Structure: The adult terrestrial gametophyte is prostrate and rosette like. It is dichotomously branched and deep green dorsoventrally. However, the aquatic species posses light green thallus. It is thin membranous. The dichotomous branching of thallus is quite close to each other and gives rosette like appearance. Each branch of thallus has a conspicuous median longitudinal groove. It is present on the dorsal side of each branch of thallus. A notch is present at the terminal end of each branch. The growing point is present in the notch. The ventral surface of thallus bears a row of the one celled scales. The scales are violet colored and multicellular. They are arranged close to each other towards the apex of the branch. The rhizoids are also found on the ventral surface of the thallus. They are unicellular and absorb nutrients and water from substratum. The rhizoids are of two types. Some are smooth walled and some are with tuberculate walls. The tuberculate rhizoids posses the peg like infolding peeping.
Internally the thallus of Riccia is differentiated into two regions. Ventral region: The ventral region of thallus consists of simple colorless parenchyma without intercellular spaces. The cells of this region make the storage tissue. They are filled up with starch grains. The ventral region has a single layered epidermis. Epidermis gives rises to several unicellular rhizoids and multicellular scales.
Dorsal region: The dorsal region of thallus consists of chlorenchyma cells. These cells have discoidal chloroplasts and are arranged in vertical rows. There are regular air canals in between each two vertical rows. This region is photosynthetic. The epidermis is single layered and it is opened outside at several places by the opening of air canals. However, the epidermis is continuous in aquatic species. The exchange of gases takes place through the air canals.
Vegetative reproduction: Vegetative reproduction in Riccia takes place by following ways.
Sometimes, a part of dichotomously branched thallus decays from the posterior end. But the terminal ends of the branches remain unaffected. These branches may grow into new thallus. Cell divisions occur in young rhizoids. It develop into gemma like structure of cells. These structures give rise new plants. Adventitious branches are produced on the ventral surface of thallus. These branches detach from the thallus and develop into new gametophyte. The apex of thallus grows downward into the soil at the end of growing season. This apex becomes thick and survives in the soil. It develops into a new plant in favorable conditions. Some species of Riccia develops vegetative structure called tubers under adverse conditions. The tubers are developed at the apices of branches. These tubers develop into new plants in favorable conditions.
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