Conceptual Overview

The rigid cell wall is a specific feature of higher plant cells. Without walls, most plant cells would rupture due to the high osmotic pressures within. The wall also determines the cell shape and the direction of its growth. Protoplasts from even very long cells revert to a more or less spherical shape when the wall is lost. Walls are typically transparent which allows for the passage of light. The primary chemical components of cell walls are polysaccharides in the form of cellulosic microfibrils that are crystalline structures, and matrix polysaccharides in the form of amorphous substances (pectic substances and hemicelluloses). The second most abundant component is water, and additional components include a wide variety of compounds such as lignin, proteins and mineral ions. Cell walls of some cell types may also contain polymeric lipids such as waxes, cutins and suberins which form hydrophobic layers or deposits on hydrophilic walls, particularly conspicuous in the epidermis, endodermis, and in cork.

Each plant cell develops its own cell wall, and neighboring cells in the plant body are held together by the middle lamella. In essence, it originates as the pectic cell plate formed at telophase. The middle lamella is very thin and often indistinguishable from the primary cell wall. The deposition of cell wall material occurs not only on the cell plate but on the whole inner surface of the cell that starts growing. The walls of dividing and growing cells are called primary walls. For many cell types, wall formation ceases with the cessation of cell growth. Such cells remain surrounded by a thin primary wall during their whole life cycle. But in other cell types the deposition of wall material continues in non-growing cells. As a result, the thickness of the wall increases, and the volume of the cell cavity decreases.

Walls formed or augmented after the cessation of cell growth are called secondary walls. For some cell types, such as tracheids, vessel members, cork cells, and some sclereids and fibers, the formation of the secondary wall is the main function of their protoplasts and the cells die as a prerequisite to fulfilling their principal function. In such a case, the secondary wall provides mainly mechanical support, and is responsible for specific features of wood, textile fiber and paper. In cells with well-developed secondary walls (S), three concentric wall layers may be distinguished: the outer (or transition) narrow S₁ layer adjacent to the primary wall, a thicker middle S₂ layer, and a thin inner layer bordering on the cell cavity (S₃). The layers differ not only in thickness, but also in chemical composition and in the angle of cellulose microfibril orientation in relation to the cell axis. The three-layered structure of secondary walls is characteristic of wood elements of conifers and some angiosperms, but in some cells the S₃ is lacking. The S₂ layer is the richest in cellulose, and is responsible for most of the properties of secondary walls. In some specialized cells (mainly those engaged in water conduction) the secondary wall is not deposited over the entire inner surface of primary wall, but appears as individual rings, continuous helices or nets.

The walls of cells with secondary wall thickenings are very often impregnated with lignin, a hydrophobic polymer composed of phenolic derivatives of phenylpropane. Both primary and secondary wall layers are lignified. Lignification results in the substantial modification of cell wall properties, e.g. the loss of elasticity, drastic increase in hardness and tensile compression, and decrease in the permeability of water as the lignin polymerizes within the cell walls.

In secondary walls of all types of cells, small areas remain where secondary wall material is not deposited. Such interruptions in the secondary wall are called pits. In cells with very thick secondary walls (e.g. stone cells) pits appear in sectional view as long radial canals extending from the cell lumina to the primary wall. The shape of such canals in surface view may be round or, less frequently elliptical or slit-like. The shape of the pit canal is tissue- and taxonomically specific. Two main types of pits are recognized: simple pits and bordered pits. In simple pits the canal typically has a cylindrical form, whereas in bordered pits the canal becomes much narrower in the process of secondary wall deposition and consequently the diameter of the pit aperture facing the cell cavity is significantly less than the diameter of the so-called pit membrane (i.e. the unthickened portion of the primary wall). In the adjacent cells, pits arise opposite each other and have a common pit membrane. Such a configuration is said to be a pit-pair. Bordered pits are characteristic of water conducting cells of wood which are dead at maturity (tracheids and vessel members). Simple pits are found in cell walls of living cells such as cells of parenchyma and some fibers. Their pit membrane is penetrated by plasmodesmata. Pits facilitate the intercellular transport of water and solutes. In addition to pits, primary pit fields are also recognized. They are thin areas in the primary walls, and they may become pit membranes when secondary wall thickening occurs.

As indicated above, individual plant cells are cemented together by the middle lamella. Therefore, if the middle lamella is dissolved by chemical agents, the cell walls become separated from each other. Such an artificial process of cell separation is called maceration. However, a more common event is natural maceration when pectins of the middle lamella are enzymatically dissolved as during the process of abscission. Very often partial maceration occurs when the middle lamella is solubilized only at certain sites, primarily at the cell corners. Cells round off due to turgor pressure, and intercellular spaces appear at these sites. These spaces enlarge and fuse with each other as cells grow, and a single branched system may appear which is filled with gases and water vapor. Therefore, intercellular spaces facilitate gaseous exchange into and out of plant cells.