Biology - An Illustrated Guide to Science (794127), страница 10
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The final ring is the femalepart called the pistil or carpel.stigma●styleovuleovarysepalreceptacleStamens and carpelsThe stamens are arranged in a ringand consist of an thers, which producepollen grains containing the malegam ete, supported on a filament.● The female part of the flower in theinnermost ring is composed of thestigm a and style. It is often so highlymodified that it does not resemblea ring.● The female gametes are completelyenclosed in the ovu le located in theovary, which is found as the lowestpart of the carpel.© Diagram Visual Information Ltd.●flower stalk57Mature stamenCONTINUITYKey wordsStamenFlower: vertical sectionanther beforedehiscence:external viewantherdehiscencegametemeiosispollenAnther before dehiscence:external viewanther afterdehiscence:external viewanther lobesline of dehiscencestamenThe male gametes●anther beforedehiscence:transverse sectionAn thers are the part of the flower thatproduce the male gam etes, which arecontained in pollen .
A special form ofcell division called m eiosis is requiredfor this process.Gross structureA stam en consists of an anther fixedon top of a filament. The filament isattached to the stalk of the flowerbelow the ring of female parts.● The anther itself has four chambers orpollen sacs arranged around thefilament. A bundle of vascular tissuecarries water and organic materials tothe anther to support the developingpollen.●Anther before dehiscence: transverse sectiontapetumouter fibrous layervascularbundleline of dehiscencepollen sacsAnther after dehiscence:transverse sectioninner fibrous layerpollen mother cell(microsporemother cell)dividing by meiosisanther lobescontainingpollen sacsAnther afterdehiscence:externalviewanther afterdehiscence:transversesectionpollenMicroscopic structureA layer of cells called the tapetum linesthe inside of each pollen sac andnourishes the growing pollen grain.Cells from this tapetum pass into thespace in the middle of the sac anddivide by meiosis to form pollengrains—often in tetrads or groups offour.● A tissue outside the pollen sac issupplied with fibrous elements thatstress the anther as it dries out—anessential part of the mechanismfor the release of the maturepollen.● When the pollen cells aremature, the anther begins todry out.
This causes the cellsto shrink as they lose water.This sets up strains in thetissues of the anther walls,which eventually split torelease the pollen. Thissplitting is called dehiscen ce.●© Diagram Visual Information Ltd.filament58CONTINUITYKey wordsdehiscencegenerativenucleushaploidmeiosispollenPollen developmentPollen develops pollen sacs from cellsthat undergo m eiosis to producehaploid cells, cells that contain singlechromosomes rather than thepairs of chromosomes found inmost body cells (see microsporemother cell diagram).● Since pollen grains are producedby meiosis, each one is unique,with a slightly different geneticmakeup than all other grains producedby the plant.● Pollen grains are microspores—thismeans they have very little storagematerial in them—unlike eggs.●Pollen formationPollen formationMicrospore mother cellAnther before dehiscence:transverse sectionFirst meiotic divisionproduces two cells.Second meiotic divisionproduces four haploidmicrospores.Pollen: external structureThe outer wall of a pollen grain is atough waterproof structure called theexine.
It is often highly sculpted(see bottom diagram).● A number of pores exist in the exine.The pollen tube that forms duringpollen germination grows out of oneof these pores.●Pollen: internal structureThe internal structure of a pollen grainis fairly simple. It is bounded bythe intine, or inner wall, andcontains two nuclei: thegen erative (sperm)n u cleu s and the tubenucleus.● The tube nucleuscontrols the productionof a pollen tube (see page59).
This structure growsout through a pore in theexine and passes between thecells of the stigma and style ofthe carpel.● The generative nucleus passes downthis tube toward the female nucleusfound in the ovule. When it uniteswith the female nucleus, the first cellof the new plant has formed.microsporemother cellPollen grainsAnther after dehiscence:transverse section© Diagram Visual Information Ltd.●Detail of pollen graingenerative (sperm)nucleusintinedetail ofpollen grain(microspore)exinetubenucleus59PollinationCONTINUITYKey wordsTypes of pollinationanthercarpelpollenstamenstigmaInsect (entomophilous) pollinationmature anthers dustpollen onto back of beebee flies toanother flowerTypes of pollinationPollination is the transfer of pollenfrom a male an ther to a stigm a of thefemale carpel.● Pollen is received by the stigma, whicharises out of the ovary.● There are two major forms ofpollination: insect pollination andwind pollination.●mature stigmatouches backof beeovaryproboscisInsect pollinationnectaryMany insects use their proboscus tocollect nectar from flowers for food.Nectar is a solution of sucrose in waterproduced by glands, called nectaries,at the base of petals.● While the insect is collecting thenectar, pollen from anthers can bedusted on to its body.
When this insectvisits another flower looking fornectar, the pollen is transferred to thestigma, thereby pollinating it.● Insect-pollinated flowers tend to haveostentatious petals, scent, and nectar,and are often highly adapted to attractparticular types of insect.●proboscisovarynectaryWind (anemophilous) pollinationpollen released from anthershanging outside flowerlarge feathery stigmaWind pollinationPlants that use wind pollinationproduce extremely large amounts ofpollen, which blow onto the stigmas ofother plants.● Wind-pollinated plants tend to havelarge numbers of inconspicuousflowers with stam en s and large,feathery stigmas that hang outside ofthe flower.●© Diagram Visual Information Ltd.pollen60CONTINUITYKey wordsembryo sacendospermfertilizationgametegenerativenucleusmicropylepollentriploidFertilization and pollination●Pollination is the transfer of pollenfrom the male anther to the femalestigma.
Fertilization then occurs whenthe male gam ete from a pollen grainfuses with the female gamete in theovule.Pollen tube developmentPollen grains contain two nuclei: thetube nucleus and the gen erative(sperm) n u cleu s containing the malegametes. When a pollen grain beginsto grow, it forms a pollen tube, whichconducts the nuclei from the pollengrain to the em bryo sac. The tubenucleus controls the growth of thepollen tube.● The embryo sac is contained withinthe ovule of the carpel. It issurrounded by thin membranes calledinteguments, which have a smallopening at the bottom called themicropyle. The pollen tube enters theovule by way of the m icropyle.Plant fertilizationFertilization and pollinationCarpel just after pollination:vertical sectionpollen graingerminatingpollen grainstyleovary wallcavity of ovaryovuleembryo sac●FertilizationDuring the movement of thegenerative nucleus down the pollentube, it has divided by mitosis.
Thepollen tube thus delivers twogenerative nuclei to the micropyle atthe base of the carpel.● The nuclei pass across into the embryosac. One of the generative nuclei fuseswith the female nucleus in the embryosac. This will become the first cell of anew plant.● The other generative nucleus fuseswith two polar bodies produced bymeiosis to form the endosperm (foodstorage material) which has a triploidnumber.stigmaintegumentsgenerativenucleusPollen grains before andafter pollinationtube nucleusmicropylestigmagenerativenucleipollen tubetube nucleusCarpel with pollen tubefully developed:vertical sectionpollen tube© Diagram Visual Information Ltd.●Fertilizationtubenucleusgenerativenucleigenerativenucleiintegumentspollen tube61Seed developmentCONTINUITYKey wordsSeed developmentCarpel afterfertilization:vertical sectionembryo sacgenerativenucleuszygoteDevelopment afterfertilizationembryo sacendosperm nucleusSeeds and fruitThe seed develops from the fertilizedem bryo sac.
It starts with the zygote,which is the first cell of the newindividual created when thegen erative (sperm) n u cleu s and theegg nucleus fuse. Repeated divisionproduces a multicelled embryo plant.● The fruit develops from the remains ofthe ovary wall. Fruits show manyadaptations to aid the dispersal oreventual germination of the seed.●zygoteendospermnucleiembryoEndospermic seedsAt fertilization, the pollen tube deliverstwo generative nuclei (male gametes)to the embryo sac (see page 60).
Onenucleus fuses with the female nucleusand will become the new plant. Theother fuses with the polar bodies anddevelops into endosperm tissue thatacts as a food store for the growingzygote. This endosperm takes up mostof the space within the seed.● As the embryo grows, it will developtwo lobes, called cotyledons. Theseare embryonic leaves that store a largeamount of food obtained by digestingand absorbing the endosperm tissue.●development afterfertilizationendosperm richin stored foodembryoThe integuments that had surroundedthe embryo sac (see page 60) toughenand become the seed coats (testa)(bottom diagram).● The developing root is called theradicle and the developing stem theplumule.●testacotyledonplumuleradicleplumulecotyledonendospermNon-endospermic seeds●Non-endospermic seed (pea):longitudinal sectionradicleEndospermic seed (castor oil):longitudinal sectionIn non-endospermic seeds thegrowing embryo takes up most of thespace. The cotyledons become swollenand filled with materials that act as thefood store for the growing plant untilphotosynthesis takes place.© Diagram Visual Information Ltd.testa62CONTINUITYKey wordsepididymisgameteseminiferoustubulespermatozoontestisvas deferensHuman reproductivesystem: maleMale reproductive systemSide viewAnatomyThe testes are the organs that producesperm, the gam etes in human males.They also produce the male hormonetestosterone.
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