An ultrastructure study of Pseudohimantidium pacificum Hust. & Krasske (Bacillariophyceae: Protoraphidaceae) with special reference to the labiate processes

Scanning and transmission electron microscopical studies were made of the marine epizoic diatom Pseudohimantidium pacificum Hustedt & Krasske to determine the relationship of the internal row of labiate processes to the external groove and pore structures described by previous authors. It was found that the external grooves were interrupted by rows of labiate processes, which extend pendantly into the valve cavity. It was also shown that these structures were unrelated to stalk extrusion at the cell ends. The combined structure is termed the "labiate groove". The phylogenetic status of this structure as well as the species with relation to the hypothetical "protoraphe" is discussed.