Distinguish between the gametophytic generation of Riccia and Marchantia

Riccia belongs to the family Ricciaceae, order Marchantiales, class Hepaticopsida and division Bryophyta. The common Indian species are Riccia siliatq, R. hitra, R. discolor, R. glauca, R. gangetica, R. melansspora, R. hitra R. crystallina. Habitat or occurrence-The geneus Riccia with about 200 species, is cosmopolitan in its distribution and commonly grows in moist soils especially during and after rains.

In Riccia the gametophytic plant body is the dominant phase in the life cycle.

i. The gametophyte is a prostrate, dorsiventrally flat, dichotomously branched, green, fleshy thallus.

ii. Each branch is either liner or wedge-shaped or obcordate and as the dichotomous branches begin to grow together from one place, the plant usually exhibits a shape of a rosette or a circular patch.

iii. Each branch is thick in the median region and thin towards the margin. Each branch shows a conspicuous, longitudinal furrow along the mid-dorsal line ending in a notch at the tip where the growing point is located.

iv. On the ventral surface are present two types of outgrowths, the multicellular scales and the unicellular rhizoids. The scales are multicellular, pink, red, violet or black and one-celled thick structures arranged in a transverse row. The scales are more crowded near the apex and overlap the growing point. In the mature portion, each scale splits up into two so that there seem to be two rows of scales along the two margins of the thallus. Scales mainly protect, the growing point and increase absorptive surface.

v. The rhizoids are unicellular, elongated, tubular hair like structures which attach the thallus to the substratum and absorb water and nutrient solution. They are analogous to the roots of higher plants. The rhizoids are of two types. The smooth-walled rhizoids having smooth inner wall with colourless contents. The tuberculate or pegged rhizoids having peg-like processes in the inner layer of the wall which project inwards into the lumen. Mature rhizoids lack protoplasm. In aquatic free-floating species (R. fluitans, R. natans) both scales and rhizoids are absent.

The gametophytic plant body reproduces by vegetative and sexual methods after attaining a certain stage of maturity.

Vegetative reproduction:

The vegetative reproduction in Riccia takes place by the following methods:

Fragmentation:

In this method progressive death and decay of the older part of the thallus from the posterior end reaches the dichotomy, the two surviving branches become separate. Then each surviving branch grows independently by epical growth and finally develops into a new plant.

Adventitious branches:

In some species (R. fluitans) special adventitious branches, similar to parent thallus, arise from the mid- ventral surface of the thallus. These branches get detached and develop into new thalli.

Tubers:

In some species (R. discolor, R. perennis), at the end of growing season, the apex of the thallus grows down into the soil and becomes thick forming a thick tuber-like body. The tuber which easily survives a period of drought resumes growth with the commencement of favourable season and develops into a new thallus.

Persistent apices:

In R. discolor, at the end of growing period, the apices of thalli grow down into the soil. The plant other than the underground apices die. Under favourable condition, these apices come up and develop into new plants.

Gemma like body:

In R. glauca gemma-like bodies are formed at the tips of rhizoid. These structures ultimately develop into new plants.

Sexual reproduction:

Sexual reproduction in Riccia is oogamous type i.e. union between a motile flagellate male gamete and a resting non-flagellate female gamete takes place. The gamete bearing organs i.e. sex organs in Riccia are multicellular and are called antheridium (male) and archegonium (female) respectively.

Both the types of sex organs may develop on the same thallus i.e., the plant is homothallic or monoecious (R. gangetica, R. glauca) or the sex organs “nay develop on different thalli i.e. the pie’:- are heterothallic or dioecious (R. discolor, R. personii).

The sex organs develop on the floor of the mid dorsal longitudinal furrow in an acropetal succession i.e., the first formed (old) sex organ is behind and the last formed (new) sex organ is near the growing apex.

The sex organs are at the first superficial in position but are soon enveloped by the outgrowth of tissues on all sides. In dioecious species out of the four spores formed after meiosis in the spore mother cells of the sporophyte, two develop into male plants and two into female plants i.e., sexes are predetermind.

In the life cycle of Riccia the haploid gametophytic generation is independent and is the main vegetative body. It reproduces both vegetatively and sexually. The asexual reproductive phase i.e. sporophytic generation is dependent upon the gemetophyte and is embedded within it. It is represented only by the sporogenous tissues which are diploid cells.