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Vessels of the Wood

("xylem") and (b) Vessels of the Bast ("phloem"), or sieve-tubes. 2. Vessels of the Cellular Tissue, or Ground Tissue, scattered through the pith and cortex of the roots and stems, and among the green cells of leaves. These three classes of Vessels differ materially in their nature, contents, and uses. The two classes of true Vessels agree in being present in all complete fibro-vascular bundles; and also in being always formed by the absorption of the walls that separate elongated cells placed in rows, either end to end, or overlapping more or less at the tapering ends, so that tubes are thus formed of indefinite length. In most Vessels it is easy to trace the cells of which they are formed by the marks that remain on the walls where the cells meet. The Vessels of the Wood differ from those of the Bast in several important respects. But before stating these it may help to elucidate the subject if a brief account of the ordinary structure of fibro-vascular bundles is given.

We shall select for description such a bundle as may be found in the stems of many Monocotyledons. In these plants the bundles usually remain separated from one another, during their whole existence, by the cellular tissue in which they are imbedded; and after they are once formed they do not undergo changes, so that the arrangement of the cells and Vessels of which they are built up is little, if at all, altered during growth. The simplest bundles of this kind consist of a group of the Vessels of the Wood (called "tracheae"), united with a group of Vessels of the Bast ("sieve-tubes"); but in most parts of plants these are accompanied by cells, some of which are elongated and slender (fibre-cells or "prosenchyma") while others are little, if any, longer than broad ("parenchyma"). Some of the cells usually remain thin-walled; but others have the walls much thickened by deposits upon the inner surface, and then the tissue is called "sclerenchyma." This tissue adds to the strength of the bundle, being arranged alongside the Vessels (as wood-fibres or hard-bast fibres), or forming a sheath surrounding the bundle entirely or in part. The commonest position of the wood and bast of each bundle, as seen in transverse section, is that in which the wood lies nearer the centre of the stem, and the bast nearer the circumference; but in some plants there is bast inside, but not outside, of the wood, or there may be bast both inside and outside, or even all round the wood. Less often the bast lies in the middle, with the wood all round it. The arrangements in leaves correspond with those in stems. If the leaf is held erect, with the upper surface next the stem, that surface is nearer the centre of the stem, and the lower surface is further from it. In accordance with this, the Wood-vessels of each bundle are usually nearer the upper surface, and the bast nearer the lower surface of the leaf. In roots the earliest bundles to appear (called the primary wood-bundles), consist entirely of Wood-vessels, which are formed successively nearer and nearer the centre of the root, increasing in size as they come nearer to the centre. Between them, and at the same distance from the centre as they are, the bast is formed; hence, the root is markedly different from the stem in the arrangements of the bundles. In the stems and roots of Monocotyledons, the bundles early assume the appearance and structure that they permanently retain; but in woody Dicotyledons and Conifers, changes occur after the first year of growth, which greatly affect their original appearance. In each bundle in the stem, the wood and the bast are separated by a layer of thin-walled cells (the "cambium") which continues to form new cells by divisions parallel to its surfaces, producing new wood to the outside of the older wood, and new bast to the inside of the older bast. The cambium forms a complete cylinder around the wood of the stems of these plants, and gives origin to ring above ring of wood, usually one in each year of growth. The bundles grow so large that they are separated only by narrow belts of cells (medullary rays), the oldest of which runs from the pith in the centre to the cortex outside the bast; while the new rays formed each year run from the inner border of the ring of wood to the cortex. The fibro-vascular bundles can scarcely be separated, after a time, from one another; but the wood and the bast are easily disunited in most Dicotyledons at the cambium, as the cells of this ring readily give way, and the bark is thus easily separable from the wood. The bast forms the innermost layer of the bark, and its connection with the wood tends to become less evident than it was before the bundles were united by the continuous cambium. In the roots of Dicotyledons and Conifers, the earliest-formed wood-bundles do not grow; but there is a layer of cambium to the inside of each bast bundle, and soon this layer begins to form wood from its inner surface, and bast from its outer. After a short time, the cambium forms a continuous layer like that in the stem, and the roots in cross-sections look much like stems, except that the pith in the centre is often small or wanting, and that a practised eye can usually detect the primary wood-bundles lying close to the centre, and free from the bundles formed by the cambium.

We must now pass to the various kinds of Vessels met with in the wood and the bast. In the wood of all bundles there are "spiral" or "annular" Vessels. These are long, slender tubes, which appear round in transverse section. They frequently show very slight traces of the cells of which they were built up. The characteristic feature in them is the existence of a peculiar thickening deposit in the vessel, which, in a longitudinal section of the bundle, is not unlike a glass tube inclosing a closely-wound spiral wire, or wire rings; or a more familiar comparison may be made with an indiarubber tube, kept open by a wire spirally coiled in its interior. Besides spiral and annular Vessels, the wood-bundles very generally contain others, in which the thickening deposits are laid down on the inner surface of the walls in the form of a more or less regular network ("reticulated Vessels"), or of the steps of a ladder ("scalariform Vessels" of Ferns and their allies), or covering the wall, so as to leave only narrow tubes or pits through the new layers ("pitted or dotted Vessels," or "Ducts"). All these Vessels show an angular form in transverse section; and they are generally wider than the spiral Vessels. They also show distinctly the boundaries of the cells of which they are built up; though the openings from cell to cell are always relatively large. Spiral and annular Vessels are very rarely formed by the cambium; hence, in Dicotyledons and in Conifers, they are present in the stems and roots usually only in the earliest-formed wood-i.e., they surround the pith; and, at one period, they were supposed to form around the pith a peculiar organ, which was called the "medullary sheath." In Dicotyledons, the Vessels in the wood formed by the cambium are almost all reticulated, dotted, or pitted. In Conifers, very few Vessels are formed by the cambium, their work being done by wood-cells, with openings from one to the other of a peculiar nature. All the characteristic Vessels of the Wood very soon lose their protoplasm, and contain only air or sap, or, more generally, both air and sap. The walls of fully-formed Wood-vessels are lignified and firm.

The Vessels of the Bast are very distinct from those of the wood. They are always present in the soft bast (though not always easily detected, except by an expert microscopist); but a few may also be found in some plants in the pith or the cortex. They assume the form of slender tubes, with thin, flexible walls, unmarked by thickening deposits. The walls separating the cells that make up these Vessels are not entirely absorbed (as is the case in the Wood-vessels). They can always be recognised as cross-partitions; but they are pierced by numerous small openings, so as to resemble a sieve, whence they are called "sieve-plates," and the Vessels are called "sieve-tubes." Often, the side walls of adjoining tubes also show sieve-plates. The sieve-tubes retain their protoplasmic contents; and the protoplasm extends through the sieve-plates. Sachs believes that the new protoplasm is largely produced in the sieve-tubes; and there is no doubt that they are the channels by which the protoplasm is chiefly, if not entirely, carried from one part of a plant to another, as may be required during growth.

The Vessels of the ground-tissue are of far less general occurrence than those of the vascular system; and they are very different from these in their nature, if we except the small, scattered bundles of sieve-tubes that traverse the ground-tissues of certain plants. The only form of ground-tissue Vessels that calls for special mention is that containing "latex," from which the Vessels themselves are called "Laticiferous Vessels." They exist only in certain orders of plants, chiefly among Dicotyledons, e.g., Campanulaceae, many Compositae, Euphorbiaceae, Ficoideae, Papaveraceae. Among Monocotyledons, they can scarcely be said to exist in their characteristic form, or with their characteristic contents. Laticiferous Vessels vary in their mode of origin in different plants. In most (e.g., Poppies, Dandelion), they are formed, like other Vessels, by the union of cells, of which the dividing-walls are entirely or partially absorbed. Thus very irregular Vessels are formed, which unite freely with one another by branches, so as to form a copious network, with free inter-communication. The walls of these Vessels seldom show thickening deposits. In a few orders (Asclepiadeae, Euphorbiaceae), the Laticiferous Vessels are probably formed, not by the union of cells, but by the elongation and branching of cells, which are not divided by cross-partitions, and which thus reach a very great length. It is believed, by many botanists, that the Laticiferous Vessels of some plants (e.g., Rhus) are really intercellular spaces, into which the latex is poured. The Laticiferous Vessels frequently accompany the sieve-tubes, and may even take their place to some extent. In those Monocotyledons that possess latex, it is contained in rows of large cells, separated by walls, in which perforations have not been clearly made out (Allium Cepa); or, in those plants (Galanthus) in which the Vessels consist of cells, with perforated walls, the contents do not resemble latex, but are only clear sap, with raphides, i.e., slender crystals of Oxalate of Lime. The latex, in Dicotyledons, is a peculiar fluid, which at once appears on breaking any part of a plant in which it exists. It is clear while in the uninjured tissues; but on exposure to the atmosphere, on a broken surface, it becomes turbid. It is then white, like milk, in most plants; but in some it is coloured yellow (Chelidonium) or orange by pigments in it. The microscope shows that it is chiefly composed of watery sap, in which float myriads of extremely minute granules: these, as in milk, are the cause of its opaque, white appearance when exposed to the air. On continued exposure to the air, or mixture with alcohol, acids, &c., masses separate from the latex in the form of "coagula," which usually become dark in colour. These coagula often afford useful products, e.g., Opium, and Caoutchouc, or Indiarubber. They vary considerably in composition, texture, and properties. The latex usually has dissolved in it small quantities of sugar, gum, protoplasm, and alkaloids; and starch granules are present in the latex of some plants (Euphorbia). In the Papaw (Carica Papaya), there is a peculiar substance ("papayotin") dissolved in the latex, which exerts a digestive action on muscular fibre. It is believed by many botanists that the Laticiferous Vessels may be of the same use in plants that veins are in animals; but the plants that possess them are comparatively few, and there is never a central organ for propelling the latex, as the heart propels the blood. The latex, like the blood, contains substances employed in the nutrition of the plant, and also substances that must be regarded as mere excretions formed during the processes of growth, and that would be hurtful if allowed to remain in the cells. Laticiferous Vessels are confined to the higher plants; but Laticiferous cells occur among some of the cellular Cryptogams, and notably in the genus Lactarius, among Mushrooms.