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the layers of cells in the cork layer of plants. The cell membranes lignify to some degree, but the cells are not impregnated with suberin. Externally, phelloid resembles cork cells. The presence of phelloid may be detected by the violet coloring that appears after boiling a sample in a mixture of strong nitric acid with potassium chlorate and treating it with zinc chloriodide (cork does not cause a change in color).