Riccia
Terrestrial, rarely free-floating aquatic, ephemeral or perennial, dioicous or monoicous. Thallus simple to repetitively forked, radiating out from a central point to form a circular outline or forming irregular group, white, green, yellow-green or blue-green, glabrous or with dense to sparse hairs on margins and sometimes on adaxial surface over capsules or rarely throughout (not in Victoria), adaxially with a narrow to broad groove along midline; margins plane incurved or undulate. Photosynthetic tissue of thallus usually adaxial to storage tissue, rarely abaxial to storage tissue (not in Victoria), divided vertically into 2–4 irregular tiers of air chambers with specialised pores, or with regular vertically oriented rows of cells separating narrow canal-like air chambers without specialised pores at surface; specialised air pores and adjacent epidermal cells often disintegrating with age to reveal upper tier of air chambers, leaving the adaxial surface appearing lacunose. Storage tissue rarely absent (not in Victoria). Oil cells absent. Abaxial scales in 1 or 2 rows, small, inconspicuous, restricted to near growing apex and often caducous, or conspicuous, semi-circular to triangular and extending to margin or beyond, hyaline or purple, rarely green and photosynthetic (not in Victoria), entire or rarely denticulate (not in Victoria). Adaxial epidermal cells chlorophyllose and thin-walled, or hyaline and thin or thick (not in Victoria) -walled, when hyaline sometimes multiple layers tall. Rhizoids with and without internal peg-like thickenings. Capsules embedded in thallus, becoming exposed to surface either adaxially or abaxially. Spores tetrahedral with a convex distal face and a proximal face with a triradiate ridge, or rarely globular without distinct faces, areolate, papillate (not in Victoria) or rarely smooth, usually with less pronounced ornamentation on proximal face, brown, reddish brown or purplish brown, shed singly or rarely in tetrads (not in Victoria).
Widespread throughout the world except polar regions, with 242 species, but most diverse in areas with Mediterranean climates such as southern Australia, USA, South Africa and the Mediterranean, and in India (Cargill et al. 2016; Söderström et al. 2016); 16 species in Victoria.
Species with their photosynthetic tissue adaxial to the storage tissue and divided into tiers of air chambers bounded by cells and specialised air pores at the thallus surface have been recognised as the subgenus Ricciella, whereas those with vertically orientated columns of photosynthetic cells separated by narrow air channels and without specialised air pores have been recognised in subgenus Riccia (Na-Thalang 1980; Cargill et al. 2016). These two photosynthetic tissue arrangements are distinct and easily distinguished in cross sections of fresh material, and are routinely utilised in the first couplet of Riccia identification keys to separate species. Despite the apparent distinctiveness between these subgenera, phylogenies of Riccia based on chloroplast and nuclear DNA regions have revealed that the Ricciella-type thallus has arisen from the Riccia-type thallus multiple times (Cargill et al. 2016). Likewise, other distinctive characters that have been used to define subgeneric groups such as the presence of hairs, appear to not define genetic lineages and consequently subgeneric classification in this genus requires re-evaluation (Cargill et al. 2016). Several of the Riccia species of Victoria also comprise several separate lineages suggesting that several unrecognised species may be harboured within these old species boundaries that have similar morphology as a result of adaptation to similar environmental conditions (Cargill et al. 2016).
Riccia species are important components of soil crusts in drier regions in sites without cover of larger vascular plants. Some species e.g. R. inflexa Taylor and R. nigrella DC, have thallus margins that become incurved upon drying to conceal the adaxial photosynthetic layers, protecting them from desiccation. As a result when dry these species are much less visible, with only the black of the abaxial scales visible as thin lines.
Cargill, D.C., Neal, W.C., Sharma, I. & Gueidan, C. (2016). A preliminary molecular phylogeny of the genus Riccia L. (Ricciaceae) in Australia. Australian Systematic Botany 29: 197–217.
Na-Thalang, O. (1980). A revision of the genus Riccia (Hepaticae) in Australia. Brunonia 3: 61–140.
Söderström, L., Hagborg, A., von Konrat, M., Bartholomew-Began, S., Bell, D., Briscoe, L., Brown, E., Cargill, D.C., Costa, D.P., Crandall-Stotler, B.J., Cooper, E.D., Dauphin, G., Engel, J.J., Feldberg, K., Glenny, D., Gradstein, S.R., He, X., Heinrichs, J., Hentschel, J., Ilkiu-Borges, A.L., Katagiri, T., Konstantinova, N.A., Larraín, J., Long, D.G., Nebel, M., Pócs, T., Puche, F., Reiner-Drehwald, E., Renner, M.A.M., Sass-Gyarmati, A., Schäfer-Verwimp, A., Moragues, J.S., Stotler, R.E., Sukkharak, P., Thiers, B.M., Uribe, J., Váňa, J., Villarreal, J.C., Wigginton, M., Zhang, L. & Zhu, R. (2016). World checklist of hornworts and liverworts. Phytokeys 59: 1–828.