Permanent Tissues: Simple and Complex

Once a cell produced by a meristem stops dividing and instead takes up a fixed shape, size, wall thickness and function, it is said to have differentiated into a permanent tissue. Permanent tissues fall into two broad groups depending on how many different kinds of cell they contain: a simple permanent tissue is made of only one type of cell throughout, while a complex permanent tissue is made of more than one type of cell, all working together as a single functional unit.

Simple permanent tissues. Three simple tissues recur through almost every organ of the plant:

• Parenchyma is the most abundant and least specialised of the three. Its cells are living, usually thin-walled (cellulose only) and roughly polygonal or oval in outline, and they are packed loosely enough to leave visible intercellular spaces between them. Because its cells stay alive and metabolically active, parenchyma performs whatever job the local tissue needs -- storage of food and water is the most common, but when a parenchyma cell contains chloroplasts (and can therefore photosynthesise) it is specifically called chlorenchyma, and when it develops unusually large air spaces for buoyancy, as in many aquatic plants, it is called aerenchyma.

• Collenchyma is also a living tissue, but its cells are elongated and packed together without intercellular spaces, and -- unlike parenchyma -- their walls are unevenly thickened: thickening is concentrated at the corners of each cell, in a mixture of cellulose and pectin, while the rest of the wall stays comparatively thin. This uneven thickening gives collenchyma mechanical strength without sacrificing flexibility, which is exactly what a young, still-growing dicot stem needs -- collenchyma typically forms a band just below the epidermis (the hypodermis) in such stems and in leaf petioles, providing support that can still bend rather than snap, and because some of its cells retain chloroplasts it can photosynthesise as well.

• Sclerenchyma, by contrast, is a dead tissue at maturity: its cells develop thick, lignified (hardened) secondary walls that leave little or no space for a lumen or living contents, and once mature they lose their protoplast entirely. What they leave behind is a rigid, mechanically strong wall, well suited to giving permanent, unbending support. Sclerenchyma occurs in two forms -- fibres, which are long, narrow, tapering cells that occur in tight bundles (the "bast fibres" used commercially for rope and textiles are sclerenchyma fibres), and sclereids, which are shorter and irregularly shaped (star-shaped, branched or roughly spherical) and occur singly or in small groups, giving a gritty texture where they occur, such as the stone cells in a pear.

Complex permanent tissues. Xylem and phloem are each built of more than one kind of cell, and it is precisely this mixed composition that lets them do their conducting job well:

Xylem conducts water and dissolved minerals from the root upward through the plant, and also contributes mechanical strength. It is composed of tracheids (elongated, dead, tapering cells with pitted walls, through which water moves cell to cell -- a comparatively slow, primitive route), vessels (short, wide, dead cells joined end to end into a continuous open tube through perforations in their end walls, allowing fast, efficient long-distance conduction, and found chiefly in angiosperms), xylem fibres (dead, thick-walled cells that add mechanical support), and xylem parenchyma (the only living component of xylem, used for lateral conduction and storage). …