In this article we will discuss about the structure of Riccia with the help of diagrams.
External Structures of Riccia:
The gametophytic plant body of Riccia is a dichotomously branched dorsiventral prostrate thallus. In terrestrial habits, the plants usually occur in a rosette form due to the presence of a number of dichotomies that grow together from one place (Fig. 6.1 A-C).
These rosettes are up to 15 cm in diameter. Each dichotomy is a linear to wedge shaped structure where the median region is thickened with a conspicuous longitudinal groove on the dorsal (upper) side ending in a notch. The growing point is located at the tip of the notch. Transverse row of scales are present on the ventral (lower) surface.
The scales are one cell in thickness that are more crowded near the apex and overlap the growing point, often pigmented violet. In the mature portion of the thallus, each scale is split into two to form two rows of scales along the two margins (Fig. 6.1 D).
The thallus also bears numerous rhizoids in the ventral surface which are unicellular, elongated, tubular and hair-like structure. Rhizoids serve the purpose of anchorage to the substratum and absorption of water and nutrients from soil.
There are two types of rhizoids: some with smooth walls and others tuberculate with internal peglike or plate-like projections of the wall (Fig 6.1 E & 6.2A). Rhizoids are devoid of protoplasm at maturity and are absent in aquatic forms.
Internal Features of Riccia:
The thallus of Riccia shows an internal differentiation of tissues (Fig. 6.2A). A vertical transverse section (V.T.S.) of the thallus shows two distinct regions (a) the ventral storage region and (b) the dorsal assimilatory (photosynthetic) region.
Storage Region:
The ventral region of the thallus is formed of a compact colourless parenchymatous tissue (Fig. 6.2A), often contains starch. This region serves as the storage region of the thallus. The scales and rhizoids develop from the basal part of this tissue.
Assimilatory Region:
This region is composed of vertical rows of green, chloro- phyllous cells that are separated by vertical air canals (Fig. 6.2A). Usually each air canal is very narrow and surrounded by four vertical rows of cells (Fig. 6.2B). However, in a few species (e.g., R. vesiculosa) the canals may be wider and surrounded by 8 rows of cells.
The canals are open on the dorsal surface so that the top of the thallus is porose, but does not show any organised air pores like Marchantia. The outermost cell in each vertical row is larger and colourless which forms an interrupted one cell thick epidermis.
Apical Growth:
The growth in length of the thallus takes place by means of 3 to 5 apical cells (initials). They are situated in the apical notch (growing point) and are more or less triangular in outline. Some of the median cells fails to divide or split vertically, as a result two separate growing points are formed, so that the thallus becomes dichotomous.