The genus Riccia has about 130 species, widely distributed in both tropical and temperate regions of the world. The plants grow predominantly on damp soil, moist and shady rocks and other similar terrestrial habitats. Riccia fluitans is, however, an aquatic species floating on stagnant or slowly running water. About 33 species of this genus have been reported from India. They occur mostly in the foot hills and plains. Riccia robusta and Riccia crystallina occur up to an altitude of 3000 meters in the Western Himalayas. Riccia ciliata, Riccia pathankotensis, Riccia robusta, Riccia gangetica, Riccia discolor, Riccia frostii, Riccia reticulata, Riccia hirsuta, Riccia melanospora and Riccia crystallina are other common Indian species. Riccia pathankotensis, Riccia discolor, Riccia melanospora, Riccia robusta and Riccia cruciata are endemic to India.
Gametophyte of Riccia
The gametophyte phage is predominant in the life cyle of Riccia, hence the main plant body is a gametophyte.
The gametophyte is flat, prostrate, dorsiventral, dark green and dichotomously branched. The branches of the thallus are linear to wedge-shaped. In terristrial forms, the plants usually take a typical rosette form due to the presence of several dichotomies close to each other. These rosettes are up to 15 cm in diameter. The thalli of Riccia reticulata are relatively thick, solcate and overlapped. In Riccia cruciata and Riccia pandei the thalli show cruciform arrangement. Riccia melanospora is characterized by the presence of several hairy epidermal outgrowths on the dorsal surface of the thallus. In terrestrial forms the thallus is approximetely 5-7 mm in length and 1-3 in width.
In most of the species each branch of the thallus has a midrip on the surface. The midrip is represented by a shallow groove, know as dorsal groove or furrow. It usually extends from the base to the tip of the thallus. In the male plants of Riccia discolor the midrip is restricted to the apical part of the thallus, whereas in the female plants it extends to the entire length of the thallus. In Riccia frostii, however, midrip is absent and upper surface of the thallus is flat or convex. At the apical portion of the thallus the midrip ends in a depression, known as apical notch. The growing point of the thallus is located in the notch.
The ventral surface ot the thallus bears many rhizoids and scales. The rhizoids, which are simply and outgrowths of lower epidermal cells, are unicellular and unbrached tubular structure. They are two type:
1.Smooth walled rhizoids – Their outer as well as inner walls are stretched and smooth, and
2.Tuberculate rhizoids – the inner wall of these rhizoids grows into peglike or plate-like ingrowths
Riccia melanospora and Riccia robusta have mostly smooth walled rhizoids.
The rhizoids attach the thallus to substratum and they also help in absorption of water and nutrients from the soil.
The scales are multicellular, one cell thick membranous structures. Their pink or violet colour is due to the presence of anthocyanin pigments. They are arranged in a single row along the margins of the thallus. In the apical region, they overlap each other and project forward to protect the growing point. The degree of persistence and size of the scale is dependent upon the habitat. Plants growing in moist terristerial habitats usually have small and ephemeral scales, whereas those of dry habitats have large and persistent scales. In some terristrial species like Riccia crystallina, Riccia cruciata and Riccia frostii the scales are either small, thin, delicae and inconspicuous or absent.
Riccia fluitans, an aquatic species, has a long, narrow, flat, ribbon-like, dichotomously brached thallus. It is about 30-50 mm in length and 1 mm: in width. It lacks scales and rhizoids, but terristrial forms of this species develop rhizoids and few small colourless scales at the anterior end of the thallus.
Interal structure or Anatomy of Riccia
Anatomically, the thallus is differentiated into a dorsal photosynthetic and ventral storage region.
The region consists of one or more vertical rows of unbranched photosynthetic filaments. All the cells of the photosynthetic filament, except the uppermost one, are similar. These cells contain mumerous discoid chloroplasts. The terminal cells are somewhat larger and colourless, and collectively form an ill defined loose and discontinuous upper epidermis. The photosynthetic filaments are separated from each other by narrow.
These air chambers open to the exterior through simple air pores on the dorsal surface of the thallus. The pores help in gaseous exchange.
In most of the species the air chambers in the photosynthetic region are usually in a single row but in Riccia robusta, Riccia cruciata and Riccia fluitans they occur in many irregular rows. In aquatic forms of Riccia fluitans, air chamber are almost completely closed by the dorsal epidermis but in terristrial forms each air chamber opens by a very minute central pore.
It lies below the photosynthetic region on the ventral side. It is made up of compactly arranged parenchymatous cells. These cells are devoid of chloroblast and do not have intercellular spaces. They contain starch as reserve food material. The innermost layer of the storage region forms the lower epidermis. Some cells of the lower epidermis give rise to rhizoids and scales.