XVI International Botanical Congess

Ultrastructural study has had a profound impact on the classification of red algae, especially at the ordinal level. The most systematically valuable cell feature, the pit plug, varies along taxonomic lines in the presence or absence of dome-shaped or thin outer cap layers, and in presence or absence of cap membranes. Molecular data now provides a means to evaluate proposals about the evolution of pit-plug character states. Application of pit plugs to systematic problems continues, and current examples, e.g. distinguishing freshwater Acrochaetiales from the chantransia stage of Batrachospermales, are described. Unicellular species and others without pit plugs have features, such as organelle associations, of systematic valuable. Cell structures that might be candidates for exploitation in systematic and phylogenetic studies are discussed.