Topics: Gametophyte, Gamete, Spore Pages: 6 (2053 words) Published: January 2, 2013
1. Riccia – Structure and Reproduction
Systematic position :Riccia belongs to the family Ricciaceae, order Marchantiales, class Hepaticopsida and division Bryophyta.The common Indian species are Riccia siliata, R. hitra, R. discolor, R. glauca, R. gangetica, R.melansspora, R. hirta, R. crystallina. Habitat or occurrence: The genus Riccia with about 200 species, is cosmopolitan in its distribution and commonly grows in moist soils especially during and after rains. Majority of the species are terrestrial, a few are free-floating or submerged aquatics e. g. R. fluitans, R.natans. The plant R. crystallina occurs at an altitude of 14,000 ft. in Western Himalayas. External morphology In Riccia the gametophytic plant body is the dominant phase in the life cycle. • The gametophyte is a prostrate, dorsiventrally flat, dichotomously branched, green, fleshy thallus. • 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. • 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. • 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. • 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. •Internal morphology: • In vertical cross section the thallus shows differentiation of tissues distinctly arranged in two horizontal zones, an upper assimilatory zone and a lower storage zone. • The upper dorsal assimilatory zone is composed of chlorophyll bearing cells arranged in isolated vertical rows or tires and are separated by narrow vertical air canals. Usually each vertical air canal •2. is enclosed by four vertical cell rows, sometimes eight rows are also present. Each air canal communicates with the external atmosphere, through air hares, present on the dorsal surface of the thallus. Each air pore is bounded by 4 to 8 colorless enlarged terminal cells of the vertical rows, which form a loose discontinuous one-celled thick upper epidermis. • The assimilatory region gradually merges into a ventral colorless region made of compactly arranged undifferentiated parenchyma tissue, the storage zone. The cells of the storage zone are thin-walled, without intercellular spaces, and containing starch granules as reserve food. The lower surface of the tissue containing small cells, compactly arranged, forming a single layer called the lower epidermis. This layer hears two types of out-growths, the multi cellular one-celled thick scales and the unicellular tubular extension of epidermal cells called rhizoids. Rhizoids are produced in the mid-ventral region whereas the scales arc produced at the margins. Reproduction: The gametophytic plant body reproduces by vegetative and sexual methods after attaining a certain stage of maturity. Vegetative reproduction :The vegetative reproduction in...
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