Reproduction in Riccia

Reproduction in Riccia

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  Reproduction in Riccia Dr.V. I. Kahalkar Department of Botany M. G. Arts, Science and Late N. P. Commerce College, Armori
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  Vegetative Reproduction • Thevegetative reproduction takes place by several ways. • By the death and decay of the older parts of the thallus. Sometimes a part of the thallus, dichotomously branched decays from the posterior end and this ways the terminal ends of the branches remain unaffected which may grow separately into new thalli. • By tubers : In some species, the tubers develop at the apices of the branches of the thallus which face adverse condition, and develop into new plants on the approach of favorable conditions.
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  • By adventitiousbranches: according to Cavers (1904) in several species of Riccia the adventitious branches are produced on the ventral surface of the thallus. These branches get detached from the thalli and develop into new gametophytes. • By gemma formation: According to Campbell the Riccia may also reproduce vegetatively by cell division of the young rhizoids which develop into gemma like structure of the cells. These structures give rise to new plants.
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  Sexual Reproduction • Majorityof the species of Riccia are homothalic i.e. monoecious, i.e. antheridia and archegonia are born upon the same thallus. The heterothallic i.e. dioecious species are also common. • 1. Sexual reproduction is oogamous. • 2. Sex organs are antheridia and oogonia. They are present in the region of mid-dorsal groove on the dorsal surface of the thallus. • 3. Most of the species are homothallic but a few species are heterothallic, e.g., R. himalayensis. • 4. Monoecious species are generally protandrous, i.e., antheridia develop first and oogonia later on.
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  Structure of Antheridium: •1. Antheridium is present in a cavity on the dorsal surface called antheridial chamber. • 2. Antheridial chamber has a narrow opening or pore on the apical side. • 3. A mature antheridium is a stalked, club-shaped or pear-shaped body. • 4. Antheridial stalk is multicellular. • 5. It remains surrounded by an outermost layer of one- celled thick sterile jacket. • 6. Inside the jacket layer are present many small androcyte mother cells. • 7. Each androcyte mother cell contains dense cytoplasm and large nucleus. It divides diagonally into two androcytes.
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  • 8. Eachandrocyte metamorphoses into a single structure, variously called antherozoid, spermatozoid or sperm. • 9. Each antherozoid is a minute, uninucleate body containing two long flagella at its anterior end. • 10. Lower flagellum is slightly larger than upper one. • 11. Dehiscence of antheridium takes place in the presence of water.
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  Structure of Archegonium: 1.The archegonium remains embedded in the archegonial cavity on the dorsal surface of pant body. 2. Upper part of the neck of archegonium generally protrudes out of the cavity. 3. An archegonium is a flask-shaped structure made up of a long, elongated neck and a globular venter. 4. Venter is sessile and surrounded by a one-celled thick layer, made up of 12 to 20 cells. 5. Neck consists of 4 to 6 neck canal cells, and remains surrounded by six vertical rows of cells. 6. Each longitudinal row consists of 6 to 9 cells. 7. At the tip of the neck are present four cover cells or lid cells. 8. Venter contains an upper, small ventral canal cell and a lower, large egg cell. 9. At the time of fertilization all the cells, except the egg, disintegrate and form a mucilaginous liquid, which gives entry to the spermatozoids. The ultimate product of the fertilization is zygote.
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  Fertilization • The wateris necessary for the fertilization. As soon as antherozoids approach the neck of the archegonium, they are attracted chemotactically by the mucilaginous substance, oozing out through the mouth of the archegonium. The antherozoids enter the venter of archegonium and only one antherozoid fuse with egg cell. By the fusion of Antherozoid and egg cell zygote is formed.
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  Sporophyte 1. It issimple and made up of only capsule or spore-sac. 2. Foot and seta are absent. 3. It remains embedded in the gametophyte, and it is a non-green structure, thus depending entirely on the gametophyte for food. 4. Inside sporogonium are present many spore mother cells which remain surrounded by a capsule wall and two-layered calyptra. 5. Spore mother cells divide reductionally, and each of them thus forms four haploid spores, arranged tetrahedrally. 6. Elaters are absent
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  Spore: 1. It isthe first cell of the gametophytic generation. 2. Shape of the spore is rounded or pyramidal. 3. Each spore is surrounded by a thick wall. 4. Wall of the spore consists of three layers: outermost exine or exosporium, which is thick and sculptured; middle thin mesosporium and: innermost, thin intine. 5. Spores are unicellular and uninucleate structures. 6. Spores germinate into gametophyte
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