AN EXPANDED SURVEY OF THE ULTRASTRUCTURE OF RED ALGAL PIT PLUGS1

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Abstract
The fine structure of pit plugs in 90 species of red algae was examined, bringing the total number of species in the continuing survey to 153. The organization of plug caps was confirmed to be a stable, predictable trait within thalli, between generations in heteromorphic life histories, and within the presently recognized orders, with one exception—the Acrochaetiales. Two forms of the outer cap were found in this group, a thin plate, as in the Nemaliales and Palmariales, and a dome, as in Batrachospermales and Corallinales. Variation of pit plug structure indicates that the Acrochaetiales are a heterogeneous assemblage and that pit plugs will be useful in reappraising their systematics. The systematic affinities of several species of uncertain affinities are clarified.Schmitziella endophloeaBornet et Batters is excluded from both orders, Corallinales and Acrochaetiales, with which it previously was allied. Although other ordinal attributions are not precluded by pit plug structure alone, pit plug structure is consistent with placement ofApophlaea sinclairiiHarvey andHildenbrandia rivularis(Liebman) J. Agardh in the Hildenbrandiales,Plagiospora gracilisKuckuck,Schmitziella endophloea,andWurdemannia miniata(Duby) J. Feldmann et Hamel in the Gigartinales, andPseudorhododiscus nipponicusMasuda in the Palmariales.