Riccia is a large genus of liverworts — small, non-vascular land plants — belonging to the family Ricciaceae within the order Marchantiales, class Marchantiopsida, and phylum Marchantiophyta. The genus comprises more than 100 recognised species and is distributed across a wide range of terrestrial and aquatic habitats worldwide.
Unlike mosses and vascular plants, Riccia species have a thalloid body plan: the plant body (called a thallus) is not differentiated into roots, stems, or leaves. The thallus is typically small, either strap-shaped and approximately 0.5 to 4 mm wide with forking (dichotomous) branches, or arranged into rosettes or half-rosettes up to about 3 cm in diameter, often growing gregariously to form dense mats.
The thallus shows dorsoventral differentiation. The upper (dorsal) surface is green and photosynthetically active, marked by a mid-dorsal longitudinal groove (sulcus); in some species, air pores break through the dorsal surface, giving it a dimpled texture. The lower (ventral) surface bears a central ridge carrying multicellular scales arranged in one or two rows; these scales are either glassy (hyaline) or violet due to the pigment anthocyanin. Two types of unicellular rhizoids occur on the ventral surface: smooth rhizoids and pegged (tuberculate) rhizoids, which anchor the plant and assist in water and nutrient absorption. Aquatic forms largely lack rhizoids.
One well-known member, Riccia fluitans (slender riccia), is particularly notable for its amphibious habit — growing on damp soil or floating freely in ponds — and is widely kept in freshwater aquariums.
Distribution
Riccia species occur across a broad global range in both aquatic and terrestrial habitats. Riccia fluitans is found growing on damp soils and floating in ponds in many parts of the world. Regional representatives include Riccia caroliniana in Northern Australia and Riccia sahyadrica in the Western Ghats of India.
Ecology
Riccia species occupy a range of moist habitats including the margins of ponds and streams, damp soil, rock surfaces, and open disturbed ground. Some species, notably Riccia fluitans, are fully aquatic or semi-aquatic, floating freely on still water or colonising wet mud. Rhizoids on the ventral surface anchor terrestrial forms and aid absorption; aquatic forms largely lack rhizoids.
Taxonomy Notes
Riccia belongs to the family Ricciaceae within the order Marchantiales, which is one of the major lineages of complex thalloid liverworts in the class Marchantiopsida (phylum Marchantiophyta). It is the largest genus in the family Ricciaceae.
