TISSUES. A tissue is a group of connected cells of like origin and structure. Tissues are there fore classified according to the form of their com ponent cells, the thickness and chemical com position of the eell-walls, the character of their contents. etc. All cells of a very young plant are alike in having thin walls, abundant proto plasm, minute vacuoles or none, and relatively large nuclei. Such cells constitute embryonic, formative, or meristematic tissue. which is found in the higher plants in three places: (1) At the tips of the shoots and roots, where it constitutes the 'growing points,' protected in the shoot by overarching leaves which form a bud, and in the root by the root-cap; (2) one or more thin layers concentric with the stem, by which new layers of wood, cork, and bast are added to it internally; (3) where wounds are made, in which cases the meristem produces tissues that heal the wound.
In all the higher plants tissues are grouped in such a way as to form tissue systems (see ANAT OMY OF PLANTS). Those there described are re duced to three by some: viz. (1) the tegumentary (equivalent to the protective) : (2) the vascular (equivalent to the conducting and in part the mechanical) ; (3) the fundamental, a sort of limbo to which are assigned all the tissues not in cluded in the other two systems. llf all tissues it may be remarked that the eell-walls are subject to thickening as they grow older. This thickening consists of material added to the surface of the primary wall. The successive additions are fre quently unlike, and in the mature wall are apt to be distinctly stratified. The thickening is seldom uniform. Sometimes minute regions escape thick ening and remain as small pits in the added layers. These pits occur at points where the protoplasm of one cell has not been completely separated from that of the other by the forma tion of a partition wall. hut remains connected there by many very slender strands. In other Only the more important kinds of tissues are here described. ( ) Parcnchyma Cells of very va
rious form, but mostly nearly equal in the three dimensions. usually with a thin cellulose wall containing (so long as they are functional) living protoplasm. and almost invariably separating more or from one another to form intercel lular spaces. In elongated organs parenchyma cells are likely to become elongated. The paren chyma of the leaf and the cortex of the stem may develop into very irregular forms (Fig. 2). In water plants and others in which the parcn ehyma develops extensive intercellular spaces (arenchyina) the cells may become branched, some even being regularly stellate (Fig. 3). Parenchyma cells sometimes•have thick walls, and thus form a transition to the sclerenehyma (see below), from which, however, they may be eases the thickening is absent from wider areas. This leaves broader and shallower pits. which are symmetrically or irregularly distributed, and give the appearance of sculpturing of the wall (2. Fig. 1). When the thickening is restricted to spiral lines or rings, the thin walls seem to be merely supported by the thickened portions, as in spiral and annular vessels (1, 3, Fig. 1).
distinguished by the presence of living cell eon tents and sometimes reserve food. Such paren chyma occurs in the rhizome of many ferns. Par enehyma forms the chief nutritive and storage regions of all plants, and is especially abundant in herbaceous plants. In fleshy lea yes. fruit:. stems, etc.. the parenchyma is greatly developed at the expense of other sorts of tissue and is there used for storage of reserve food. The outer walls of parenchyma cells. which form, the surface ( i.e. the epidermis), undergo a peculiar change. being infiltrated with cutin. by which they are rendered almost impervious to water. Parenchyma cells of tabular form arise from the phellogen, which have all their walls cutinized. They constitute a tissue known as cork ( Fig. 4).