Page 60 - Medicinal Plants_PharmD general

A secondary wall is formed sometimes after living cells have stopped
enlarging, and the complete mature cell wall may finally come to have a
thickness many times as great as the primary wall. The secondary wall may
be of cellulose or of cellulose impregnated with other substances. Some of
these substances notably lignin, add to their hardness and lend decay
resistance to wood. Others, like suberin and cutin, are fatty or waxy
substances that protect leaves and stems against water loss. In addition,
certain other materials may enter into the composition of the cell wall such
ass tannins, minerals, pigments, proteins, fats, and oils. It should be
emphasized that in mature, hard tissues, such as wood, lignin may be
deposited not only in the secondary wall but also in the primary wall and
middle lamella.

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