Exotheca

An exotheca refers to the outer layer or covering surrounding the spore-bearing structures in certain plants, particularly those in the mosses and hornworts. It is the outermost layer of the capsule wall (also called the sporangium) in these bryophytes. The exotheca provides protection to the developing spores inside the capsule. Once the spores mature, the exotheca may split or dehisce in a specific manner, facilitating spore dispersal. The structure and dehiscence mechanisms of the exotheca can be important features for taxonomic identification in some species. Variations in the exotheca may include differences in cell shape, thickness, ornamentation, and the type of dehiscence. The exotheca lies external to the amphithecium-derived layers of the capsule wall, such as the operculum and annulus (where present), and internal to any calyptra that might be present.