Ultrastructure of the ascus top and the ascospore wall in

Morphological and developmental studies on the ascus top and the ascospore wall of Fimaria theioleuca, F. cervaria, Pseudombrophila obliquerimosa, and P. deerata were carried out with light and electron microscopy. Ascus tops are found with roughly delimited ascostomes and opercula, without indentations, preformed weakened zones, or subapical rings. Dehiscence of the ascus takes place in an eroded, slightly thinner zone of the outer layer and next to a cor-responding wrinkled region in the inner layer. This corresponds with the struc-ture and dehiscence mechanism found in genera of Pyronemataceae studied thus far. During ascospore development in all species at first a smooth electron-dense secondary wall is formed. In F. theioleuca and F. cervaria this layer is permanent, but in P. obliquerimosa and P. deerata it usually breaks up to form an ornamentation. The presence of smooth and rough ascospores in the same species is explained by assuming a common process of development followed by a further final ripening in the rough-spored ones. The presence of two types of septa is recorded from excipular cells and paraphyses ofF. theioleuca