This is the phellogen, Periderm.
The phellogen derives its name from the fact that its external product is the characteristic tissue known as cork.
The internal tissue formed by the phellogen is known as phelloderm, and consists usually of ordinary parenchyma.
The extent of development of the phelloderm is dependent upon whether the phellogen has a superficial or a deep-seated origin.
E, epidermis; q, phellogen; 1, cells, and ~1, the pheliogen of the lenticel; k, cortical parenchyma, containing chlorophyll.
Phellogen is replaced by successive new phellogenic layers of de~per and deeper origin, each forming its own layer of cork.
The complex system of dead and dying tissues cut off by these successive periderms, together with the latter themselves in fact, everything outside the innermost phellogen, constitutes what is often known botanically as the bark of the tree.
A peculiar modification of periderm is formed by the phellogen in the submerged organs (roots or stems) of many aquatic or marsh-loving plants.
Such meristematic layers are called secondary meristems. There are two chief secondary meristems, the cambium and the phellogen.