Riccia

a genus of liverworts of the family Ricciaceae. The dichotomously branched thallus of basal and palisade tissue usually forms rosettes reaching 2 cm in diameter. The rosettes have linear or cordate segments, with rhizoids and amphigastria on the lower side. The palisade tissue consists of air chambers that often open to the top or of vertical rows of cells with air canals between them. Antheridia, archegonia, and a sporogonium (in the form of a rounded theca without a stalk or foot) are embedded in the upper part of the thallus. There are about 130 terrestrial or, less commonly, floating, species. They are distributed on all continents, most often in southern latitudes and the southern hemisphere. The USSR has about 20 species.