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The typical SCW is a cellulose-rich structure synthesized by cells after expansion of the primary wall has ceased. In its best-studied forms, the SCW is a multi-layered structure composed of three major polymers: cellulose, hemicellulose and lignin. Such SCWs are found in tracheids and vessels of the xylem, in fibers of the xylem, phloem and interfascicular bundles, and in diverse sclereids distributed throughout the plant body. The production of such walls entails expression of hundreds of genes for the synthesis, targeting and assembly of these three complex polymers to form a coherent cell wall, controlled by a complex network of transcription factors (Zhang et al. 2014).

But not all SCWs are multilayered or lignified. For example, cotton fibers*, which are outgrowths of epidermal cells of the seed coat, make a nonlignified SCW made of nearly pure cellulose (Haigler et al, 2012). Parenchyma in the xylem and other tissues may form relatively thin SCWs with relatively low lignin content (Esau, 1977). Thus SCWs are found in a diversity  of forms in the plant kingdom.

Globally, SCWs are produced on a vast scale; much of the carbon and energy captured during photosynthesis on land is stored in SCWs, which form an important component of the global carbon cycle. They also have vast economic significance as the raw materials for the production of paper, lumber, textiles and a variety of chemicals. SCWs are burned for heating, cooking, and electricity generation and they are the biological sources of second-generation biofuels.  

MAJOR FUNCTIONS: SCWs in the xylem and in fiber bundles are essential for mechanical support of the stem, leaves and other organs. In the xylem they prevent collapse of the water-filled conduits when high transpirational demand results in substantial negative pressures.  SCWs of sclereids form the tough, protective shells of nuts, stone fruits and seed coats. Special forms of SCW appear in tension wood and compression wood


General reviews: Mellerowicz & Sundberg, 2008; Strabala and MacMillan 2013. 



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