[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.
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.