The floating liverworts of Essex
Riccia fluitans L. emend Lorbeer
An aquatic light green to yellow green liverwort found floating, often in dense masses at or just
below the surface of ponds, stagnant abandoned canals, gravel and clay pits, or scattered on mud
amongst marginal vegetation in both acidic and basic mesotrophic to eutrophic waters (Hill et al.).
Often occurring in ponds that dry up in summer, leaving the thalli stranded. The thalli are strap-
shaped and repeatedly bifurcate at an angle of around 45°, so that they curve back on themselves
after several dichotomies. Unlike /?.. rhenana, they retain their shape when stranded on mud. They
gradually increase in width towards the apices, ranging from 0.3 to 2.0mm wide and l-5cm long .
Most of the thallus consists of tiny air chambers up to 0.3mm long, with a single layer of cells
above and below, and with minute pores opening to the upper surface, but in the centre the tissue is
thicker below with smaller air chambers above. The apices are grooved and the surface of the
straps have a reticulate appearance due to the air chambers. Semi-lunate scales occur sparsely
along the mid-line on the underside, increasing in size towards the apices. Nationally it occurs
mainly in the south-east of England, with only scattered records for the west country, Wales and
Ireland and is absent from Scotland (c.140 x 10km sq records for England and Wales). On the
continent, however, it extends northwards to southern Scandinavia, and is found widely in temperate
Europe and Asia, extending south eastwards through Indonesia to Japan and New Zealand, and is
also found in temperate North America. It can persist at the same locality for many years, the plants
dying back to just the swollen apices, which sink to the bottom, and rise up again in the spring.
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Essex Naturalist (New Series) 21 (2004)