The plant cell protoplast is enclosed by a fibrous wall that grows as the cell expands to its mature size, but which becomes cross-linked and eventually limits the growth of the cell, defining and supporting the cell and collectively providing support for stems and lives. Some cells, like photosynthetic and storage cells, only have a thin primary cell wall and other cells have both a primary wall and a thick, lignified and therefore rigid secondary cell wall, either to retain the cell’s shape against the tension of water movement through the plant, as in xylem cells or to provide concentrated regions of support or protection as in fiber cells or sclerids. The trunk of a tree is made up of concentric layers of water-transporting cells with secondary walls that serve both water-carrying and support functions.
The cell walls in wood are important structures. Unlike the lumen, which is a void space, the cell wall itself is a highly regular structure, from one cell type of another, between species and even when comparing softwoods and hardwoods. The cell wall consists of three main region that are the middle lamella, the primary wall and secondary wall. In each region, the cell wall has three major components that are cellulose microfibrils, hemicelluloses and a matrix or encrusting material, typically pectin in primary walls and lignin in secondary walls. Besides, the cell wall has been found to consist of many very thin layers (1, 2, 11), which seem to indicate a periodicity in the growth in thickness. Each layer is considered as a deposition of new cell wall material upon the surface of an older layer and while each thin layer may perhaps have two surface available for deposition of new material, we are considering here only innermost layer of the wall and the surface which is in direct contact with the cytoplasm of the cell, that is the inner surface of the cell wall. Measurements of these layers show that their thickness is equal to several hundred or perhaps several thousand glucose molecules. The x-ray studies indicate that each of these thin layers of microscopic visibility is made up of still thinner layers which are invisible and which are only one glucose residue in thickness. The inner surface of the cell wall would be, then one side of such a layer of glucose residues.
When considering the growth of a cell wall, three stages in the process may in general be recognized. Firstly, the formation of the original or first layers of cellulose. Second, the surface area growth of the wall as the cell becomes larger. Lastly, the increase in thickness of the wall. An attempt has been made here to show that inherent in the molecular structure of the wall are forces which must share very largely in the process of building up the wall itself in thickness at least. It was pointed out that crystallization forces alone could not account completely for the transformation of glucose to cellulose, that a third factor which seemed to be associated with living protoplasm was necessary.
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