A thick secondary wall is laid interior to the primary wall after the cell has reached its maturity. It is laid down in series of at least three layers frequently termed s1, s2 and s3. It expands in thickness by accretion (apposition) that is, deposition of materials over the presented structures. The central layer (S2) is generally the thickest layer. In various cells though, the number of layers might be more than three. The formation of secondary wall is not regular in all cells. This outcomes in the differentiation of different types of cells, like collenchyma sclerenchyma, parenchyma, fibers and tracheids.The micro fibrils of the secondary wall are compactly arranged with various orientations in distinct layers embedded in a matrix of hemicellulose and pectin. Substances such as suberin, lignin, minerals waxes, resins gums, tannins, and inorganic salts like calcium carbonate, and calcium oxalate, silica and so on might be deposited in the secondary wall. The secondary wall is strongly anisotropic and layering can be noticed in it.
Fine structure of the cell wall specifically that of the secondary wall has been intensively studied. This study was stimulated since of its significance to the fiber, paper and other industries. The cell wall is built of a system of microscopic threads that is the micro fibrils that are grouped together in bigger bundles. The layering seen in the secondary wall is frequently the outcome of the dissimilar density of the micro fibrils. The secondary wall includes of two continuous interpenetrating systems one of which is the continuous system of microcapillary spaces and the other, the cellulose micro fibrils. Such spaces might fill with cutin, lignin, suberin, hemi-cellulose and other organic substances and at times even some mineral crystals.
The cellulose molecules comprise of long chains of linked glucose residues. The chain molecules are arranged in bundles that are usually referred micellae. The hypothesis of the existence of micellae was introduced by Nageli. According to Frey-wyssling and Muhlethaler the thread like cellulose molecules are arranged in bundles. All such bundle that forms an elementary fibril comprises of about 36 cellulose molecules. The elementary fibril is mainly crystalline.
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