The root, which is equal in surface to a third part of the tree above ground, is covered with a pierced rind. The cold of winter contracts the whole of this, the parts are drawn closer together, and the mouths of these innumerable vessels are shut or nearly so, by this contraction; a very little of the half-congealed moisture of the ground gets into them, but this suffices for the service of the tree, when there is little heat to cause any perspiration, and at a time when in the deciduous trees, the very organs of the greatest perspiration, the leaves, do not exist.

The warmth of the spring arrives, the fluids of the earth grow thinner, every part of the root expands; this opens the mouths of the vessels, and the torrent of nutrition rushes in. By these means, every coat of the rind, and the interstitial spaces thereof, are rendered supple, and may be easily separated from the under coverings.

In roots, the colour of the rind varies very much, being white in some, brown in others, &c. Every root, according to Grew, after it has arrived at a certain age, has a double skin, the one coeval with the other parts, and exists in the seed; a ring is afterwards sent off from the bark, which forms the second skin; thus in the root of dandelion, towards the end of May, the original or outer skin appears shrivelled, and is easily separated from the new one, which is fresher, and adheres more firmly to the bark. Perennial plants are supplied in this manner with a new skin every year; the outer one always falls off in the autumn and winter, and a new one is formed from the bark in the succeeding spring.