A Detailed Morphological Study of the Type Species of Osmundea (Rhodomelaceae, Rhodophyta)

Abstract

Details of vegetative and reproductive development of Osmundea osmunda (S. G. Gmelin) Nam et Maggs (Rhodomelaceae, Rhodophyta), the type species of the resurrected genus Osmundea Stackhouse, are presented based on an examination of liquid-preserved and herbarium specimens collected from the British Isles. Each axial segment bears one trichoblast and two pericentral cells on either side of basal cell of the trichoblast. Spermatangial development follows the typical filament type in which spermatangial filaments derive from apical and epidermal cells in pocket-shaped pits with ostiole-like upper openings. Procarp-bearing segments of female trichoblasts have five pericentral cells, of which the fifth acts as the supporting cell of the carpogonial branch. Tetrasporangia are produced from random epidermal cells in apical pits of branchlets, and are arranged parallel to the stichidial axis. Two presporangial cover cells also exhibit parallel arrangement to the stichidial axis as seen in surface view. Among the known Osmundea species, O. osmunda is characterized by having a strongly compressed thallus with distichous branching, a discoid holdfast, lack of secondary pit connections between epidermal cells, presence of lenticular medullary thickenings, non-palisade-like arrangement of epidermal cells in a transverse section of branchlets, pocket-shaped spermatangial pits in adaxial series between a branchlet and its parent axis, clustering terminal sterile cells in spermatangial filaments, procarp-bearing segments with five pericentral cells in female trichoblasts and cystocarps with non-protuberant ostioles. A discussion on phylogenetic implications of these vegetative and reproductive features is also included.