pericoecium

Pericoecium is a term used in bryology, the study of mosses and liverworts, to describe the outermost layer of protective tissue surrounding the developing sporophyte in some mosses. It is essentially a modified calyptra, a cap-like structure that covers the sporangium (capsule) of a moss sporophyte. The pericoecium is derived from the archegonium, the female reproductive organ of the moss.

In many mosses, the calyptra is a simple, delicate covering that detaches and falls off as the sporophyte matures. However, in certain species, the archegonial tissue surrounding the base of the calyptra and the young sporophyte becomes thickened and persistent, forming the pericoecium. This structure can vary in size and shape depending on the moss species. It often encloses the base of the seta, the stalk that supports the capsule, and sometimes extends partially or completely around the capsule itself.

The pericoecium serves a protective function for the developing sporophyte and its sporogenous tissue, shielding it from desiccation and mechanical damage. Its presence and morphology are often important taxonomic characters used by bryologists for species identification. The term is less commonly used than calyptra and is typically applied only when this protective layer is notably developed and persistent.