Abstract
R. duplex Lorbeer ex Muller thallus branches elongate rapidly in the vegetative phase and much more slowly in the reproductive phase. Slow growth and sporophyte formation are brought about by low nutrient conditions, in particular low nitrate availability. In such conditions increasing daylenghth reduces the time for sporophyte formation. Two out of 4 clones of R. fluitans L. and 1 of 2 clones of R. rhenana Lorbeer ex Muller formed archegonia when cultured on solutions providing low nutrient conditions. Antheridia were not formed on either of these 2 latter species under laboratory conditions. R. rhenana Lorbeer ex Muller is not known to be fertile in nature.