calyptra

noun

(Gr. kalyptra: cover) A layer of cells that covers the developing sporophyte of mosses, liverworts, clubmosses, horsetails, and ferns. In mosses it forms a hood over the capsule and in liverworts it forms a sheath at the base of the capsule stalk. In mosses, a cap-like structure covering or partly covering the capsule and derived from the neck of the archegonium; a membranous covering of haploid tissue over the developing sporophyte, derived largely from the archegonial venter; the venter generally ruptures near the base (see epigonium), is carried upward by elongation of the seta, and frequently expands to form a protective covering over the capsule. In liverworts the structure is strictly an epigonium that generally ruptures near the apex and remains at the base of the seta, see vaginula. In a flower, a cap covering the stamens and carpels in the bud and formed by fusion or cohesion of perianth parts.