Riccia: Identification and Classification
- Division: Bryophyta
- Class: Hepaticopsida
- Order: Marchiantales
- Family: Ricciaceae
- Genus: Riccia
There are about 200 species of Riccia found all over the world, though predominantly in the southern hemisphere. They belong to the Division Bryophyta and Order Marchiantales. Most of them grow in wet terrestrial habitats and damp soil, while R. reticulata is xerophytic (desert species) and R. fluitans and R. natans are aquatic (free floating and submerged).
Plant body is a prostrate, flat, green, ribbon-shaped and dichotomously branched thallus. Branching determines the shape of the plant or plant cluster exhibited as circular, rosette, cruciform, reticulate or overlapping.
The thallus is branched and has a dorsal median groove and ventral median ridge. The mid rib along the dorsal median groove may be a deep channel in some species, which ends in the apical notch.
Rhizoids and scales are present on the ventral median ridge.
Rhizoids are numerous, unicellular, elongated, tubular and hair-like and are found as simple rhizoids or tuberculate rhizoids.
Scales are arranged in a transverse row and may be membranous, multicellular, single celled, and with colours ranging from pink, red, violet and black. Scales contain anthocyanin pigment and when in water absorbs chlorophyll making it look green.
Vertical C.S. of the thallus indicate tissue differentiation into an upper assimilatory region and a lower storage region.
a). The upper assimilatory region is photosynthetic. It consists of numerous unbranched photosynthetic filaments arranged in vertical rows with a single row of cells, ending in the upper epidermis.
Upper epidermis is discontinuous and has non-green cells. All other filament cells contain chloroplast which help manufacture food for the plant. Spaces between the filaments are called air spaces, air chambers, air canals or air channels. Air spaces may be compactly or loosely arranged.
b). The lower storage region is a continuation of the upper assimilatory region and is parenchymatous. Cells are wider, devoid of chloroplast and are compactly arranged without air spaces. They act as food storage cells. The lower part of the storage area is limited by the lower epidermis which are made up of small cells and give rise to the single celled rhizoids or multicellular scales. Rhizoids and scales serve as anchors for the plant. Rhizoids grow out from the mid section of the lower epidermis, while scales grow out from the margins.
- Fragmentation: Older portions of the thalli die till it separates the two lobes of the thallus apart, each giving rise a new bi-lobed thallus each, by apical growth.
- Adventitious branching: Thalli branches similar to the parent thallus that grow out from the mid-ventral section of the thallus are called adventitious branches. Each branch gives rise to a new thallus.
- Tubers: A few Riccia species give rise to penetrating tubers from the apical lobe cells. these tubers, under favourable conditions, gives rise to a fresh thallus.
- Persistent apices: These are the apical cells that grow into the soil and in time give rise to a fresh plant.
- Rhizoid extention: In R. glauca, the terminal end of the rhizoids multiply and give rise to new thalli.
Riccia reproduce extensively by sexual reproduction. Well defined and distinct sex organs are present. The male sex organ is the antheridium while the female sex organ is the archegonium. When male and female organs are present on the same thallus, they are said to be monoecious or homothallic, and when they are present on different thalli, they are said to be dioecious or heterothallic. Sex organs develop in acropetal succession, ie. newer organs are formed at the apical lobe while older ones are pushed behind. Sex organs lie embedded in the mid dorsal section of the thallus between air spaces and filaments.
- Foot and seta are absent
- The capsule is globose.
- Capsule wall is one layered, disorganized and abortive.
- Amphithecium forms the jacket of the capsule.
- Endothecium forms the archesporium.
- Archesporium gives rise to the spores and nurse cells.
- No special mechanism of dehiscence. Spores are set free after the death and decay of the thallus.
Common Bryophyte Terminology
- Thallus: A plant body that is not differentiated into stems and leaves, and lacks true roots and a vascular system. The plant body may be round with extended leaves or may be flat and ribbon-like.
- Rhizoids: Hair-like structures on the thallus similar to roots
- Dichotomous branching: Branching into two arms, or forking into two
- Antheridium: Male sex organ -club shaped which contains androcytes. Each androcyte changes to a small, motile, biflagellate antherozoid.
- Archegonium: Female sex organ – flask-shaped. Archegonium contains an egg or oosphere.
- Zygote: The result of fertilization by the penetration of the large, non-motile oosphere by the small, motile antherozoid.