Plants forming mats, prostrate with erect laterals to 20mm long, densely branched, especially in distal portions; attached by a holdfast consisting of a mass of multicellular rhizoids issuing from cortical cells, or secondary attachments from tips of branched indeterminate axes; main axes polysiphonous, terete, to 0.5mm wide, with laterals arising mostly at every axial cell; laterals usually polys