meristematic tissue located in the bark and that generates the phelloderm on the inside and the phellem on the outside. Cork cambium.

Merriam-Webster Online Dictionary
phellogen (noun)
a secondary meristem that initiates and in the periderm of a stem or root - phellem phelloderm called also cork cambium
phellogen (Wikipedia)
Cork cambium of woody stem (Tilia). It is different from the main vascular cambium, which is the ring between the wood (xylem) on the inside (top) and the red bast (phloem) outside it.

Cork cambium (pl. cambia or cambiums) is a tissue found in many vascular plants as a part of the epidermis. It is one of the many layers of bark, between the cork and primary phloem. The cork cambium is a lateral meristem and is responsible for secondary growth that replaces the epidermis in roots and stems. It is found in woody and many herbaceous dicots, gymnosperms and some monocots (monocots usually lack secondary growth). It is one of the plant's meristems – the series of tissues consisting of embryonic disk (incompletely differentiated) cells from which the plant grows. The function of cork cambium is to produce the cork, a tough protective material.

Synonyms for cork cambium are bark cambium, pericambium and phellogen. Phellogen is defined as the meristematic cell layer responsible for the development of the periderm. Cells that grow inwards from there are termed phelloderm, and cells that develop outwards are termed phellem or cork (note similarity with vascular cambium). The periderm thus consists of three different layers:

  • phelloderm – inside of cork cambium; composed of living parenchyma cells
  • phellogen (cork cambium) – meristem that gives rise to periderm
  • phellem (cork) – dead at maturity; air-filled protective tissue on the outside

Growth and development of cork cambium is very variable between different species, and is also highly dependent on age and growth conditions, as can be observed from the different surfaces of bark, which may be smooth, fissured, tesselated, scaly, or flaking off.

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