The periderm is formed of three types of tissues. They are:
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Phellogen or cork cambium
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Phellem or cork
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Phelloderm or secondary cortex
Cork cambium: It is the secondary meristem which is formed from the permanent cells. It consists of a single layer in which the cells are rectangular in shape and flattened. The phellogen usually arises from the epidermis in certain plants it also arises from the cortex in rare it even originate from the phloem. Due to the periclinal activity of the cork cambium it forms the outer and inner derivatives. The outer derivatives give rise to cork or phellum and inner derivatives give rise to secondary cortex or phelloderm.
Cork: During the process of maturation of the phellum cells it loses it living content and the cell wall get thickened due to the depositions of fatty substances known as suberin. The cell become elongated and becomes impervious to water and gases.
Secondary cortex: These cortical cells are living, iso diametrical in shape and thin walled. The cell wall is formed of cellulose. The stem performing the photosynthetic function will have the chloroplast and starch in it.
