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.
In the former case the formation of phelloderm is trivial in amount; in the latter, considerable, since this tissue has to replace the cast-off cortex, as a metabolic and particularly a storage tissue.
Both cork and phelloderm may be differentiated in various ways.
The development of periderm was a constant feature, and this tissue attained a great thickness, consisting chiefly of a phelloderm, produced on the inner side of the formative layer, and no doubt subserving a mechanical function.