Let us make an in-depth study of the formation, growth and chemical nature of the cell wall of plant cell.
Most of the plant cells are provided with tough rigid cell wall and this is taken as an outstanding point of difference between plant and animal cells.
Naked protoplasts are noticed in lower groups of plants. The reproductive cells of higher plants as a rule lack walls.
The cell wall was actually noticed by the early workers long before the more important matter protoplasm was recognised. Naturally it received considerable attention then. With the discovery of protoplasm the wall obviously lost much of its importance.
In recent years the cell wall has again attracted serious attention of the investigators. In fact, spectacular advances in the technique of investigations, in application of X-rays, polarized light photography and electron microscope, and increasing commercial importance of cellulose have given new impetus in investigations on the wall.
The cell wall is usually looked upon as a secretory product of the protoplast. In that sense it ought to be a non-living membrane surrounding the protoplast. It protects the protoplast from adverse external influences, delimits the same, gives mechanical stability to the cells and also imparts shape to the cells.
But the very fact that wall is secreted by protoplast and that an intimate relationship exists between the two have led some workers to believe that it may not be completely non-living.
Formation of the Cell Wall:
Formation of the cell wall and the process thereof has got to be sought in the method of cell division. During mitosis two daughter nuclei are constructed after having gone through a series of complicated changes.
The nuclear division is then followed by cytoplasmic division (cytokinesis). In this process protoplasmic matters accumulate in the equatorial region in form of small droplets, which cohere in course of time and form a continuous plate called cell plate (Fig. 508).
This plate undergoes physical and chemical changes and is ultimately converted into the intercellular substance, the middle lamella. In fact, middle lamella is the ‘cement’ between the two cells.
By proper chemical treatment middle lamella may be dissolved, when the cells lose the cementing material and become isolated. Middle lamella is optically inactive and colloidal in nature. It is composed of pectates of calcium and often magnesium.
The protoplast goes on secreting cell wall materials on the middle lamella, and ultimately a soft, delicate and plastic wall is formed. This is primary cell wall (Fig. 509). It consists mainly of cellulose and pectic compounds and may also contain other polysaccharides.
This is really the first formed cell wall, which may persist in many cells as the only wall. The primary wall is optically active.
Primary cell wall gets more and more stretched during the growth of the cell. In course of time secondary cell wall materials are deposited on the primary wall. It may happen that primary walls of the adjacent cells become fused with the intercellular substance and form the common layer between the two cells.
In that case middle lamella is not purely the cementing material. Some workers have attributed the term compound middle lamella to such layer. The secondary wall is mainly composed of cellulose which
may undergo other modifications.
In many cells it is usually three-layered, the layers differing in physical and chemical properties, of which the middle one is the thickest (Fig. 510).
Some authors have suggested that another layer with a different chemical composition may be present in addition to the inner layer of secondary wall, and preferred to call this layer tertiary or terminal one (Frey Wyssling).
While secondary material is deposited on the primary wall small thin areas are left out. These areas are called primary pit fields (Fig. 509) through which fine fibrils of cytoplasm pass from one cell to another and thus establish the organic continuity of protoplasm.
These fibrils are known as plasmodesmata (Fig. 516). As the secondary walls are laid down after the cell has ceased to enlarge, it does not grow like the primary wall.
So it may be regarded as a collection of supplementary layers making the wall thick and massive and mainly giving mechanical support. Thus the cell wall is made of at least three layers—intercellular substance, primary and secondary wall.
The submicroscopic nature of the cellulose wall has attained prominence in recent years as a result of studies made with X-rays, polarized light and electron microscope. It is believed that the wall is made up of a porous matrix of cellulose.
The most minute units are some extremely delicate thread-like bodies called microfibrils which are grouped together in larger bundles and anastomose to form a three dimensional network with an interfibrillar system of microcapillaries.
The fibrils are the aggregates, referred to as miscelles, of a large number of long chain-like cellulose molecules arranged along the longer axis of the fibril. Colloidal pectic compounds and other non-cellulosic substances fill up the microcapillaries of the anastomosing system.
Growth of the Cell Wall:
It has been stated that primary cell wall is soft and plastic. It is composed of cellulose and pectic substances. The pectic matters are hydrophilic colloids; they can absorb and hold water.
Due to softness and plasticity the primary wall gets considerably stretched during the growth of the cell, when new cell wall materials are deposited on the primary wall.
Two processes of the growth of cell wall have been described, viz., growth by intussusception and growth by apposition. The first method, though rather difficult to explain, involves wedging in or interpolation of new cell wall materials between the existing ones of the stretched primary wall. This process brings about growth in surface area.
During growth by apposition new materials are laid down layer after layer on the inner side of the primary cell wall like the pages of a book. This method causes growth in thickness mainly on secondary walls.
Though both the methods of growth are possible locally, growth by apposition is more common. Thus secondary walls are placed in layers, except in the region of pits, and consequently the wall becomes quite thick and massive.
Growth by apposition is usually centripetal, occurring from outside. In unicellular bodies like pollen grains and spores, growth is centrifugal, which explains the prominently thick and spined exine.
The contents of the degenerated cells surrounding the spores (tapetal cells) are possibly involved in the formation of the exine. Electron microscopic studies have led to the enunciation of two theories regarding the methods of growth, viz., mosaic growth and multinet growth.
The former demands that new microfibrils are pushed into the thin areas penetrated by cytoplasm. The second one, a more recent concept, maintains that new lamellae with denser microfibrils are added centripetally on the previous ones.
Thus the classical controversy as regards the growth of the wall by intussusception and apposition is still continuing.
In many plants cells the primary wall may persist up to the permanent stage. In that case wall remains comparatively thin and more or less uniform. In woody parts of the plants, particularly in the water-conducting portions (xylem) cells and vessels have massive and peculiarly thickened walls. Here secondary matters are localised to certain regions on the primary cell wall, instead of being deposited all over the same.
Due to localised nature of thickening the following peculiar designs or patterns (Fig. 511) are produced:
(i) Annular or ring-like thickening is noticed in the protoxylem elements where secondary matters are placed centripetally in form of rings at regular intervals (Fig. 511 A & B).
(ii) Spiral thickening is also found in protoxylem elements, secondary wall being deposited in form of spiral (Fig. 511 C & D).
Secondary matters took like the rungs of a ladder here (Fig. 511 E & F).
The secondary matters here assume the form of a network (Fig. 511G).
In this case secondary cell wall materials are deposited practically all over the primary wall, only leaving some small thin areas here and there. These unthickened areas are the pits (Fig. 511 H & I).
The pit fields present on the primary wall, may develop into pits with growth and maturation of the cell, or the pits may arise over primary wall parts which do not bear pit fields. Pits, in fact, are the areas through which diffusion of fluids takes place from cell to cell. In front view they look like circular, oval or pentagonal, areas on the wall.
Pits are usually formed in pairs, i.e., if a pit is present on one cell, a complementary pit will be formed in the adjacent cell. The picture becomes clear in sectional view (Fig. 512).
So at the region of the pit the only wall between the two adjacently-lying cells is the primary cell wall together with the middle lamella. It is referred to as the pit membrane or closing membrane. There are two types of pits. The one just described is a simple pit. In cells with thick walls the simple pits may break into two or more cavities and look like branched canals. These are called ramiform pits (Fig. 513).
The second type, where the secondary wall arches or overhangs from both sides, is called a bordered pit. In this case due to the said overhanging of the materials a dome-shaped body is formed, which may be represented by two circles, the smaller one at the opening indicating the pit aperture (Fig. 514) and the larger one inside showing the pit cavity or pit chamber.
In front view these pits exhibit two circles, the pit cavity forming a border round the pit aperture, and hence the name. In bordered pit the pit membrane is, as usual, composed of primary walls of two cells and the middle lamella.
The central part of the pit membrane swells up. The swollen portion is called torus. The torus may shift its position. Due to changes of pressure it may swing and come to the mouth of the pit, and thus block the pit aperture (Fig. 514D).
Presence of torus is characteristic in the bordered pits of the gymnosperms—Coniferales, Ginkgo and Gnetales. They occur rarely in other groups of vascular plants. Normally pits are of either simple or bordered types.
In some cases it may be a combination of the two—a bordered pit being complemented by a simple pit (Fig. 515). This type of pit is called half-bordered. Here the torus is either poorly developed or lacking. A large pit on one side may be complemented by two or more small pits on the opposite side. It is known as unilateral compound pitting. Blind pit, a pit without the complementary one, is rather rare. It may occur, when, for example, a pit is placed opposite an intercellular space.
In the tracheary elements of secondary wood of some families of dicotyledons like Leguminosae, Cruciferae, Myrtaceae and Caprifoliaceae minute outgrowths are formed from the free surface of the secondary walls of the bordered pits in form of sculpturings.
These pits, which are thus rendered sieve-like in appearance, are called vestured pits. The projections forming the vesture are refractive and of diverse types. In half-bordered pits they occur only on the bordered side of the pair.
They are found in phylogenetically more advanced xylem and are considered to be an advanced form of pit.
In the secondary wood of conifers rod-like or bar-like sculpturing are common in the lumen of the tracheids extending across from one tangential wall to the other. These are known as trabeculae, which usually occur in long radial series of cells.
In stained preparations of the tracheids of some gymnosperms bar-like or crescent-shaped thickenings of intercellular materials and primary walls are noticed on outer and lower margins of pit pairs partially encircling them.
These are characteristic thickenings of the middle lamella and primary walls which are visible through the secondary walls in stained preparations. They are called crassulae.
Crassulae represent the borders of the primary pit fields of young cells from which they have developed.
The terms ‘bars of Sanio’ or ‘rims of Sanio’, after Sanio who extensively worked on wood of conifers in the nineteenth century, had been applied to the crassulae for a long time. But it was considered rather confusing, because these terms were also applied to trabeculae.
Chemical Nature of the Cell Wall:
The cell wall is mainly composed of an insoluble polysaccharide, cellulose, with the empirical formula (C6H10O6)n. Pectic compounds, hemicelluloses and other polysaccharides may remain associated with cellulose.
The middle lamella, as already reported, is mainly made of pectates of calcium, and often magnesium. The primary and secondary walls are fundamentally composed of cellulose. Other substances may be added to cellulose or the latter may be modified in various ways.
Cellulose is the most complex insoluble carbohydrate. It is the principal material in building up the framework of a plant. Normally cellulose wall is not digestible; excepting in case of some fungi and bacteria it is not used as food.
Cellulose wall is permeable to water and solutes. Economically it is of great importance. Cotton fibres, commercially valuable fibres of flax and hemp and paper pulp are plainly cellulose materials.
Rayons, cellophene, nitrocellulose and some plastics are also manufactured from cellulose.
It gives the following microchemical reactions:
(i) Violet colour with chlor-zinc-iodine solution,
(ii) Blue colour with iodine followed by 50% sulphuric acid.
Lignin occurs in the walls of the woody tissues. It can give sufficient strength and rigidity to the plant members. Cellulose may be modified into lignin or may be associated with the same.
As a result of infiltration of this complex substance the walls become quite thick and hard. With gradual lignification of the wall most cells lose protoplasm and ultimately become dead.
It appears in the middle lamella and the primary wall and later may even be found in the secondary wall. Like cellulose lignin is also permeable to water and solutes. It gives requisite strength and rigidity to the plant members, so that they can stand against strains like tension and compression. Most of the mechanical tissues have lignified walls.
Lignified walls turn (i) bright yellow when treated with aniline sulphate solution; (ii) red with phloroglucin and hydrochloric acid and (iii) yellow with chlor-zinc-iodine solution.
It is a waxy or fatty modification occurring on the outer walls of epidermal cells of aerial organs. Cutin remains associated with cellulose, and often with pectic materials. Walls impregnated with cutin are said to be cutinised.
Cutin is impermeable to water and so cutinised walls can greatly check evaporation of water from plant surface. Though normally it is localised to outer walls of epidermal cells, often subepidermal cells are also cutinised.
Impregnation of cutin associated with deposition of other waxy materials results in the formation of a strong waterproofing layer on the outer surface of aerial organs. This effective layer is called cuticle. Some fruits also possess strong cuticle for retaining water.
(i) Yellow, if treated with caustic potash solution,
(ii) Yellowish-brown with iodine followed by sulphuric acid and
(iii) Pinkish-red with Sudan III.
It is another fatty substance resembling cutin in many properties. It is an important constituent of the walls of cork cells.
Walls made of suberin, suberised walls as they are called, are relatively more impermeable to water and hence can effectively check water loss from the surface.
Though normally restricted to the peripheral layers, suberin may also occur in the internal cells of endodermis and exodermis.
Suberin turns (i) yellowish-brown with chlor-zinc-iodine solution,
(ii) Brown with caustic potash solution.
Mucilage and Gum:
Mucilages and gums are substances related to carbohydrates of the cell wall. They can absorb water quite quickly and can retain the water. Mucilage is present in the seed coats and aquatic plants.
The seeds of common ‘topmari’—Plantago ovata of family Plantaginaceae is a nice example. In plants inhabiting dry regions like deserts mucilage is definitely helpful to germination, what is really the most serious problem there. In aquatic plants mucilage protects them from insects.
Mucilaginous hairs turn violet, when treated with iodine solution followed by sulphuric acid.
Deposition of various inorganic salts is not uncommon on the cell wall. Silicon particles are infiltrated on the wall of the plants belonging to grass and sedge families and horse-tails. Oxalates of calcium are present on the walls of-some plants.
Calcium carbonate crystals of peculiar shapes occur in the leaves of plants of families Moraceae, Urticaceae, Cucurbitaceae, Acanthaceae, etc. Cystoliths are conglomerate crystals of calcium carbonate deposited on a stalked projection of the cellulose cell wall.
Due to strong incrustation of lime they attain irregular shapes and often fill up the volume of enlarged cells (Fig. 507). Besides these mineral matters, various organic substances like resins, tannins, fatty matters, aromatic oils, etc., may also be impregnated on the wall.
Delimitation of the protoplast in plant body due to presence of sharp wall is by no means complete.
The primary wall is hardly uniform. Small breaks or meshes on the wall are traversed by extremely delicate and fine strands of cytoplasm to establish the connection between two adjacently-lying cells.
These threads are called plasmodesmata (Fig. 516). They are characteristic of all living cells. Plasmodesmata have been found in red algae, liverworts, mosses, pteridophytes and sper- matophytes.
They appear to remain aggregated in the primary pit fields (Fig. 509). After formation of secondary walls they possibly remain restricted to the closing membranes of the pits. On account of their extreme delicacy plasmodesmata cannot be normally demonstrated.
With some special techniques they may be readily seen in the endosperm of seeds with profuse stored food like date-palm —Phoenix (Fig. 504) and in the cotyledons. They are very thin in gymnosperms.
Though the presence of plasmodesmata definitely establishes the close relationship between the protoplast and the wall, their origin and development have been debatable.
Some workers believe that at die early stage of growth the wall is penetrated by cytoplasm, which becomes gradually narrower as a result of accumulation of microfibrils and pectic matters and ultimately form thin plasmodesmata threads.
Others are of opinion that plasmodesmata exist at the early cell-plate stage, and they also increase with the increase of the wall surface and often split. Various suggestions have been put forward from time to time about their functions.
Though nothing has been definitely known, it has been suggested that:
(i) They may be instrumental in conduction of stimuli —external and internal, through plant tissues;
(ii) may be mainly concerned with translocation of food, particularly in the regions like endosperm; and
(iii) may serve as channels for movement of some viruses.
Their occurrence in the haustoria-like bodies of parasites like Cuscuta and Orobanchi is suggestive of their roles in the movement of food and virus.
The cells usually remain compactly arranged in the meristematic condition. During gradual differentiation the close contact between the adjacently-lying cells may be partly broken down, so that some spaces appear between them.
These are known as intercellular spaces. They usually remain filled up with air or water and constitute a continuous system for aeration or conduction. The spaces vary to a wide extent in shape, size and arrangement.
They may be very small and, at the same time, some of them may be fairly large, so that the tissues become loose and spongy. The larger ones are usually called chambers and the elongated ones are known as canals.
The intercellular spaces are usually formed by separation of the walls along the areas of contact followed by contraction of separated parts. Spaces thus formed are called schizogenous (schizo —split), because these were formerly believed to be formed as a result of splitting of the middle lamella.
But it has been conclusively proved by Martens in 1937-38 and others that the process is much more complex, particularly in case of dividing cells. According to them during the division of a cell the new middle lamella, formed by physical and chemical changes in the cell plate, between newly-formed cells at first comes in contact with the primary wall of the mother cell (Fig. 517) and not with the middle lamella of that cell.
Thus new and old middle lamellae are separated by cellulose wall. A small cavity, referred to as intra-wall cavity, which is usually triangular in cross-section, appears at the point of contact. Now the parent cell wall dissolves opposite the cavity and the latter gradually reaches the outer middle lamella and thus the space is formed.
A separate cavity is often formed within the old middle lamella, which is truly intercellular, either before the formation of the intra-wall cavity, or during its formation or even following its formation. The two cavities may fuse up giving rise to a large space.
Aquatic plants possess particularly large spaces which often extend like canals through the internodes. Schizogenously formed spaces often constitute specialised structures, the secretory ducts in conifers and some spermatophytes.
The resin ducts of Pinus (Fig. 518A) and the secretory ducts of some members of families Compositae and Umbelliferae are the examples.
Here the cells lining the duct cavity are secretory and the products are released into the canals. The secretory cells are called epithelial cells.
Another type of intercellular space may arise as a result of destruction of the cells formerly occupying the position of the space. These are known as lysigenous (lysis = loosening) spaces.
Large air-spaces of some aquatic plants and of some monocotyledonous roots like maize, and the oil-cavities of Citrus fruits (Fig. 518B), Eucalyptus, etc., are formed by this method.
In case of secretory cavities the cells that are destroyed release their secretions in the space and they themselves remain in collapsed condition around the cavity.
Some cavities, though less common, are formed by the combination of both the methods. They are referred to as schizo-lysigenous ones. Protoxylem lacunae (Fig. 518C) of plants like maize are the examples.