Periderm
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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