Periderm is a secondary dermal tissue which arises inside the stem ground tis-sue, closer to the surface. Like the other dermal tissue (epidermis), it is a complex tissue. It includes three layers (starting from surface): phellem (cork), phel-logen (cork cambium) and phelloderm (Fig.5.4). Phellem consists of largedead cells with secondary walls saturated with suberin, and is the main, thick-est component of periderm. Phellogen is a lateral meristem, like cambium; it often arises fragmentarily (and also temporarily) and does not cover the whole stem under-surface. But when phellem starts to grow, all peripheral tissues (like epidermis) will be separated from water transport and eventually die. Phel-logen makes phellem towards the surface, and phelloderm towards the next layer (phloem). Phelloderm is a minute tissue, and does not play significant role in the periderm.
In older plants, phellogen arises deeper, sometimes inside phloem and separates outer layers of phloem from vascular cylinder. All this mixture of tissues (phel-logen, phellem, phelloderm, epidermis and upper layers of phloem) considered as a bark.
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