The cell wall - Its essential features

The Cell Wall 

The primary wall is laid down at cytokinesis or cell-division. In many lower plants, and in the formation of spores in many plants of all grades, the cytoplasm divides by furrowing, that is by the formation of a cleavage plane which deyelops imnrds from the old wall tmyards the centre of the cell. In the great majority of cell diyisions, on the other hand, the two daughter cells are separated by a cell plate, which forms between the two daughter nuclei towards the end of the nuclear division process. The cell plate first appears near the middle of the cell and grows outwards until its edges touch the old all and join it. 

The cell plate is at first a fluid film, but it is soon solidified by the deposition in it of pectin and protein. The latter soon disappears from new cell walls in mature tissues, but it seems to be more persistent in the cell walls of meristematic tissues. This first solid layer forms the intercellular matrix or middle lamella. Its subsequent changes of composition are not yet clear, but it seems probable that the pectin or pectinic acids first present become combined with cellulose to produce an insoluble substance closely resembling pectose, and that Calcium later enters into its composition, perhaps in the form of Calcium pectate. In woody tissues, hmyever, the middle lamella may become intensely lignified (about 70 per cent.) and pectic substances disappear or'are at least masked. 

While the middle lamella is still quite young a thin cellulose layer is deposited on each side of it, and the three layers together constitute the primary wall. No further deyelopment of this wall takes place until the cell ceases to divide, except that the new middle lamella grows into the surrounding walls and becomes continuous with their middle lamellae. 

When a cell has ceased to grow and divide it may develop a secondary wall. This also consists of three layers, of which the middle one. is much the thickest. All three contain cellulose, but the thin outer and inner layers contain more pectic substances than the middle layer. When lignification of the wall occurs the lignin substances are laid down chiefly in the thick cellulose layer, in the form of longitudinal radial plates, rich in lignin, divided by layers rich in cellulose. Eyen in the walls of a woody cell there is thus a large amount of cellulose, with which the lignin is mixed or perhaps chemically combined. 

The deposition of the secondary wall layers is seldom, if ever, perfectly uniform; there are commonly a number of unthickened spots, which are called pits, at which the cells are separated only by the primary wall. Through this thin partition there frequently run extremely fine protoplasmic threads, the plasmodesma or intercellular connections. Although the plasmodesma are characteristic of the pit areas they are not necessarily confined to them, and in some cells with very thick secondary walls, for example in the storage tissues of some seeds, they may be seen to penetrate the whole thickness of the wall. 

How these plasmodesma are formed is still uncertain. Formerly it was supposed that they were the remains of protoplasmic fibrils which connect the two daughter nuclei after a division. If this were true they could only occur between sister cells, which is certainly not the case. On the other hand, it is difficult to believe that such fine threads could bore their way through a wall already formed. It would therefore seem probable that they are established at an early stage, when the cells are separated only by the soft matrix or middle lamella. The presence of protein in the young lamella probably indicates that it is permeated by the cytoplasm. At this stage they may be very numerous, but they occur in groups, and although some may be obliterated as the cell matures, these groups are, in general, the sites at which pits are formed. That the plasmodesma actually provide protoplasmic continuity from cell to cell is shown by the fact that infecting viruses will only pass through cell walls where plasmodesma are present. 

The cellulose wall is doubly refractive, and it may therefore be concluded that it is crystalline in structure. The nature of the molecular arrangement is not, however, indisputably settled. The most generally held view is that which originated in the researches of Nageli in 1863, who called the crystal­line units of the cell wall structure micellae. Analysis of cellulose with X-rays by Preston and others shows a fibrillar structure, due to the linking together of B-glucose molecules into chains about 500 Angstrom units long by about 50 Angstrom units thick. As the single glucose molecule measures about 5 Angstrom units in length, a chain of 1,000 would be about 5,000 Angstrom units or 0.5u Iong, which is within the microscopic range. Adjacent chains lie parallel to one another and about 100 of them constitute one micelle. The latter in turn are aggregated into "fibrils," which give rise to the striations visible in the wall, especially after treatment with a swelling agent, the striations being probably slip-planes due to mechanical displace­ment of the fibrils. In addition to cellulose there seems to be some pectic material and some wax present, mixed with the cellulose and probably forming a matrix around the cellulose units. The orientation of the fibrils is spirally around the cell at varying degrees of inclination to the long axis, the thickness of the wall being apparently built up of a series of successive sheets of micelles or of fibre units, the inclination of the spiral being different in successive sheets and changing as the proportions of the cell change during growth. The angle of the spiral is more nearly longitudinal in the thick layer of the wall and more nearly transverse in the thin layers, though it is very variable. The molecular pattern exposed at the inner surface, where it is in contact with the cytoplasm, apparently acts as a molecular mould for the building in of fresh glucose units into the cellulose pattern. 

The growth of the cell in length does not involve any stretching of the existing cellulose chains, for the forces holding their glucose units together are much too great for this to be possible. It is due either to the intercalation of new sets of fibrils bet\yeen those already existing or to their growth in length by the addition of new glucose molecules. Growth by this inter­calation process is called intussusception and is characteristic of the young phase of the cell, while the addition of successive thickening layers, con­centrically, is called apposition and is a feature of cell maturity. 

Secondary cell walls often show a visible structure of concentric layers. These are not, of course, single sheets of cellulose molecules, but seem to represent variations in the amount of cellulosic and non-cellulosic material present. In the ,yall of the cotton fibre it has been possible to correlate these concentric layers with the daily metabolic cycle, the denser layers representing deposition by day, and the lighter layers being formed during the night. They are analogous to the growth rings in trees, and are similarly affected by variations in the environment at different times. They can be definitely dated, and the corresponding layers in the cell walls in neighbouring plants are found to be similar. 

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