ESSEX   BOTANY   AND   MYCOLOGY  GROUPS 

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Riccia cavernosa Hoffm.                                                                              Caverous Crystalwort

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Most of our Riccia species that form rosettes, are winter annual colonists of bare moist ground, typically in arable fields, where they develop during the autumn, winter and spring, surviving the summer as spores. This species however, is an autumn annual that develops on the wet mud around reservoirs, lakes and pools that have dried out during the summer and refill over the late autumn and winter. A large plant, up to 3cm in diameter, bifurcating into (2)3(5) rather truncate lobes, with thick vertically-rounded margins, and very wide shallow grooves that fizzle out a short distance back from the apex. The basal pad of isodiametric cells gives rise to a layer of irregular vertical columns of chloroplast containing cells, topped by colourless oval epidermal cells; but early on in development, large air spaces develop at several levels in this tissue due to the collapse of some of the epidermal and chlorenchyma cells. The light reflecting inside these cavities gives the thallus a characteristic glistening appearance. It is monoecious, and usually produces up to 20 sporophytes per rosette, that are buried deep in the thallus, the clusters of dark reddish-brown spores can usually be seen deep down in the spongy thallus. As in other Riccias the sporangia remain immersed in the gametophyte thallus. The spores have an irregular honeycomb pattern of ridges on the surface that divide up the hemispherical outer face of the spores into 4-6 areas known as  alreolae. The closely related rather similar R. crystallina (confined to horticultural land in Cornwall and the Scilly Isles) with which it was formerly confused, has up to 10 such alreolae across the curved face. R. cavernosa is said to prefer a neutral pH, and to withstand a high level of nutrients. At the Hanningfield Reservoir site, the lagoon involved is used for dumping water treatment chemical waste, and the concentration of aluminium sulphate is probably very high.