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. 146 Essex Naturalist (New Series) 21 (2004)