SEXUAL REPRODUCTION IN FLOWERING PLANTS 

1.Angiosperm is a flowering plant.

2.Flower is the reproductive unit.

3.Male reproductive organ: Androecium.

4.Female reproductive organ :Gynoecium.

5. ANDROECIUM :

   ●It consists of a whorl of stamens .

    ●Stamen is consist of filament ( long stalk ) & anther .

   ●Anther is bilobed structure .

   ●Each lobe has two theca (covering ) ie dithecous.

   ●A longitudinal groove separates this theca.

  ●Each lobe has two microsporangia

  ●One anther has four microsporangia.

  ●Microsporangia further develop into pollen sac.

6.Longitudinal groove is the site of dehiscence when microsporangia is matured.

7.A microsporangia is surrounded by four wall layer .

●From outside to inside these are –

•The Epidermis.

•Endothecium.

•Middle layer.

•Tapetum

8.Outer three layers are protective in nature & helpful in dehiscence.

9.Tapetum is a nutritive in layer.

●Tapetum nourish the developing pollen grain.

●Tapetal cells are binucleate or may show polyploidy

10.A young anther is consist of homogenous tissue called sporogenous tissue.

11. MICROSPOROGENESIS –> Formation of Microspore

●Each cell of sporogenous tissue is a potential pollen mother cell {PMC}/ microspore mother cell.

●Cells of the sporogenous tissue lying in the centre behave as PMC.

●PMC undergoes meiosis & divide to form microspore tetrad.

●PMC is diploid cell represent the sporophyte

●Microspore is haploid & represent the 1st cell of gametophyte.

●Microspores dissociate from tetrad after anther maturity & dehydration & develop into pollen grains.

12.Size of pollen grain is around 25 -30 micrometer in diameter.

13.Pollen grain has two -layered wall.

●The hard outer layer is Exine which has sporopollenin.

●Sporopollenin is most resistant organic material which can withstand against high temperature & strong acids and alkali.

●Till now ,there is no enzyme is found that degrade the sporopollenin.

●Pollen grain exine has pores where sporopollenin is absent (Germ pore)

●During germination pollen tube emerge from germ pore.

●Pollen grains are preserved as fossils because of sporopollenin.

●There are different patterns & designs present on exine.

●The inner wall is called INTINE which is made up of cellulose & pectine.

●Intine is thin & continuous layer whereas exine is discontinuous at germ pore.

14.A mature pollen grain has two cells namely big vegetative cell & small generative cell (spindle shape )

15.Generative cell float in cytoplasm of vegetative cell. (Cell with in cell)

16.In 60 % of angiosperms –>

●Pollen grain shed at two celled stage

●Rest of 40 % shed thei r pollen grain at three cell stage .

17.Pollen grains of many species cause allergies & bronchial problems leading to chronic respiratory disorders like Asthma ,bronchitis.

18.Parthenium (carrot grass ) causes pollen allergy.

19.Pollen grains are rich in nutrients. Pollen tablets and pollen syrups are used as food supplements .

●It is claimed that pollen consumption increase the performance of athletes and horses.

20.Pollen grains of rice and wheat lose viability within 30 minutes of shedding.

21.Pollen grains of Some members of Solanaceae, leguminoceae and Rosaceae maintain viability for months.

22.Pollen grains are stored in liquid nitrogen (-196o C ) for years. (Cryopreservation)

●This pollen bank is useful for crop breeding programmes.

23.Pollination is the transfer of pollen grain from anther to the stigma of pistil of flower.

24.In angiosperm ,both male and female gametes are non motile so pollination is the mechanism which brings them close.

25.Depending on the source of pollen ,pollination are of three types.

●Autogamy

●Geitonogamy

●Xenogamy

26. Autogamy :

oHere pollination is achieved in same flower of a plant.

oTo felicitate self- pollination in autogamous flower, it requires .-

1) Synchrony in pollen grain release & stigma receptivity.

2) The anther & stigma lie close to each other

27.In oxalis & commelina ,there are two types of flower.

Cleistogamous flower : •

●which never open to expose stigma & stamen to assure autogamy.

●Cleistogamous flowers produce assured seed set even in the absence of pollinator.

●In cleistogamous flower there is no chance of cross pollination.

in cleistogamous flower, Pollination is take place in bud stage.

Cleistogamous flowers expense least energy in pollination.

Chasmogamous flower : open & expose stigma & anther.

28. GEITONOGAMY

In geitonogamy, stigma of flower receive pollen grains of same plant instead of same flower.

Geitonogamy requires pollinating agent as functionally it is cross pollination.

Autogamy & Geitonogamy –> Not contribute in evolution as they are Genetically self- pollination. They bring inbreeding depression.

29. XENOGAMY :

Transfer of pollen grain from anther to stigma of different flower.

This pollination brings genetically different pollen grains to stigma.

Cross pollinating plants spend lot of energy in this process.

30. AGENT OF POLLINATION :

oGeitonogamous & Xenogamous flower need pollinating agent.

oWind & water are the two abiotic agent.

oAnimals are the biotic agent of pollination.

31.Pollination by biotic agent contribute more than abiotic agents.

32.Pollination by abiotic agent (wind and water ) is a chance factor .

oSo to increase the probability of pollination ,these plants produce enormous pollen grains.(high expenditure of energy )

33. WIND POLLINATION : (Anemophily)

o Wind pollination is more common than water pollination.

oWind pollinated pollen grains are light & non sticky.

o Flower has well exposed stamen ( for easily dispersal ),feathery stigma ( to catch the pollen grains ).

o Wind pollinated flower often have single ovule in each ovary & numerous flowers packed into a inflorescence.

o Maize & grass are wind pollinated plant .

o In maize cob , long thread like structure, tassel are stigma and style.

34. HYDROPHILLY :

oPollination by water.

oOnly about 30 genera show hydrophily.

oVallisneria, Hydrilla (fresh water ) and marine sea grass zostera are example of hydrophily

oIn vallisneria pollination is done on surface of water. (Epihydrophily)

oIn seagrasses like zostera pollination is done under water. Their pollen grains are ribbon like. (Hypohydrophily)

oHydrophillous pollen grains have mucilaginous sheath for protection.

35.Water hyacinth and waterlily are aquatic plants but pollinating agent is insects.

36. POLLINATION BY ANIMAL :

●Bees are the dominating pollination agent among animals.

●Use UV radiation for observation

●To ensure pollination ,animal pollinated flowers adapted themselves.

●Insect pollinated flowers are large ,colorful, fragrant & rich in nectar .

●They have sticky pollen grain to adhere on animal body. (Pollenkitt )

●In case of small flower ,numerous flowers are clustered in an inflorescence to make it conspicuous.(sunflower )

●Sunflower is not a flower , It is inflorescence. □

The flowes pollinated by flies and beetles secrete foul odour to attract them. (Best example -> Rafflesia “Largest flower” )

●Amorphophellus (tallest flower ) provide space inside the flower to lay eggs of its pollinating agent.

●Yucca plant and moth need each other to complete their life cycle.

●Moth lays egg in locule of ovary and flower get pollinated in this act .Larva comes out of the egg when seed start developing.

37. OUTBREEDING DEVICES:

●These are the devices present in plant to prevent self-pollination.

●Continuous self-fertilization result into inbreeding depression

●expression of recessive character. ●Prevention of self-pollination is achieved through:

●Non synchronisation of pollen grain & stigma maturity in flowers (dichogamy )

●The anther and stigma are placed at different level.( Heterostyly)

●Self -incompatibility  It is a genetic mechanism

•inhibit germination of pollen grain on stigma of same flower or growth of pollen tube in style in autogamy

oDioecism –> Male and female flowers present on different plant

e.g. Papaya,

38.Monoecious plant can prevent autogamy but not Geitonogamy whereas Dioceious plant can prevent both autogamy and geitonogamy.

oCastor and maize are monoecious plant.

39. POLLEN PISTIL INTERACTION :

oAll events from landing of pollen grain on the stigma until the pollen tube enters the embryo sac (in case of compatibility ) or pollen inhibition in case of incompatibility.

40.Pistil and pollen grain have the ability to recognise the compatibility/incompatibility. This interaction is mediated through chemicals.

oIn case of compatibility, pollen grains germinated on stigma and pollen tube emerge from germ pore.

41.If pollen grain shed at two celled stage, the generative nucleus divides inside pollen tube and two male gametes are produced.

o60% plants shed pollen grains at two celled stage.

42.Pollen tube enters in ovule through micropyle.

43.Inside Ovule, Filiform apparatus of synergid cell of embryosac guide the entry of pollen tube in embryo sac.

44.Pollen germination can be seen in a drop of 10% sugar solution @ Hanging drop

45.Artificial hybridisation is done to get desired hybrid between species which is commercially superior.

46.For artificial hybridisation emasculation and bagging is required to protect stigma from contamination from unwanted pollen.

47.Emasculation is removal of anther from hermaphrodite flower in bud stage.

48.THE PISTIL ,MEGASPORANGIUM (OVULE ) AND EMBRYO SAC.

oThe gynoecium is female reproductive part

Consist of pistil or carpel.

oThe gynoecium may consist of either one pistil (monocarpellary) or more than one pistil (multicarpellary).

oIn multicarpellary condition,

if carpels are fused SYNCARPOUS (in Papaver ).

if carpels are free  APOCARPOUS ( in Michaelia).

49.Pistil is divided in to three part namely:

   o Stigma  receives pollen .

  o Style a hollow tube which connect stigma with ovary.

  o Ovary basal bulge part which has ovule with embryo sac & egg

50.Inside the ovary there is ovarian cavity (locule) where placenta is present.

o Integumented megasporangia arises from placenta. @ Ovule

51.Wheat, paddy and mango have one ovule in an ovary.

52.Papaya, orchid and water melon have many ovules in an ovary.

53. STRUCTURE OF OVULE:

oFunicle is the stalk of ovule which connect ovule with placenta.

oHilum is junction between ovule and funicle.

Here body of ovule fuse with funicle.

o Integument is the protective layer of ovule.

o Micropyle is the area where integument is absent.

Pollen tube enters in the ovule through micropyle.

oCHALAZA is the opposite end of micropyle.

o Nucellus is the tissue present inside the integument .O

Nucellus is diploid in nature.

o From nucellus a megaspore mother cell (MMC ) is differentiated towards micropylar end.

o MMC has dense cytoplasm and prominent nucleus.

o MMC undergoes meiosis and four haploid megaspore is formed .

o Generally only one megaspore is functional (monosporic ) from which female gametophyte is developed.

o Formation of megaspore from MMC is called mega-sporogenesis.

54.Megaspore is first cell of female gametophytic generation

55.Haploid megaspore divide mitotically and form female gametophyte.

o After 1st mitotic division two daughter nuclei are formed which move towards opposite side.

o After second and third mitotic division total 8 nuclei is formed .

o All the three mitotic division in megaspore is strictly free nuclear division.

o After the 8 nucleate stage, cell wall is laid down.

56.

Embryo sac is 7 celled 8 nucleated in which central cell has two polar nuclei.

57.In an embryo sac ,3 cells are arranged as egg apparatus which consist of two synergids and one egg towards micropylar end.

58.Synergid has special cellular thickening towards micropylar end called filiform apparatus which guide the entry of pollen tube into the synergid

59.There are three antipodal cells towards chalaza end which degenerate after fertilization.

60. Monosporic 7 celled 8 nucleated is also called Polygonum type embryosac.

61. DOUBLE FERTILIZATION & TRIPLE FUSION :

●Pollen tube brings two male gametes in embryo sac.

●Pollen tube discharge the two male gametes in one of the two synergids

●One male gamete fuse with egg and form zygote (Syngamy)

●Zygote develop into embryo.

●The other male gamete is fused with two polar nuclei (Triple fusion)

●This central cell is called primary endosperm cell (PEC)

●PEC develops into ENDOSPERM

●Endosperm provide nutrition to developing embryo

●Nuclear endosperm is the most common endosperm

●Coconut water is free nuclear endosperm while white kernel is cellular endosperm.

62.Double fertilization & triple fusion is characteristic of angiosperm only

63.On the basis of presence/absence of endosperm

oseeds are of two types.

NON-ALBUMINOUS OR EXALBUMINOUS SEED:

•endosperm is absent in seed

•e.g. Pea, Groundnut and Beans.

ALBUMINOUS SEED :

•seed has endosperm

•e.g.  wheat, maize, barley and castor seed.

64. EMBRYO:

o Zygote divide to form embryo which after seed germination give rise a new plant.

o A typical dicot embryo has an embryo axis. it has two cotyledon

65.EPICOTYLE –>The portion of embryonal axis above the level of cotyledon which terminate with Plumule ( future stem ).

66.HYPOCOTYLE –>The portion below the cotyledon junction is called hypocotyle which terminate into radicle or root tip. Root tip has root cap.

67.Monocotyledon embryo has only one cotyledon.

68.In grass family this single cotyledon is called scutellum which is present lateral side of embryonal axis.

69.COLEORHIZA is the undifferentiated sheath which enclose the radicle and root cap of monocot embryo.

70.COLEOPTILE is the hollow foliar structure which encloses plumule and leaf primordia in monocot embryo.

71. SEED:

oFertilized ovule is called seed.

o Seed is consist of seed coat (integument),cotyledon and embryonal axis.

oThe micropyle remain as pore which allow the entry of oxygen and water during seed germination.

72.PERISPERM is the remains of nucellus in some seeds.

oExample  black pepper and beet.

73.Angiospermic seed shows two generations :

oParental sporophyte (seed coat )

oFuture sporophyte (embryo).

74.Seed dormancy –> At maturity of seed , embryo exhibit slow metabolic activity, reduced water content in seed (10 to 15 % of mass ) which make the embryo almost inactive (seed show very reduced metabolic activity )

o When adequate water,air and temperature available, seed dormancy is broken down and seed germinates.

75.Seeds offer several advantages to angiosperm .

76.In angiosperm, pollination and fertilization are independent of water so seed formation is more dependable.

77.Seed dispersal techniques are helpful in distribution of species to new habitat.

78.The hard seed coat protect the young embryo.

79.Seed is the product of sexual reproduction so it offer genetic recombination leading to variation.

80.Seeds are basis of agriculture.

81.Dehydrated and dormant seeds are stored for food and agriculture.

82.Lupinus arcticus (lupine) is the example of oldest viable seed which was excavated from arctic tundra. It germinated and flowered after 10000 years of dormancy.

83. Phoenix dactylifera (date palm) is example of 2000 years old viable seed which was excavated from king’s Herod palace near dead sea.

84. APOMIXIS :

o Apomixis is formation of seed without fertilization.

o Apomixis is form of asexual reproduction that mimic sexual reproduction in product.

85.There are several ways through apomictic seeds are formed –

o In some species, diploid egg is formed without meiosis and divide to form embryo.

o some of the nucellus cells protrude in embryo sac and divide to form embryos as in mango and citrus.

86.POLYEMBRYONY –>Presence of many embryos in a seed .

oexample-mango ,citrus

87.Hybrid varieties are commercially important  –>Apomictic seeds do not show segregation of hybrid characters in progeny

oThere are lots of researches going on to transfer of apomictic genes into hybrid varieties.

88. Fruit :

Normally after fertilization, ovary develop into fruit which contain seed (fertilized ovule ).

Seed and fruit develop simultaneously.

Pericarp :The wall of the ovary develop into fruit wall .

Example of fleshy fruits are guava, orange and mango.

oExample of dry fruits are groundnut and mustard.

oTrue fruit : which develop from only ovary.

oFalse fruit :in this type of fruit thalamus also contributes in fruit formation with ovary. Examples are apple, strawberry and cashew.

89.Parthenocarpic fruit –>fruits develop without fertilization .

These fruits are seedless

●develop artificially by application of growth hormones.

●Banana and grapes are example of Parthenocarpic fruit.