Tunica (biology)

In biology, a tunica (ˈt(j)uːnɪkə, UKˈtʃuːnɪkə; : tunicae) is a layer, coat, sheath, or similar covering. The word came to English from the Neo-Latin of science and medicine. Its literal sense is about the same as that of the word tunic, with which it is cognate. In biology one of its senses used to be the taxonomic name of a genus of plants, but the nomenclature has been revised and those plants are now included in the genus Petrorhagia. In modern biology in general, tunica occurs as a technical or anatomical term mainly in botany and zoology. It usually refers to membranous structures that line or cover particular organs. In many such contexts tunica is used interchangeably with tunic according to preference. An organ or organism that has a tunic(a) may be said to be tunicate, as in a tunicate bulb. This adjective tunicate is not to be confused with the noun tunicate, which refers to a member of the subphylum Tunicata. In botany there are several contexts for the term. As a general, but not comprehensive, descriptive term in botanical anatomy John Lindley in the 19th century defined tunica as "any loose membranous skin not formed from epidermis". The apical meristem, in particular in Angiosperms, has an outer layer of cells called the tunica; its role in growth and development differs from that of the inner meristem, or corpus. Bulbs and corms often have protective outer coverings of dead material. Particularly in fields such as horticulture and taxonomic description, such bulbs and corms are said to be "tunicate" — having tunics. Usually such a tunic is derived from the bases of sacrificial dead leaves, often cataphylls specialised for the purpose and dying in place. Such tunics may be delicate, brittle membranes such as those around the bulbs of onions, but the many variations reflect a wide range of adaptations in various species. Many of the larger Amaryllidaceae in particular, for example Boophone species, accumulate thick layers of rot-resistant leaf-base material around their bulbs.