Permanent Tissue

Permanent tissue is made of cells that have finished dividing and, through a series of physiological, morphological and functional changes during maturation, have settled into one fixed size, shape and job for the rest of their life (which, for some permanent tissues, means the cell itself is dead). Depending on how many different cell types are present and working together, permanent tissue is split into two broad categories: simple permanent tissue, built from only one kind of cell, and **complex permanent …

Simple permanent tissues are each made of a single cell type carrying out a single kind of function, and may be either living or dead depending on the type. Three simple permanent tissues occur in plants — parenchyma, collenchyma and sclerenchyma.

Parenchyma is the least specialised and most abundant of the three. Its cells are thin-walled, living, and vary in outline from isodiametric or rounded to oval, polygonal or somewhat elongated; the wall is essentially pure cellulose. Each cell has a prominent nucleus, a thin peripheral layer of cytoplasm and one large central vacuole, and small intercellular air spaces are typically visible at the corners where several cells meet. Because it is the least specialised tissue, parenchyma can also dedifferentiate — for example, giving rise to vascular cambium and cork cambium when secondary growth begins.

Collenchyma is also a living simple permanent tissue, but its cell wall is cellulosic with an important twist: cellulose and pectin are deposited unevenly, building up especially at the corners where cells meet, and the walls may show pits. In cross-section the cells appear circular, oval or angular, and — unlike parenchyma — they are packed tightly with no intercellular gaps, so the tissue looks compact. Functionally collenchyma is a living mechanical tissue: because its thickening is flexible rather than rigid, it lets a young stem or a leaf's petiole bend and be pulled without tearing, while still giving it real mechanical support, and it also accommodates the elongation that accompanies organ growth. It is characteristically absent from monocot stems and from the roots of dicots. …

Complex permanent tissues are heterogeneous — made of more than one type of cell — but all the component cells function together as a single unit, and their shared job is conducting sap and food from a source region to a sink region. Xylem and phloem are the two complex tissues found in plants.

Xylem is a dead complex tissue (also called hadrome) made of four kinds of cells: tracheids, vessels, xylem parenchyma and xylem fibres. Its main jobs are conducting water and dissolved minerals and giving the plant body mechanical rigidity — this is why leaves, epicarps and seeds stay firm and resist falling or collapsing. Tracheids and vessels are the conducting elements proper. Tracheids are elongated, tubular, dead cells with oblique, tapering ends and thick, unevenly lignified walls; the wall may show one of several thickening patterns of increasing structural sophistication — annular (separate rings), spiral (a continuous helical band), scalariform (ladder-like rungs), reticulate (a net-like pattern), or pitted (small circular areas, either simple or bordered, the most advanced pattern). Tracheids make up around 95% of the wood in gymnosperms but only about 5% in angiosperms, and remain the sole conducting elements in pteridophytes and gymnosperms — with the notable exceptions of the pteridophyte Selaginella and the gymnosperm Gnetum, both of which possess true vessels. Vessels are longer than tracheids and are formed when the end walls of a row of vessel elements dissolve away, fusing them into a single continuous tube with a wider lumen than a tracheid's; they are the principal conducting elements of angiosperms, and their shape in cross-section differs by group — rounded in monocots, angular in dicots. The earliest-formed vessels of a bundle, the protoxylem, are narrow with annular or spiral thickenings (allowing them to stretch as the surrounding organ elongates), while the later-formed metaxylem vessels are wider with reticulate or pitted thickenings. Where protoxylem sits toward the centre (pith) and metaxylem toward the periphery, the arrangement is called endarch (the pattern typical of stems); where the arrangement is reversed — protoxylem toward the periphery — it is called exarch (the pattern typical of roots). The remaining xylem cells are non-conducting: xylem parenchyma, small living cells associated with the tracheids and vessels, is in fact the only living component of xylem, and it stores food (as starch) and sometimes tannins while also carrying out lateral or radial movement of water and sap; xylem fibres are elongated, tapering, lignified sclerenchymatous cells (also called wood fibres) that add purely mechanical support. …