Minute monoecious flowers, stalkless, auxillary. Fruit rarely produced, smooth or slightly warty. Branched stems with numerous whorls of 6-8 rigid, dark-green leaves. Leaves 1-2 times forked, segments fine-toothed. Stems to 1m.
Still or slow-moving lakes, ponds, canals. Rare, very local distribution in Ireland. Identified by Paul Green
Above: Rigid Hornwort growing with Spiked Water-milfoil. The Gearagh, Co.Cork. August 2007