ESSEX   BOTANY   AND   MYCOLOGY  GROUPS 

A central European taxon, very close to R. fluitans, and believed to be its autodiploid, that occurs occasionally in this country, either as a presumed introduction from the contents of an aquarium tipped into a pond, or possibly sporadically on bird’s feet from the continent. The floating form generally has a thallus wider than R. fluitans, typically 0.8 – 1.2mm wide, with very large air chambers (0.5-1.0mm long) giving the surface an obvious reticulate appearance, and a translucent margin, the whole strap-shaped thallus being proportionately thinner than in R. fluitans. When stranded on mud, unlike R. fluitans it forms semi-rosettes to 1.5cm across with the thalli up to 2.5mm wide.  Antheridia, archegonia and sporophytes appear never to have been recorded. Damsholt suggests growing  the thalli of doubtful plants on mud to see if they form rosesettes.[ Both R. rhenana and R. fluitans differ from R. canaliculata in the absence of a groove along the upper surface of the thalli].