Riccia

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Riccia is a genus of liverworts in the order Marchantiales.[1]

Description

These plants are small and thalloid, i.e. not differentiated into root, stem and leaf. Depending on species, the thallus may be strap-shaped and about 0.5 to 4 mm wide with dichotomous branches or may form rosettes or hemirosettes up to 3 cm in diameter, that may be gregarious and form intricate mats.[1][2]

The thallus is dorsiventrally differentiated. Its upper (dorsal) surface is green and chlorophyll-bearing, with a mid-dorsal longitudinal sulcus (furrow or groove). Air pores occasionally break through the dorsal surface, giving the thallus a dimpled appearance. In exceptional members such as Riccia caroliniana [3][4] of Northern Australia and Riccia sahyadrica [5] of Western Ghats, the photosynthetic region is confined to the lower half of the thallus.[6]

The lower (ventral) surface has a mid-ventral ridge bearing multicellular scales that originate as a single row but normally separate into two rows as the thallus widens. The scales are multicellular and hyaline (glassy) in appearance, or violet due to the pigment anthocyanin.[1]

Rhizoids are nearly lacking in aquatic forms, but there are usually numerous unicellular rhizoids of two types on the ventral surface.[1] One type is called smooth and the other type is the pegged or tuberculated rhizoids; these help in anchorage and absorption. The inner surface of the smooth rhizoids is smooth while that of the tuberculate rhizoid will have internal cell wall projections.

Distribution and habitat

One of the more than 100 species in this genus is the "slender riccia" (Riccia fluitans), which grows on damp soil or, less commonly, floating in ponds,[6] and is sometimes used in aquariums.

Reproduction

Plants are usually monoicous, and sexual reproduction is by antheridia and archegonia. Asexual reproduction occurs by spores, by fragmentation of the rosettes, and by formation of apical tubers. Spores are large (45 to 200 μ) and formed in tetrads.[1]

The sporophyte of Riccia is the simplest amongst bryophytes. It consist of only a capsule, missing both foot and seta, and does not perform photosynthesis.[7]

Species

The following species are recognised in the genus Riccia:[8]

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References

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  1. ↑ a b c d e Script error: No such module "citation/CS1".
  2. ↑ Script error: No such module "citation/CS1".
  3. ↑ Na-Thalang, O., 1980. A revision of the genus Riccia (Hepaticae) in Australia. Brunonia, 3(1), pp.61-140.
  4. ↑ Cargill, D.C., Neal, W.C., Sharma, I. and Gueidan, C., 2016. A preliminary molecular phylogeny of the genus Riccia L.(Ricciaceae) in Australia. Australian Systematic Botany, 29(3), pp.197-217.
  5. ↑ Cargill, D.C., Manju, C.N., Deepa, K.M., Chandini, V.K. and Rajesh, K.P. 2019. A New Indian Species of Riccia with connections to Northern Australia. Journal of Bryology 42, DOI. https://doi.org/10.1080/03736687.2019.1611249
  6. ↑ a b MacVicar, Symers M. 1971. The student's handbook of British hepatics, p. 12-13. Hitchen Whelden & Wesley, Ltd.; Authorized reprint of the 1926 edition published by Sumfield & Day, Ltd.
  7. ↑ Text Book Of Botany Diversity Of Microbes And Cryptogams
  8. ↑ Script error: No such module "citation/CS1".

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