Structure of Riccia (Explained With Diagram)

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 dorsiventral prostrate thallus that is dichotomously branched. Due to the presence of a lot of dichotomies that arise from a single location, terrestrial plants are typically found in rosette form (Fig. 6.1 A-C).

These rosettes have a maximum diameter of 15 cm. Each dichotomy is a linear to wedge-shaped structure with an enlarged middle region and a prominent longitudinal groove terminating in a notch on the dorsal (upper) side. The growth point is situated at the notch’s tip. There is a transverse row of scales on the ventral (lower) surface.

The scales are one cell thick, are denser near the apex, overlap the developing point, and are frequently coloured violet. Each scale in the mature section of the thallus is divided into two to generate two rows of scales along the two borders (Fig. 6.1 D).

Structure of Riccia

Figure: Structure of Riccia

The ventral surface of the thallus is covered with many unicellular, elongated, tubular, hair-like rhizoids. Rhizoids fulfil the functions of substratum anchoring and absorption of soil water and nutrients.

There are two types of rhizoids: those with smooth walls and those with tuberculate walls that have internal protrusions that resemble pins or plates (Fig 6.1 E & 6.2A). At maturity, rhizoids lack protoplasm, which is also absent in aquatic forms.

Internal Features of Riccia

The thallus of Riccia displays internal tissue differentiation (Fig. 6.2A). A vertical transverse section of the thallus reveals two separate regions: (a) the ventral storage region and (b) the dorsal assimilatory (photosynthetic) region.

1. Storage Region

The ventral part of the thallus is composed of a dense, colourless parenchymatous tissue (Figure 6.2A), which frequently contains starch. This region acts as the thallus’ storage zone. The scales and rhizoids emerge from this tissue’s basal portion.

2. Assimilatory Region

This region consists of vertical rows of chlorophyllous cells separated by vertical air canals (Fig. 6.2A). Typically, each air channel is flanked by four vertical rows of cells and is quite thin (Fig. 6.2B). In a few species, such as R. vesiculosa, the canals may be broader and encircled by eight rows of cells. The canals are exposed on the dorsal surface, making the top of the thallus porous, although it lacks the organised air pores of Marchantia. Each vertical row’s outermost cell is bigger and colourless, creating an epidermis that is one cell thick and interrupted.

3. Apical Growth

The thallus grows in length thanks to three to five apical cells (initials). They are located in the apical notch (growing point) and have a triangular form. Some median cells fail to divide or split vertically, resulting in the formation of two distinct growth sites, causing the thallus to become dichotomous.