Histology, Vegetable, is the study of the tissues of which all multicellular plants are composed. A tissue is an aggregate of cells or vessels obeying a common law of growth and, as a consequence, resembling one another. A true tissue is the result of cell-division; a false tissue, of the approximation of cells of independent, origin. Tissues may be merismatic or meristem, when their cells retain their protoplasm and the consequent power of cell-division; or permanent, when they have lost this. Again, they may be parenchymatous, when their cells are not much elongated, when there are generally marked intercellular spaces; or they may be prosenchymatous, with elongated elements. Thirdly, tissues may have the walls of their constituent cells iinthickencd or thickened, and in the latter case the term sclercncltyma is applied when the walls are hardened by a ligneous deposit, and collenchyma when the corners of cells are thickened mucilaginously. Of these kinds of tissue, the embryo and the growing points of stems or roots exhibit a meristem known as primary, or in the latter case as apical, from which at an early stage three or four primitive tissue systems, the dcrmatogen or primitive epidermis, the plerome, or primitive vascular axis, the periblent, or primitive cortex, and in roots the calyptrogen, or primitive root-cap, are commonly differentiated. In roots and stems, zones of tissue remaining merismatic until a late period are known as secondary meristem. Of this the cambium-ring of the exogenous stem, the pcricambium, of roots, and thephcllogen, or cork-cambium, are examples.
Tissues may also be grouped under six systems. (1) The fundamental or ground-tissue, from which all the others have been differentiated and which remains eis a sort of connective tissue between them, is permanently represented by the inner or primary cortex, the liypoderm, often collenchymatous, immediately below the epidermis, and the endoelcrmis or bundle-sheath. (2) The epidermal, or limitary system is usually a single layer of somewhat flattened, colourless cells, with hairs and, on those parts exposed to air, stomata (q.v.) and sometimes excreting a waxy bloom or a separable outer thickening or cuticle. (3) The fascicular or fibro-vasculav system originates in isolated merismatic strands of the plerome which are then known as procambium, and from this isolated origin its members are termed bundles. It forms the veins of leaves and, in the higher plants, the bulk of the stem. Each bundle consists of two groups of elements, xylem, orwood, and phloem, or bast, either exclusively, as in the closed bundles of leaf-veins or of the stems of monocotyledons; or with a zone of cambium between them, as in the open bundles of the exogenous stem. Xylem consists of three or four kinds of elements, viz. vessels, or cell-fusions with thickened walls; tvachcids, or elongated vessel-like but unfused cells; woody fibre; and wood-parenchyma. Phloem consists of sieve-tubes, the protoplasm-containing vessels of the bast; bast-fibres, the long flexible elements that form the bulk of the whole phloem; and bastparenchyma, including the elongated cambiformeells. (4) The medullary system consists mainly of unthickened and often parenchymatous pith. (5) The cortical system, in addition to the primary cortex, originating directly from the periblem and often consisting of thin-walled parenchymalfilled with starch, comprises the phelloyen, or cork cambium layer, the 2>eriderm, or secondary cortex, of muriform parenchymalwith cuticularised walls filled with air, which is formed by the phellogen, and the phelloderm, or hypodermal chlorophyll-containing layer. (6) The secretory system comprises those more or less disconnected elements imbedded in tissues of other systems but devoted to special secretions, such as oil-cavities, resin-passages, and laticiferous tissue. Many of these tissues and tissue-systems are separately described.