Riccia – Structure and ReproductionSystematic 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 geneus Riccia with about 200 species, iscosmopolitan 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 morphologyIn Riccia the gametophytic plant body is the dominant phase in the life cycle. • The gamtophyte 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
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 colourless 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 colourless 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 multicellular 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 acertain 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 aresting 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 ofsex organs may develop on the same thallus i.e. the plant is homothallic or monoecious (R. gangetica, R.glauca) or the sex organs may develop on different thalli i.e. the plants are heterothallic or dioecious (R.discolor, R. personii). The sex organs develop on the floor of the mid dorsal longitudinal furrow in anacropetal succession i.e. the first formed (old) sex organ is behind and the last formed (new) sex organ isnear the growing apex. The sex organs are at the first superficial in position but are soon enveloped by theoutgrowth of tissues on all sides. In dioecious species out of the four spores formed after meiosis in thespore mother cells of the sporophyte, two develop into male plants and two into female plants i.e. sexes arepredetermind.Antheridium: • A mature antheridium of Rictia is a pearshaped body within an open antheridial chamber which is formed by the overarching tissues. The antheridial chamber communicates with the dorsal surface by a pore. • The antheridium is attached to the base of the antheridial chamber by means of a few-celled stalk. • The pearshaped antheridal body has got a flat broad base and a conical apex. • The antheridial body is surrounded by a single-layered wall or jacket made of thin-walled cell. • A central mass of cuboidal cells enclosed by the jacket layer are the androgonial cells or androcyte mother cell. • Each androcyte mother cell, on maturity, divides diagonally to produce two traingular androcytes. • Each androcyte ultimately metamorphoses into a single biflagellate antherozoid or spermatozoid. • During metamorphosis cell walls of the androgonial get disorganised to form a semifluid mucilaginous content in which the mature antherozoids float freely. • Next gelatinization of jacket cells towards the apex marks it more breakable. • When water enters into the antheridial chamber the gelatinized jacket cells absorb it and swell and finally break open. • Then the semifluid mucilaginous content of the antheridium containing the antherozoids, oozes out of the antheridial chamber to the dorsal surface of the thallus.Archegonium: • A mature archcgonium is a flask-shaped body embedded within a chamber called archegonial chamber formed similarly like antheridial chamber, which communicates with the dorsal surface by a pore. • The archegonium is attached to the base of the archegonial chamber by means of a short few-celled stalk.
• The flask-shaped archegonium is differentiated into a basal swollen part the venter, and an elongated protruding tubular portion, the neck. • The venter consists of a single layered wall having more than six cells in perimeter and encloses a lower large egg or female gamete with an upper small ventral canal cell. • The wall of upper tubular neck consists of 6-9 tires of elongated cells arranged in 6 vertical rows which encloses a narrow central canal consisting of 4-6 neck canal cells in a single row. • The tip of the neck is covered by four specialized cells called cover cells. • When the archegonium is matured, the canal cells (neck and ventral canal cells) degenerate, leaving a mucilaginous mass. • Shortly before fertilization, when water enters into the archegonial chamber, the mucilaginous mass imbibes water, swells and sets up a force which pushes the cover cells apart. Thus a free neck canal is formed from the apex of the archegonium to the egg.Fertilization:For fertilization water is necessary. Water helps liberation of antherozoid by the rupture of the antheridium.Water also acts as a medium for transportation of antherozoid towards the egg. After rain water is retainedas a thin film in the dorsal furrow of the thallus and acts as a medium for the movement of antherozoids.Prior to fertilization the mucilage that is out of the archegonium attracts the antherozoids towards thearchegonium. The antherozoids thus attracted, arrive near the egg travelling down through the neck canal.Finally a single antherozoid (n) fuses with the egg (n) and forms a diploid (2n) zygote. The zygote is thefirst cell of the sporophytic generation.Structure of the sporophyte: • The mature sporophyte of Riccia is a globular capsule, embedded within the gametophytic plant tissue and is without foot and seta. • After fertilization the zygote secretes a wall and increases in volume until it nearly fills the cavity of the venter. • Simultaneously the cells of the venter divide to form a two layerd venter enclosing the developing sporophyte, a structure called calyptra. • The zygote now divides in both vertical and transverse planes and produces a more or less spherical mass of 20-40 cells. • The spherical mass then differentiates into a peripheral cell layer, the amphithecium and a central mass of cells, the endothecium. • The amphithecium forms the jacket or wall of the sporophyte. • The endothelial cells divide and form a sporogenous tissue, the archesporium.
• The archesporial cells are finally differentiated into sporemother cells with dense thick cytoplasm and nurse cells with watery vacuolated cytoplasm. • The spore mother cells now undergo meiosis reduction division resulting in haploid (n) spore tetrades (four spores). • In the meantime the nurse cells, amphithecial layer and also inner layer of the venter degenerate to form a nutritive viscous fluid. This fluid supplies nourishment to the developing spores. • The spore tetrad usually remain attached to one another and are finally separated. • The mature sporophyte, commonly, designated as sporogonium, is a more or less rounded structure containing the mature haploid spores, embedded within the gametophytic thallus. • The mature sporogonium does not contain a single diploid cell the envelope formed from the outer layer of calyptra, haploid spore, and encircling gametophytic tissue haploid. • The spores are liberated only by the decay of venter wall and surrounding gametophyte tissue.Structure of spore:Spore is the first cell of the gametophytic generation. Each spore is pyramidal or tetrahedral in shape with aclear triradiate mark at the proximal face. A mature spore shows three layers of wall—the outermost thinand cuticularised exosporium, the middle cuticularised mesosporium and the innermost endosporium.Germination of spore and formation of the new haploid gametophyte:The spore germinates under favourable moist conditions. During germination the spore takes water andswells up, as a result the massive black exosporium bust and the thin endosporium enclosing the sporecontents protrudes out in the form of a tubular outgrowth called the germ tube. The germ tube elongatesand divides to form an eight celled germ disc. The rhizoid emerges out near the base of the germ tube. Thecells of the germ disc soon divide and re-divide to form a multicellular thallus which remains fixed with thesoil by rhizoids.Life cycle:In the life cycle of Riccia the haploid gametophytic generation is independent and is the main vegetativebody. It reproduces both vegetatively and sexually. The asexual reproductive phase i.e. sporophyticgeneration is dependent upon the gemetophyte and is embedded within it. It is represented only by thesporogenous tissues which are diploid cells. Mature sporophyte or sporogonium is made up of haploid cellsonly, it is a peculiar condition found only in Riccia. Dr. Jayakara Bhandary M. Associate Professor- Botany GAS College , Karwar – 581301 Karnataka, India.