The prostrate system of the Gelidiales: diagnostic and taxonomic importance.

Despite numerous recent studies on the Gelidiales, most taxa belonging to this order are still difficult to distinguish when in the vegetative or tetrasporic state. This paper describes in detail the morphological and ontogenetic features of the prostrate system of the order with the aim of validating its diagnostic and taxonomic significance. Observations were made on fresh, liquid preserved and cultured thalli. The morphology of the attachment system and the characteristics of rhizoid ontogeny have proven to be useful diagnostic and taxonomic characters. Species belonging to Gelidium, Pterocladia and Pterocladiella bear true attachment organs consisting of cells of both exogenous and endogenous origin. In the family Gelidiellaceae, in contrast, attachment to the substratum is effected by single independent exogenous rhizoids. The attachment rhizoids of the Gelidiales, both exogenous and endogenous, are of the same cell type, and are the so-called hyphae, historically considered typical of the family Gelidiaceae only. A new subdivision of the Gelidiales into three families is proposed here, with the amendment of both Gelidiellaceae and Gelidiaceae, and the Pterocladiaceae fam. nov. [ABSTRACT FROM AUTHOR]