noun, plural: periderms
(botany) An outer covering that replaces the epidermis of certain plants
(embryology) Epitrichium, i.e. the outermost layer of certain animal embryos or fetuses, especially up to the sixth month of gestation
(zoology) Perisarc, i.e. the outer layer of certain marine animals, such as those of hydroids
The term periderm etomolygically means “about, around” (from the Greek perí) and “skin” (from the Greek dérma). Thus, it generally pertains to an outer protective layer. In botany, the term periderm is the outer covering of certain plants, especially woody plants. It is the outermost layer of the bark made up of cork cells, cork cambium, and phelloderm. It replaces the epidermis of the stems and roots of woody plants. It is more resistant to mechanical injuries and more efficient at preventing water loss. It is because of the cork cells located at the outermost. These cells are dead cells impregnated with materials such as tannins and suberin. The cork cells come from the cork cambium, which is the meristematic component of the periderm. Apart from the cork cells, the cork cambium also provides new cells that form the inner layer of the periderm, phelloderm.
The term periderm is also used in embryology and zoology. In embryology, it pertains to the outermost layer of certain embryos and fetuses at sixth month gestational age. This layer, also called epitrichium, usually disappears before birth. In zoology, the periderm is another term for the perisarc, which is the hard outer covering of many hydroids.