Art. XXII.—On the Structure of the Leaf ofPhormium Tenax.
(Read before the Auckland Institute, October 18, 1869.)
In the present paper an attempt has been made to describe the structure of the leaf of our native Flax, so as to form a basis for the examination and comparison of the manufactured fibre, as dressed by different machines, and prepared by different processes, on which subject I hope we shall receive communications from many of our members. I have also added some observations on the gum secreted by the leaf, and which is generally looked upon as the bête noire of the manufacturer, but which I believe not to be so black as it is painted. While it was in progress, I saw in the newspapers a short abstract
of a paper on the same subject, read by Mr. Nottidge, to the Philosophical Institute of Canterbury. Not having as yet had the pleasure of reading this paper, I am not able to refer further to it; but judging from the very short abstract that appeared in the papers, we seem to be pretty well agreed upon all points, except as to the existence of small canals in the fibro-vascular bundles, formed by the inter-spaces between the ultimate fibres, which, I venture to suggest, must have been caused by the knife tearing apart the bundles of fibres, when making transverse sections.
Development of leaf.—The young leaf of the Flax plant (Phormium tenax), when about a quarter of an inch in length, is composed of loose parenchy-matous tissue, covered both on the inner and outer surfaces, near the centre, by an epidermis of elongated rectangular cells, the edges of the leaf being crenulated by the jutting-out of the young growing cells. Imbedded in this cellular tissue lie about twenty-three vascular bundles, eleven on each side of the midrib. These, in this young state, are composed entirely of spiral-vessels, which contain a single spiral fibre, easily unrolled with a needle. These bundles are 0.0013 inch broad, and about 0.004 inch apart from one another. (Fig. I.) They lie longitudinally in the leaf, towards the central part of it, the outer growing edge being composed only of cellular tissue. The upper end of each bundle runs into that lying next to it towards the centre, and they thus get shorter and shorter as they recede from the midrib. When the leaf gets about an inch long, the vascular bundles are still seen to branch and anastomose with one another (Fig. II.); but this soon ceases, and at all later stages they are very nearly parallel, converging slightly towards the point of the leaf. A layer of thin elongated tubes now makes its appearance, surrounding the bundles of spiral vessels. This is the first state of the fibres from which the plant has got its celebrity; but as yet they are exceedingly tender, and break with the slightest touch. When, however, the leaf has attained a length of four inches, the fibres in the upper part of it have acquired considerable strength, while those at the base are still quite weak. When the leaf is about nine inches long, it commences to exude gum, and appears then to be fully formed.
Description of full-grown leaf.—When fully grown the leaf attains a length of from three to ten feet, according to soil and variety. The colour varies from light yellow-green to deep blue-green, with yellow or red margins and midribs, while the lower part of the leaf is usually pink. In shape the leaf is linear-lanceolate, and keeled, with an acute point. At a point rather less than half way down from the tip, the two superior or inner surfaces of the two halves of the blade begin to coalesce at the midrib, and this coalescence gradually increases until one-half of the leaf is joined together. The coal-escence then gradually decreases, until it occupies only about a fourth of the breadth of the half blade, which breadth it keeps to the bottom of the leaf, the two half blades being closely appressed as far as the base, where the two marginal edges closely overlap one another, and form a sheath through which the younger leaves grow. It is only the inner surfaces of the lower portion of the leaf, below the place where the coalescence of the two half blades begins to decrease, that exude gum.
For the sake of convenience, I shall, in this paper, call the upper part of the leaf, the blade; that portion where the coalescence of the two half blades reaches its maximum, and which is about half-way between the two ends of the leaf, the butt; and the lower portion, the base of the leaf.
The full-grown leaf is composed of parenchymatous tissue, in which fibro-vascular bundles lie imbedded, and remain isolated from one another as fibrous cords, some of which run from one end of the leaf to the other. This cellular tissue is covered, on both surfaces of the leaf, by an epidermis composed of
elongated, rectangular cells, of considerable consistence, but without chlorophyll, or other colouring matter.
No stomata or other openings are found on either surface, and the gum appears to be exuded by exosmosis.
The fibro-vascular bundles, which form what are commonly called the fibres of the plant, are composed of elongated tubular liber-cells, enclosing a centre of spiral vessels, and vary much in shape in different parts of the leaf. They are surrounded by a layer of elongated cells, that at once turn blue with iodine, and are probably cambium cells. The bundles are thickest at the base, and taper gradually to the point of the leaf.
The spiral vessels are about 0.001 inch in diameter, and contain a single, rarely a double, spiral fibre, which can be unrolled with a needle. They are filled with air, and have no strength, but are readily broken across.
The liber-cells, on the contrary, have great strength, and form the true fibre of the leaf. They are very long, probably an inch or more, while their thickness never exceeds 0.0006 inch, and is sometimes only 0.0003 inch.* They taper each way to a more or less blunt point (Fig. III.), each cell being distinct from the rest, and not joined together end-ways. Notwithstanding their strength they are of very delicate structure, colourless, translucent, and almost devoid of secondary deposits, the cell-wall being from 0.0001 inch, to 0.0002 inch, thick. They are hollow, and filled only with air, but are highly hygroscopic, imbibing water quickly, and during the operation twisting about in all directions. The fibres, although round when separated, are not so when collected into bundles (Fig. XI.), being then compressed into ovals, polygons, etc., so as to fill up the whole space, and leave no interstices. They have the same thickness in all parts of the leaf.
At the base of the leaf the vascular bundles are scattered through the cellular tissue, in about four irregular rows. They are arranged in ovals of two sizes; the largest, whose major axis is about 0.02 inch, and minor axis 0.014 inch, enclose, in their centres, a rhomboidal cluster of spiral vessels (Fig. X. s. v.); while the smaller, whose major axis is 0.015 inch, and minor axis 0.007 inch, contain none. Near the outer surface of the leaf four other irregular rows of fibre bundles, without spiral vessels, are found, whose section is lanceolate, thus making eight rows altogether in the base of the leaf. The fibres here are generally much weaker than in any other part, and can be easily torn across with a needle.
Near the outside of the leaf the cellular tissue, in which these bundles are imbedded, is lax and translucent, but towards the interior it is open and spongy, the cells being arranged in single rows, enclosing irregular inter-cellular spaces (Fig. X. h.), which are of a tubular form, the tubes running parallel with the vascular bundles. These inter-cellular passages begin to be developed when the leaf is about four inches long, and they are found only in those parts where gum is exuded. At present I have always found them empty, but it is possible that they may be gum-canals, or the glands or ducts by which gum is secreted.
In the thick part of the butt of the leaf, the internal rows of bundles are reduced to one, the others having died out. (Fig. IV.) They are here club-shaped, with a constriction in the middle (Fig. V. a.), and enclose a rhomboidal bundle of spiral-vessels (Fig. V. s. v.). Besides these there is also a marginal row of bundles, the largest of which (Fig. V. b.) alternate with the bundles of the interior row. They are of an elongated clavate form, constricted in the
[Footnote] * What are called the cotton-like, or hair-like, filaments of the flax, are far from being the ultimate fibres, but are single bundles containing perhaps 100 or more liber-cells; the ultimate fibres are all but invisible to the unassisted eye.
middle, and with a rhomboidal bundle of spiral-vessels (Fig. V. s. v.) in the thick part of the club. Between these occur three smaller bundles, of an oval form, also constricted in the middle. The central one of these, which is situated just opposite to the interior bundle (Fig. V. c.), is rather more than half the length of the largest, and encloses near its inner end a rhomboidal bundle of spiral-vessels. The other two are of the same form as the central one, but not much more than half its length (Fig. V. d.), and the bundles of spiral-vessels, which are also near the inner end, are in section like the sector of a circle, or a boy's kite. The cellular tissue which encloses these bundles is rather lax, and of a light-green colour, becoming close, and densely filled with chlorophyll, towards the outer side. All traces of the inter-cellular passages have disappeared, but in their place two cylindrical bundles of large, loose, translucent cells (Fig. V. f.) appear near the end of each of the smallest marginal bundles, and these continue to the point of the leaf. The cells that surround the interior bundles, and the rounded interior ends of the largest marginal bundles, are filled with a red fluid, as are also those of the midrib and margins of the leaf.
The vascular bundles in the free part of the leaf, at the butt, are rhomboidal in section (Fig. VI. b.), reaching the epidermis on the outer or inferior surface, but separated by several rows of cells from the inner or superior epidermis; they enclose bundles of spiral-vessels (Fig. VI. s. v.) that are also rhomboidal in section. Midway between these, and near the superior surface, there sometimes occurs another small cylindrical bundle (Fig. VI. e.) of liber cells, without any spiral-vessels, but more often this is absent. Between each of the vascular bundles there is also a cylindrical bundle of large loose cells (Fig. VI. f.), similar to those in the thick part of the butt. The midrib (Fig. VII.) is formed by an elongated bundle of fibres, enclosing a rhomboidal bundle of spiral-vessels near its inner end, and it is surrounded by three circular bundles (Fig. VII. e.) of fibres alone.
Higher up in the blade, as the free parts of the leaf get larger, the interior bundles of the butt die out, and the medial-sized marginal bundles elongate, until they, as well as the larger ones, reach almost across the leaf (Fig. VIII.) from the outer to the inner surface, the smaller ones (d.) retaining their relative size. All have, however, now altered in shape; the larger ones are still clavate, but are swollen, instead of constricted, in the middle; and the swollen parts contain their bundles of spiral-vessels, which neither in shape nor in relative position have changed with the liber-cells. The smaller, or inter-mediate bundles (Fig. VIII. d.) have become more clavate; but their spiral-vessels have still retained their shape of the sector of a circle. Occasionally on the inner side of the leaf small round bundles (Fig. VIII. e.), composed altogether of fibre, are found, alternating with the bundles that cross the leaf.
The average thickness of these bundles on the superior or inner surface of the leaf is 0.005 inch, and on the inferior or outer surface, 0.0027 inch, and their distance from one another is 0.018 inch, or fifty-five bundles to the inch. There are, therefore, about two hundred and fifty bundles of fibre in the whole breadth of the leaf, not including the intermediate bundles, which would give about as many more in the lower part of the blade near the butt; but towards the point of the leaf these intermediate bundles die out, leaving only the large ones, that go the whole way across. These also get finer and closer together the nearer they get to the point, so that there are almost as many of them close to the tip of the leaf, as there are near the butt. On the inferior or outer surface, the bundles of fibres reach to the epidermis, but on the superior or inner surface, several rows of cells intervene. Each bundle of fibres is separated from the next to it, in the interior of the leaf, by a mass of lax
translucent cells (Fig. VIII. f.) similar to those found in the butt of the leaf. The rest of the cellular tissue is densely filled with chlorophyll.
The Maoris, in cleaning the fibre, cut through the outer or inferior surface of the leaf, as far as the lax tissue in the centre, then tearing out that part which contains one half of each of the bundles that cross the leaves, and the intermediate bundles, with the small quantity of cellular tissue between them, they throw away the inner or superior surface, together with the other half of the large bundles of fibres. The cellular tissue, already broken up by tearing it away from the rest of the leaf, is easily got rid of, and thus the fibre is both finer and cleaner than that prepared by the machines. Of course the system is very wasteful, and it would never do for us to follow it, but I would suggest that it might be very advantageous to split the leaf longitudinally, so as to divide the inner and outer surfaces before putting it through the machine, as by this means the bundles of fibres would be split into two, and a finer, as well as a cleaner, article obtained.
Gum.—The gum of the flax plant, when first exuded, is a thickish, sticky, colourless fluid, that runs down the leaf when it is cut. It gradually hardens into a semi-solid, jelly-like, viscid substance, and ultimately into a thin tough pellicle, which can be easily peeled off the leaf, and which generally retains the markings of the cell-walls of the superior epidermis. When pure it is colourless, or pale yellow. It shews no microscopical structure, but generally contains small pieces of vegetable tissue, etc., and sometimes several animals, which live in it. On exposure to the sun, it shrinks greatly, and after a few days hardens into a tough solid substance, not easily broken.
In its first, or fluid state, in which alone it exists in the interior of the plant, it readily mixes with water. Both in its usual semi-solid state, and after hardening in the sun, it softens and intumesces in cold water, but only partially dissolves. In boiling water, it dissolves readily, when in the semi-solid state, but with difficulty after having been dried. Alcohol fails to dissolve it, but turns it white. It is unaffected by the caustic alkalies, but dissolves easily in acids. Iodine colours a solution of it yellow, without any trace of blue.
Neither alcohol nor neutral acetate of lead produce any effect upon a solution in water; but it is precipitated, of a yellowish-white colour, by tribasic acetate of lead, by protochloride of tin, and by nitrate of mercury.
These reactions show that it differs from all the gums, by not being precipitated by alcohol, and further, from the gum-arabic group by its insolubility in cold water; from the cherry-tree gum group, by its being precipitated by nitrate of mercury; and from the gum-tragacanth group by its insolubility in caustic alkali, while it is allied to this latter by its intumescing and partly dissolving in water. On the other hand, on all these points, except the solubility in cold water, it agrees with carrageen and linseed mucilage; and the latter, after having been dried, intumesces in water, and only partially re-dissolves. From all the mucilages, however, our flax gum differs in its behaviour with neutral acetate of lead, which proves that it contains little or no pectin, and in this respect it is like the true gums. It appears, therefore, that flax gum is intermediate between the gum-tragacanth, or Barsorin group of true gums, and the mucilages, and ought, strictly speaking, to be called a gum mucilage.
We are thus led to the following conclusions:—
That the object to be aimed at, in manufacturing the fibre, is the separation of the fibro-vascular bundles from the cellular tissue and epidermis of the leaf, and not, by any means, the breaking up of the bundles into their ultimate fibres, which would entirely destroy their strength.
That the bundles of fibres in the leaf are of different sizes.
That no woody fibre exists in the leaf, but the liber-cells have thin walls of delicate construction, which probably accounts for the quick deteriora-tion of the fibre by over-bleaching, or by the use of chemicals.
That the gum appears to offer no peculiar obstacle to the manufacturer, provided the leaves are above the butt, for in the state in which it alone exists in the interior of the plant it readily mixes with cold water.