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Summary
The apothecial wall of the cup-fungus Urnula craterium consists of three layers, an outer black, non-elastic, leathery one, a nearly colorless, gelatinous, water-holding, medullary one, and an inner brown hymenial one.
The apothecia open as the result of pressure exerted by the paraphyses, which on swelling are changed from branched septate filaments to bead-like chains of ellipsoidal cells. This change is confined to a zone at the rim. Swelling results from increase in osmotic concentration, a consequence of the transformation of reserve glycogen in the hymenial layer to reducing sugar. Apparently melanin in the outer apothecial wall functions to absorb heat to the extent of initiating the activity of the glycolytic enzyme system. Essential moisture is supplied by the medullary layer.
The apices of the asci are turned upward, a phototropic response.
This mechanism for opening the cup to expose the hymenium is presently known to occur only in Urnula craterium and U. Geaster.
Addendum—Mycelial cultures of Urnula craterium originating from ascospores were planted on decaying branches of oak that had been sterilized. After approximately 6 months, black heaps of conidia had formed. This conidial stage appears to be Strumella corynoidea Sacc. and Wint., which Davidson (Mycologia 42: 735–742. 1950) suspected of being the conidial stage of Urnula craterium.