In this article we will discuss about the morphology of riccia with the help of diagrams.
Riccia a cosmopolitan genus with over 200 species, all species are terrestrial (Fig. 2.1 A) except for Riccia fluitans which is free-floating or submerged aquatic. This species is also capable of growing on land (Fig. 2. 1 B). In India, there are about 30 species of genus Riccia, with a maximal representation in eastern Himalayas and the hills of southern India.
The plant body is a thallus with an indistinct to distinct midrib and dichotomous branching (Fig. 2.1 A). The extent of dichotomy determines the growth pattern. More often, due to repeated dichotomies one finds a typical rosette (Fig. 2.1C) as seen in R. pathankotensis.
At times the rosette is masked; only two dichotomies result in a cruciate form, as seen in R. cruciata (Fig. 2.1D).
During rainy season (July to October) one can spot many species growing in plains, such as R. discolor (Fig. 2.1E), R. billardieri (Fig. 2.1F), R. gangetica and R. melanospora (Fig. 2,1G) The other very common species found from October to March are R. cruciata (Fig. 2. 1D), R. discolor (Fig. 2.2E) and R. frostii. The first two occur in garden beds and the third one grows along river banks.
Each thallus is usually linear to wedge-shaped, with a thickened midrib. On the dorsal surface, along the midrib is a conspicuous median longitudinal furrow which ends in a depression at the apical region; forming an apical notch.
The growing point of the thallus is situated in this apical notch. The deepest point of the furrow is towards the apical point, further down the furrow it gets reduced. The furrow is meant for retaining water; and is needed for fertilization. Rarely, on the dorsal surface are seen hairy epidermal outgrowths (Fig. 2.1G) as in R. melanospora.
On the ventral surface are rhizoids and scales (Fig. 2.2A). Towards the apex are transverse rows of scales, which are one-cell thick. These uni-stratose structures project forward and overlap the growing point; providing it protection as well as moist environment, by capillary conduction of water.
Posterior to the growing point an increase in width of thallus splits the scales into two parts. Therefore, the older parts of thallus have two rows of scales along the two lateral margins (Fig. 2.2A). Also on the older parts of thallus, some superficial cells of thallus project out into long unicellular structures; the rhizoids. A rhizoid helps to attach the thallus to substratum and helps in absorption of water and solutes from the soil.
The rhizoids (Fig. 2.2B) are of two types- (a) simple rhizoid, colourless with smooth wall (Fig. 2.2B) and (b) tuberculate rhizoid (Fig. 2.2B) with rod-like or plate-like ingrowths of the wall which project into the cell lumen. Tuberculate rhizoids are at times brown to dark brown. The scales are variously coloured; pink, red, violet to black. Prominent scales and tuberculate rhizoids are an indication of xerophytic environment.
In the species confined to moist habitats scales are at times light green, or are absent. Associated with this suppression of scales, the rhizoids present are smooth-walled or with faintly developed tubercles. This reduction reaches culmination in aquatic species.
R.fluitans, where rhizoids as well as scales may be absent. However, this species is also capable of growing on soil. The land-form (Fig. 2. 1B) of this plant is a thickened thallus, with numerous rhizoids and pale green to violet scales. These features are good enough to tempt a taxonomist to describe it as a new species.
