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Ergastic substances include crystals, oil drops, gums, tannins, resins and other compounds. Some of these substances help the organism function, keep up cell structure, or are just stored food material such as oil or fat. During the life of a cell, they can appear and disappear. Ergastic substances may appear in the protoplasm, in vacuoles, or in the cell wall.
Ergastic substances are usually absent in tissue with dividing cells, such as meristem tissue in plants. Plant cells often store noxious substances in leaf cells as a kind of defence against herbivory. In animals storing noxious substances is done as defence against predation. Storing venom for use in biting or stings is common in some groups of animals.
In general, if animals need quantity of a liquid substance they produce it with special cells in a gland, and send the product to where it is needed or stored.