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the internal layer of the periderm in plants, which originates as a result of the division of phellogen cells in a direction parallel to the surface of the organ. Phelloderm cells resemble phellogen cells in shape, and their membrane and contents are similar to those of parenchyma cells of the primary cortex or of fibrous parenchyma. Unlike phellogen and parenchyma cells, however, phelloderm cells are always arranged in radial rows. In roots with secondary thickening, such as pine, the phelloderm often proliferates and is transformed into storage tissue.