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ABSTRACT
Ultrastructural aspects of development of hat-shaped ascospores and centrum organization in O. cucullatum were studied and compared with other species in Ceratocystis sensu lato apparently having similar ascospores. The ascospore wall consisted of a reniform primary and ornamented secondary wall. Secondary wall ornamentation incorporated three equally spaced and parallel ridges along the length of the ascospore that gave the ascospore a triangular outline in transverse section. Brim appendages developed on the secondary wall at the rounded ends of the primary wall resulting in the hat-shaped appearance of the ascospores in side view. These appendages differed considerably from those of the symmetrically shaped and hemispherical bowler-hat ascospores previously observed in C. moniliformis and C. fimbriata. Centrum organization in O. cucullatum was, however, similar to that previously illustrated of O. davidsonii. In both of the latter species, young asci were arranged in a cluster at the ascomatal base. This was different to that in C. moniliformis where young asci lined the inner ascomatal wall.