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organic fibres. He found that, in suspending weights to threads of the same diameter,
Silk supported a weight equal to 34
7 That even the most delicate woody tissue consists of tubes, may be readily seen by examining it with a high magnifying power, and also by the occasional detection of particles of greenish matter in its inside. A very different opinion has nevertheless been held by some physiologists, who have thought that the woody tissue is capable of endless divisibility. “ When,” says Duhamel, “ I have examined under the microscope one of the principal fibres of a pear tree, it seemed to me to consist of a bundle of yet finer fibres; and when I have detached one of those fibres, and submitted it to a more powerful magnifying power than the first, it has still appeared to be formed of a great number of yet more delicate fibres.” (Physique des Arbres, i. 57.) To this opinion Du Petit Thouars assents, conceiving the tenuity of a fibre to be infinite, as well as its extensibility. (Essais sur la Végétation, p. 150.) These views have doubtless arisen from the use of very imperfect microscopes; under low powers of which such appearances as Duhamel describes are visible; but with modern glasses, and after maceration, each particular tube can be separated with the greatest facility. Their diameter is often very much less than that of the finest human hair; the tubes of hemp, for example, when completely separated, are nearly six times smaller. It must, however, be observed, that the fibres of this plant, as used in linen-making, are by no means in a state of final separation, each of the finest that meets the naked eye being in reality a bundle of tubes. While some do not exceed zooo of an inch in diameter, others have a diameter as considerable as that of ordinary cellular tissue itself; in Coniferæ the tubes are often do or sho, and in the Lime they average about išo. Link states (Elementa, p. 85.) that they are very large in trees of hot countries. The sides of woody tissue become thickened, as they advance in age, by the successive deposits of layer after layer in their interior (see fig. 1.); this is particularly observable in the liber, and hence, perhaps, the reason why the toughest kinds of fibre are obtained from that part.
There are two distinct kinds of Pleurenchyma :
1. That in which the walls are not occupied with either granules or glands sticking to them, or in which the former are of very rare occurrence. (Fig. 7.) This is the finest and the commonest of all; and is also the most genuine state of woody tissue.
2. The second kind of woody tissue is the glandular. This has hitherto been examined chiefly in Coniferæ, in which it uniformly occurs. Its dimensions are more considerable than that of the last-mentioned form, and it has been described as perforated with pores. The markings of the tubes are vesicular, and usually transparent, with a darkened centre (Plate II. fig. 3.), which last is what has been described as a pore, the vesicle itself being considered a thickened rim. fig. 8.
Kieser figures the glands as pores in Pinewood (fig. 8.), in Ephedra, and other cases. They may be most conveniently found by examining with a microscope a thin shaving of common Pinewood
(Pinus Strobus), when they will be seen in the form of transparent globules, having a dark centre, and placed upon the walls of the pleurenchyma.
The structure of coniferous glands has of late attracted the attention of many anatomists, and, at last, Professor Mohl seems to have discovered their real nature. He states them to be circular spaces, thinner than the rest of the tube, and placed opposite each other, so that when the walls are examined by transmitted light they appear more transparent than the rest of the tube. A plan of this structure is given in a cross section of two tubes in Plate II. fig. 4., where a a represent depressions in the centre of the elevation, which depression is supposed to cause the appearance of a central pore. With patience sections may be obtained so as to show the glands in profile, and then they are seen to project considerably above the wall of the tube. In a specimen of Pinus Strobus I have found them surrounded with an irregular elevated rim, as at Plate II. fig. 7. and 8., as if the lining of the tube was growing over them. It is not at all uncommon to find them in what may be supposed a nascent state, merely looking like tumours, with a pore in the middle, as is shown at Plate II. fig. 5.
If the disks of coniferous wood are examined with a good eighth of an inch object glass, and a low ocular, they will be distinctly seen marked with concentric circles as represented at Plate II. fig. 6. The highest oculars with a lower objective will not separate the circular lines. M. Valentin, who first noticed them (Repertorium, vol. i. t. i.), considers them to be the projecting edges of numerous layers of woody matter concentrically deposited round a space which they gradually close up, except a narrow opening into an air chamber, the layer next the centre being the youngest. I have not succeeded in obtaining any section which will show this structure.
It has been imagined that this glandular Pleurenchyma is confined to Gymnosperms, but Dr. Brown long since remarked it in Tasmannia, and Mr. Griffith finds it common in aromatic trees. At Plate II. fig. 20. is a view of it as I see it in Sphærostema.
The nature of the disks has been examined by M. Guillemin, who, in a paper laid before the Academy of Sciences, Dec. 19, 1836, considers them to be tumours, and calls them Edemata. He supposes them to be flattened vesicles, the central circle being either a pore or minute cell; and he imagines them to be filled with a colourless volatile oil, which changes to turpentine when it has been exuded from the central luminous point. He also adverts to the existence of similar appearances in aromatic woods, especially Drimys chilensis, but says they are not to be confounded with @demata. (Comptes Rendus, iii. 761.)
Pleurenchyma constitutes a considerable proportion of the ligneous part of all plants ; it is abundant in liber, and forms the principal part of the veins of leaves, to which it gives stiffness and tenacity.
Sect. IV. Of Vascular Tissue, or Trachenchyma.
This consists of simple membranous tubes tapering to each end, but often ending abruptly, either having a fibre generated spirally in the inside, or having their walls marked by transverse bars arranged more or less in a spiral direction.
Such appears to me to be the most accurate mode of describing this kind of tissue, upon the exact nature of which anatomists are, however, much divided in opinion; some believing that the fibre coheres independently of any membrane, others doubting or denying the mode in which the vessels terminate; some describing the vessels as ramifying; and a fourth class ascribing to them pores and fissures, as we have already seen has been done in cellular and woody tissue. It will be most convenient to consider all these points separately, along with the varieties into which vascular tissue passes.
There are two principal kinds of vascular tissue; viz. spiral vessels (Plate II. fig. 3. b. 9. 11.), and ducts (Plate II. fig. 12. c. f. 15, 16. 18. 20.)
SPIRAL VESSELS or TRACHEÆ are membranous tubes with conical extremities; their inside being occupied by a fibre twisted spirally, and capable of unrolling with elasticity. To the eye they, when at rest, look like a wire twisted round a cylinder that is afterwards removed. For the purpose of finding them for examination, the stalk of a strawberry leaf, or a young shoot of the Cornus alba (common dogwood) may be conveniently used; in these they may be readily detected by gently pulling the specimen asunder, when they unroll, and appear to the naked eye like a fine cobweb.
Very different opinions have been entertained as to the exact structure of spiral vessels. They have been considered to be composed of a fibre only, twisted spirally, without any connecting membrane; or to have their coils connected by an extremely thin membrane, which is destroyed when the vessel unrolls; or to consist of a fibre rolled round a membranous cylinder; or even, and this was Malpighi’s idea, to be formed by a spiral fibre kept together as a tube by interlaced fibres. Again, the fibre itself has been by some thought to be a flat strap, by others a tube, and by a third class of observers a kind of gutter formed by a strap having its edges turned a little inwards. Finally, the mode in which they terminate, has been asserted to be a continuation of cellular tissue.
With regard to the presence of an external membrane within which the spiral fibre is developed, an examination of it externally, by means of longitudinal sections of the surrounding parts, is scarcely sufficient to settle that point. The best mode of examination is to separate a vessel entire from the rest of the tissue, which may be done by boiling the subject, and then tearing it in pieces with the points of needles or any delicate sharp instrument: the real structure will then become much more apparent than if the vessel be viewed in connection with the surrounding tissue. From some beautiful preparations of this kind by Mr. Valentine and Mr. Griffith, it appears
that the membrane is external : in the root of the Hyacinth, for example, the coils of the spiral vessel touch each other, except towards its extremities; there they gradually separate, and it is then easy to see that the spiral fibre does not project beyond the membrane, but is bounded externally by the latter, which would not be the case if the membrane were internal: a representation of such a vessel is given at Plate II. fig. 9. Another argument as to the membrane being external may be taken from the manifest analogy that a spiral vessel bears to that form of cellular tissue (p. 16.), in which a spiral fibre is generated within a cellule: it is probable that the origin of the fibre is the same in both cases, and that its position with regard to the membrane is also the same. Sections, moreover, may be obtained through the centre of spiral vessels, and then it is manifest that the fibre is internal, because it projects beyond the inside of the vessel, at every turn.
It is more difficult to determine whether the fibre is solid, or tubular, or flat like a strap; and Amici has even declared his belief that the question is not capable of solution with such optical instruments as are now in use. When magnified 500 times in diameter, a fibre appears to be transparent in