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amphithecium (noun, pl. amphithecia;ascocarp term; fromGreek "amphi-" for both, on both sides, around) – an anatomical term more or less equivalent to the thalline exciple, i. e., the lateral, thalline part surrounding an ascocarp of lichenized ascomycetes. The term has sometimes been used in conflicting ways. The Greek syllable amphi- in the sense of "both" might suggest that amphithecium could be used in the broadest sense to include all lateral layers, even the parathecium (or proper exciple). However, the two main hyphal layers surrounding an ascocarp of some lichenized fungi have a distinctly different origin and must not be lumped together: (1) the inner layers originate from the ascocarp itself; (2) in contrast, all outer layers are derived from the thallus as additional, secondary layers. To avoid confusion the Greek syllable amphi should therefore be used in a restrictive sense meaning "on both sides" or "around". This concept implies that the term amphithecium only refers to hyphae surrounding the ascocarp itself, including the hyphae from the parathecium. In this case, the parathecium no longer can be regarded as an outer layer. Instead the amphithecium becomes the outermost layer, embracing the parathecium. Additional outer hyphae forming an amphithecium are always absent in non-lichenized fungi. Some lichenized ascomycetes show no tendency form an amphithecium, others have an indistinct or poorly developed amphithecium. In apothecioid ascocarps amphithecial hyphae form a thalline margin. If these ascocarps remain immersed within the thallus, it is often difficult to evaluate whether this thalline margin is indeed present. In perithecioid ascocarps hyphae from the amphithecium may develop into an involucrellum, typically forming a shield like structure on the surface, rarely, however, surrounding even the base of the perithecioid ascocarp. A general term referring to all enveloping layers surrounding the ascocarp is exciple. In a broad sense this term includes both the proper exciple, i. e., the parathecium, and the thalline exciple, i. e., the amphithecium. Terms describing parts of the amphithecium (thalline exciple): cortical exciple, medullary exciple, amphithecial cortex. Related terms: proper exciple, lecanorine, involucrellum. This is:part of ascocarp.

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