19 - National Institute Of Open Schooling

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MODULE - 3Reproduction in PlantsReproduction andHeredity19NotesREPRODUCTION IN PLANTSReproduction is one of the most important characteristics of all living beings. It isthe production of ones own kind. It is necessary for the continuation of the specieson earth and also to replace the dead members of the species. The process by whichliving organisms produce their offsprings for the continuity of the species is calledreproduction.The modes of reproduction vary according to individual species and availableconditions. It may be simply by division of the parent cell as in unicellular organisms,by fragmentation of the parent body, by formation of buds and spores, or it maybe very elaborate involving development of male and female reproductive organs(stamens and pistils). Irrespective of the mode of reproduction, all organisms passon their hereditary material to their offsprings during the process of reproduction.In this lesson, you will study about the process of reproduction in plants.OBJECTIVESAfter completing this lesson, you will be able to :zdefine reproduction;zdifferentiate between vegetative, asexual and sexual reproduction;zdescribe the methods of asexual and sexual reproduction in unicellular lowerplant (Chlamydomonas) and filamentous green alga (Spirogyra);zdescribe the mode of reproduction in flowering plants;zexplain the parts of a dicot flower and their functions;zdescribe stages of microsporogenesis;zdepict with the help of diagram the structure of ovule and mention the stepsof megasporogenesis;BIOLOGY1

MODULE - 3Reproduction andHeredityReproduction in Plantszdescribe the stages of development of male and female gametophytes inflowering plants;z state the types of pollination, their significance and various modes ofpollination;zNotes zzzzzzzzexplain the steps involved in fertilization, (syngamy and triple fusion), embryodevelopment, endosperm development, formation of seed;differentiate between structure of dicot and monocot seeds;explain the formation of fruit and parthenocarpy;describe seed germination;define vegetative reproduction;differentiate between natural and artificial propagation;explain the advantages and disadvantages of vegetative propagation;describe the role of tissue culture technique in micropropagation;state the advantages of micropropagation;19.1 MODES OF REPRODUCTIONThe various modes by which plants reproduce are of three types –(a) Vegetative(b) Asexual(c) SexualIn Asexual and vegetative mode of reproduction, offsprings are produced from avegetative unit formed by a parent without any fusion of gametes or sex cells.–A single parent is involved–Offsprings are genetically identical to the parent.(a) Vegetative reproduction may be of the following types—(i) Vegetative reproduction : It involves formation of new plantlets fromvegetative (somatic) cell, buds or organs of the plant. Here, a vegetativepart of the plant (Root, stem, leaf or bud) gets detached from the parentbody and grows into an independent plant. It is similar to asexualreproduciton in that it also requires only mitotic division, no gameticfusion is involved, and newly-formed plants are genetic clones of theparent plant.We will discuss the different types of vegetative reproduction in angiospermslater in this lesson.(ii) Fragmentation : In filamentous algae, an accidental breaking of thefilament into many fragments, each fragment having atleast one cell, maygive rise to a new filament of the algae by cell division e.g. Spirogyra.(iii) Fission : It Occurs in unicellular organisms like bacteria and yeasts wherethe content of the parent cell divides into 2, 4 or 8 daughter cells andaccordingly the fission is known as binary or multiple fission. Each newlyformed daughter cell grows into a new organism.2BIOLOGY

MODULE - 3Reproduction in Plants(iv) Budding : It also occurs in unicellular plants. A bud-like outgrowth isformed on one side of the parent cell and soon it separates and growsinto a new individual e.g. in yeast.Reproduction andHeredity(b) Asexual Reproduction : Takes place by asexual spores which may be flagellateor nonflagellate.Spore formation : In lower plants including bryophytes and pteridophytes,special reproductive units develop asexually on the parent body. These are called Notesspores. They are microscopic and covered by a protective wall. When they reachthe suitable environment they develop into a new plant body e.g. in bread moulds,moss, fern. In higher plants like pea, maize and gymnosperms, asexualreproduction is always heterosporous. Here, spores are produced after meiosis.The small male spores called microspores give rise to male gametophyte. Thelarge female spores are called megaspores, and they give rise to femalegametophytes.(c) Sexual reproduction involves fusion of male and female reproductive cells(gametes) which are haploid and are produced by male and female reproductiveorgans. This fusion is known as fertilization and results in the production ofa zygote (diploid). Further development of zygote gives rise to a new individualwhich is diploid.Here, at some stage of the life history meiosis is involved and the offspringsare not genetic clones of their parents, but are genetically different and generallyexhibit mixed characters of their parents.19.1.1 ApomixisApomixis is a unique mechanism of asexual reproduction in certain plants (e.g.dandelions) which produce seeds without pollination and fertilization. (In Greek,apomixis means ‘away from act of mixing’). Since there is no fusion of male andfemale gamete, any somatic cell of ovule which is diploid, gives rise to the embryoand then ovule matures into a seed. The seeds are then dispersed. The interestingfact is that apomixis is an asexual process but disperses its seeds like those of plantsthat undergo sexual reproduction.No PollinationOvulesSeedsNo FertilizationINTEXT QUESTIONS 19.11. Define reproduction.2. How is asexual reproduction different from sexual reproduction?.BIOLOGY3

MODULE - 3Reproduction andHeredityReproduction in Plants3. What is a gamete?.4. Name two types of vegetative reproduction.5. Choose the correct optionNotesApomixis is:(a) Development of plants in darkness(b) Development of plants without fusion of gametes(c) Inability to perceive stimulus for flowering(d) Effect of low temperature on plant growth19.2 REPRODUCTION IN LOWER PLANTSWe will study the different types of reproduciton in two lower plants, one unicellularalga (Chlamydomonas) and the other multicellular filamentous alga (Spirogyra).19.2.1 Chlamydomonas (A Unicellular Alga)(i) It is a haploid unicellular alga found in fresh water ponds:(ii) The plant body is pear-shaped with two flagella attached at the narrow end.(iii) On one side of the cell, a light sensitive eye spot is present.(iv) A large cup-shaped chloroplast is present.(v) Towards the centre, a definite nucleus is present.(vi) Chloroplast contains a single pyrenoid. (Fig. 19.1a).PapillaBasal bodyFlagellumContractile vacuolesCytoplasmEye spotCup shapedchloroplastNucleusPyrenoidStarch grainsCell wallFig. 19.1a A Chlamydomonas cell4BIOLOGY

Reproduction in PlantsReproductionMODULE - 3Reproduction andHeredityA. Asexual reproduction takes place with the help of zoospores, aplanosporesor hypnospores depending upon the availability of water for swimming.Asexual Reproduction by Zoospores :z If plenty of water is available for free swimming, Chlamydomonas reproducesby flagellate thin-walled spores, called zoospores.zChlamydomonas cell loses flagella and becomes non-motile.zIts protoplasm (cytoplasm and nucleus) divides mitotically and forms 2-16daughter protoplasts, each of which develops flagella, and is called a zoospore.zThe parent cell wall is ruptured and zoospores are released.zEach zoospore develops a cell wall and grows into an adult cell.(Fig. 19.2-b, c)zAfter release of zoospores the parent cell does not exist, any more.NotesA. Asexual Reproduction by Aplanospores and Hypnospores :z If a thin-film of water is available where swimming is not possible,Chlamydomonas produces thin-walled, non-flagellate daughter protoplasts,called aplanospores.zThe parent cell loses flagella and becomes highly extended. Its protoplast dividesrepeatedly to produce 100 or more daughter protoplasts, each of which is calledan aplanospore.zThe whole structure containing groups of non-motile aplanospores resemblesa non-motile Colonial alga, called Palmella, and so this is called palmella stageof Chlamydomonas.zIf plamella-stage is flooded with water, each aplanospore develops flagella,comes out of the parent cell wall and grows into a normal independent plant.zIf water suddenly dries up, some of the aplanospores develop thick-wall, eachof which becomes dark brown or black, and is called a hypnospore. Whenfavourable conditions are present and water is available for swimming, eachhypnospore ruptures to release protoplast that develops flagella, becomes azoospore and grows into normal Chlamydomonas-plant.B. Sexual ReproductionChlamydomonas reproduces sexually by isogamy, anisogamy or Oogamy dependingupon the species :Sexual Reproduction by Isogamyz Isogamy is exhibited by Chlamydomonas eugametos and C. eherenburgii.zThe male and female cells become non-motile by losing their flagella.BIOLOGY5

MODULE - 3Reproduction andHeredityNotesReproduction in PlantszThe protoplasm of each cell divides mitotically into 32-64 daughter cells.zEach daughter cell develops flagella and is released in water by the rupture ofmother cell wall. Each of these cells acts as a gamete.zThe gametes are morphologically identical in structure but differ physiologicallyor chemically.zGametes released in water from two different mother cells fuse in pairs formingquadriflagellate zygotes.zWhen the contents of the two gametes fuse, they form a zygote (diploid). Thisis the only diploid stage in the life cycle of Chlamydomonas.zThe zygote develops a thick wall around itself and develops brown to blackcoloured pigmentation to tide over unfavourable conditions (zygospores).zOn the return of favourable conditions (temperature, food and water) the diploidnucleus of the zygote divides by meiosis and forms four haploid zoospores. (Fig.19.2 d-i)zEach zoospore grows into a new adult Chlamydomonas plant.Sexual Reproduction by Anisogamyz Anisogany is exhibited by Chalamydomonas braunii.zMale and female cells lose flagella and become non-motile.zIn male cell, protoplast divides repeatedly to produce 32-64 biflagellate gametesbut in female cell, protoplast divides to produce 8 to 16 biflagellate gametes.zBoth male and female gametes are released in water.zWhen larger female gametes lose flagella and become non-motile, each one isfertilized by a smaller motile male gamete.zAfter fertilization, the fusion product loses flagella, becomes spherical anddevelops thick wall to become a resting zygote.zOn return of favourable conditions of water, temperature and light, the zygoteundergoes meiosis and produces four haploid zoospores each of which growsinto an independent Chlamydomonas plant.Sexual Reproduction by Oogamyz Oogamy is exhibited in Chlamydomonas coccifera and C.ooganum.z Here, female and male cells lose flagella and become non-motite.z All the contents of female cell act as female gamete or egg, but the protoplasmof male cell divides to produce 32-64 biflagellate gametes.zThe biflagellate gametes are liberated in water and swim around in search offemale gamete.z Two or more flagellate gametes enter each female cell having nonmotile eggbut only one fertilizes the egg and others degenerate, contributing nutrition tothe young zygote.6BIOLOGY

MODULE - 3Reproduction in PlantszThe fusion product of egg and a motile gamete is called zygote that developsa thick, pigmented wall to enter into resting phase.zOn return of favourable conditions of water, temperature and light, the zygoteundergoes meiosis to produce four haploid biflagellate zoospores, each of whichon liberation from zygote, grows into an independent plant of Chlamydomonas.Reproduction andHeredity(d)Notes(e)(h)(f)(g)(i)(a)(b)(c)(a) Mature cell (b) 4 daughter cells (Zoospores formed by Asexual reproduction)(c) Zoospore after it escapes from the parent cell (d) Palmella-stage of Chlamydomonas(e, f, g) Free swimming gametes and fusion of gametes (h) a resting zygote (i) 4 cellsformed after meiosis of the zygote cell (zygospores)Fig. 19.2 Asexual and Sexual reproduction in ChlamydomonasINTEXT QUESTIONS 19.2l.2.3.4.5.Define the term isogamy. Which species of Chlamydomonas exhibits isogamy.Where does meiosis occur in Chlamydomonas ?.Give the method of asexual reproduction in Chlamydomonas. What is thefunction of zoospores in Chlamydomonas.Name the speicies of Chlamydomonas that reproduces by Anisogamy and thespecies that reproduces by Oogamy.Define the term zoospore and aplanospore.BIOLOGY7

MODULE - 3Reproduction in PlantsReproduction andHeredityProducestwo to eightfemale gametes(through mitosis)Produces16 to 32biflagellatemale gametes(through mitosis)n nNotesFemalecelln nMalecell2n2nResting zygotegerminates bymeiosis to producefour haploid zoospores2nRestingzygoteFig. 19.3 Sexual Reproduction by Anisogamy in C.brauniillale ceof fem vum orstnetoonee conAll th ff to act as meteoadgneulrofemaFemale cellhaving justone ovumFemalecellProduces 32to 64 biflagellategametes bymitosisMalecellTwo or more malegametes enter thefemale cell throughcrack that develops inthe wall of the cell2nRestingzygoteUnder favourable conditions of waterand temperature Oospore germinatesthrough meiosis producing 4Parent wall gets degradedor more biflagellata zoosporesby bacterial action and theeach of which grows intothick-walled zygote (oospore)a male or female celllies freen nOne male gametefuses with theOvum and othersdegenerateprovidingnutrition to theyoung zygoteFig. 19.4 Sexual Reproduction by Oogamy as in C.oogamum and C. coccifera8BIOLOGY

MODULE - 3Reproduction in PlantsReproduction andHeredity19.2.2 Spirogyra (A Multicellular Alga)Structure(i) It is a free floating alga found in fresh waterponds.Cell wallChloroplastPyrenoid(ii) The body has a row of cylindrical cells joinedend to end (filamentous alga).VacuoleNucleus(iii) Each cell depending upon the species, may have1 to 14, spiral ribbon shaped chloroplasts withmany uni-seriately arranged pyrenoids.NotesStrands ofcytoplasmCytoplasm(iv) Central region of the cells has a large vacuole.(v) The single nucleus is present in the centre of thecells supported by cytoplasmic strands. (Fig.19.5)Fig. 19.5 Spirogyra : Singlecell from the filament.ReproductionA. Vegetative Reproduction by fragmentation:(i)The filament breaks into small fragments, at the point of transverse septumfollowing a physico-chemical change.(ii) Each fragment having at least one complete cell grows into a new filamentby repeated mitotic cell division.B. Sexual Reproduction : It takes place by scalariform and lateral conjugation.Scalariform Conjugation (conjugating filaments give a ladder-like appearance).(Fig. 19.6)zTwo filaments come to lie very close to each other so that the cells of the twofilaments pair septum to septum and face to face.zThe pairing cells of the two filaments form a contact with the help of a tubecalled the conjugation tube.zCytoplasmic contents of each cell round off to act as a gamete.zGamete from one cell (male) passes to the other cell (female) through theconjugation tube, by amoeboid movement.zThe cells of each filament acts either as male or female.zThe contents of two gametes fuse in the female cell and form a diploid zygote.Consequently, after the sexual fusion of gametes, all the cells of male fitamentare empty whereas each cell of the female filament has one thick-walled diploidzygospore.BIOLOGY9

MODULE - 3Reproduction andHeredityReproduction in PlantszThe zygospore develops a thick wall around itself and develops dark brown toblack pigment to tide over the unfavourable period.zOn the return of favourable conditions the diploid nucleus divides by meiosisinto four haploid nuclei. Three of these nuclei degenerate.zOn germination, wall of the zygospore ruptures and a small tube like structure,containing one haploid nucleus comes out.Notes z The small tube develops into a long filament by repeated mitotic cell divisions.Lateral Conjugationz Here, cells of only one filament are involved in conjugation wherein, male andfemale cells are arranged in alternate pairs i.e., two male cells alternate with twofemale cells all along the length of a filament.zConjugation tube is formed lateral to the septum separating a male and a femalecell. Protoplasm of male cells migrate into female cells.zAfter fertilization, a filament would show two empty cells alternating with twocells each having thick-walled diploid zygospore.zThe zygospore under favourabe conditions, germinates as in scalariformconjugation to produce only one independent plant, because 3 haploid nucleiafter meiosis, degenerate.Conjugating tube(a)(b)(c)DegeneratingNuclei(e)(g)Young filament(d)(f)Zygosporewall(h)Fig. 19.6 Life cycle of Spirogyra: Sexual reproduction-Scalariform conjugation. (a) TwoFilaments lie close, (b) Formation of conjugation tube, (c) Transfer of gamete from the donorto the recipient cell, (d) Zygospore within the recipient cell, (e) Zygospore released fromfemale filaments, (f) Meiotic division in zygospore produces haploid nuclei, (g) 3- haploidnuclei degenerate, (h) formation of young filament10BIOLOGY

Reproduction in PlantsThe cell in the main plant body form the gametes without meiosis, thereforeChlamydomonas and Spirogyra are gametophytes (haploid).MODULE - 3Reproduction andHeredityINTEXT QUESTIONS 19.31. Vegetative reproduction in Spirogyra takes place by means of.2. Name the kind of sexual reproduction that occurs in Spirogyra.3. When does meiosis occur in Spirogyra ?.Notes4. How many filaments are involved in lateral conjugation ?.19.3 REPRODUCTION IN ANGIOSPERMS (FLOWERING PLANTS)Angiosperms reproduce both by vegetative as well as by sexual methods. In thissection we will study the sexual reproduciton in angiosperms. As you know sexualreproduction occurs by fusion of male and female gametes produced in the flower.Thus, flower represents the reproductive unit of a flowering plant.How frequently do plants flower? There is great variation shown by the angiospermicplants in this respect.Angiosperms can be classified

BIOLOGY 3 Notes MODULE - 3 Reproduction and (iv) Budding : It also occurs in unicellular plants. A bud-like outgrowth is Heredity formed on one side of the parent cell and soon it separates and grows into a new individual e.g. in yeast. (b) Asexual Reproduction : Takes place by asexual spores which may be flagellate or nonflagellate.

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