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A modified form of cellulose found in the endosperm tissue of seeds. The cell walls of the endosperm tissue become thickened with hemicellulose until they occlude most of the lumen (i.e. they cells become completely filled with hemicellulose). These thick-walled endosperm cells become very hard when dry, with hard, bony texture similar to ivory. The large seeds of several palm tree species are rich in hemicellulose, and this is the source of vegetable ivory.

Like wood, vegetable ivory is essentially composed of thick-walled dead cells; however, unlike grainy hardwoods it has a texture and hardness similar to ivory. Vegetable ivory is remarkably dense, with a rating of about 2.5 on the scale of mineral hardness. (3.5 for a copper penny and 10 for diamond). Ivory-nuts can be polished in a stone tumbler, as you would polish agates and quartz: by using tin oxide and a buffing wheel.
Hemicellulose is a large polysaccharide which helps give structure to plant cells. Along with pectin, it forms an amorphous matrix in which cellulose fibrils from the plant cell walls are embedded. It has the chemical formula of C6H10O5 and consists of a chain of D-oxylose sugar units which has side chains of units of other sugars branching off it.

From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.

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