RHODOPHYCEAN PIT PLUGS: AN ULTRASTRUCTURAL SURVEY WITH TAXONOMIC IMPLICATIONS

The fine structure of pit plugs was examined in 63 species of red algae representing 34 families. The number of plug cap layers was found to be a taxonomically reliable character when tested against a recent revision of the Rhodymeniales which established the order Palmariales. In a broader survey involving all the orders of red algae possessing pit plugs, three basic plug cap configurations were detected. These differ in having one, two, or no plug cap layers. Some taxa have plug caps in which the outer cap layer is greatly elaborated, however, this layer was shown to be positionally and cytochemically identical, thus probably homologous, to the outer cap layer of the typical two-layered plug cap. The pit plug character confirms the validity of the order Palmariales and indicates that the Nemaliales and Cryptonemiales are heterogeneous assemblages. Revision of these groups along lines indicated by the plug character is supported by significant features of morphology and reproduction. These correlations lead us to propose that Rhodophysema and Coriophyllum be transferred to the Palmariales, that the Gelidiales be resurrected, that the Bonnemaisoniales (including the Naccariaceae) is deserving of recognition, that the Batrachospermaceae be raised to a higher rank which will also receive the Lemaneaceae and the Thoreaceae, and that the Corallinaceae and Hildenbrandiaceae both be raised to ordinal status.