Ascocarp development in Rhytisma acerinum and R. punctatum

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Abstract
In Rhytisma acerinum, the apothecial primordium differentiates simultaneously into three layers: (1) roof, (2) hypothecium, and (3) basal stroma. By contrast, the primordium of R. punctatum differentiates initially into two layers, the upper one becoming the roof, the lower then differentiating further into hypothecium and stroma. Asci and paraphyses of both species originate in the hypothecium. In R. punctatum, uni- or bi-nucleate ascogenous initials occur near the bases of the paraphyses, binucleate initials apparently resulting when two uninucleate elements fuse. A binucleate initial may then (1) develop into a single ascus, (2) elongate slightly and produce several outgrowths which become asci, or (3) develop into a septate or aseptate ascogenous hypha bearing a number of asci. These three developmental patterns also occur in R. acerinum though ascocarps of this species were too mature to show unproliferated initials. Conjugate division in proliferating ascogenous elements is inferred from the frequent occurrence of nuclei in closely associated quartets or pairs. The systematic position of Rhytisma is evaluated and it is proposed that the genus be removed from the Hypodermataceae Rehm sensu von Arx and Müller and placed in the Rhytismataceae Chev.

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