1 9.2 - Sexual Reproduction in Flowering Plants. At the end of this lesson, students should be able...

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9.2 - Sexual Reproduction in Flowering Plants

At the end of this lesson, students should be able to :

a. Overview a typical life cycle of flowering plants.b. Describe general structures & functions of

reproductive organs in flowering plants.c. Describe the development of a pollen grain &

formation of male gamete.d. Describe the development of ovule, embryo sac &

formation of female gamete.e. Describe the process of pollination.f. Explain double fertilization in the formation of seed.

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Learning outcomes

9.2 : Sexual reproduction in flowering plants (a) Overview a typical life cycle of flowering plants.

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Life cycle of flowering plants

• The life cycles of angiosperms & other plants are characterized by an alternation of generations, in which:

haploid (n) gametophyte generationsdiploid (2n) sporophyte generations

• Both generation alternate & producing each other.

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Alternation of generations


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• The diploid plant, the sporophyte (2n), produces haploid spores (n) by meiosis.

• These spores divide by mitosis, giving rise to multicellular male & female haploid gametophyte (n).

• The gametophyte produce gametes – sperms (n) & eggs (n).

• Fertilization results in diploid zygotes (2n), which divide by mitosis & form new sporophyte.

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Life cycle of flowering plants

In angiosperms, the sporophyte (the plant we see growing in our environment) is the dominant generation. The sporophyte is diploid generation (2n).

In angiosperms, the sporophyte produce a unique reproductive structure, the flower; sex organs that produces haploid (n) spores (by meiosis) that will germinate to form the gametophyte generation.

The gametophyte (n) generation is very much reduced to special structures, the pollen grain (male) & the embryo sac (female), that are specifically in the process of sexual reproduction.

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Male gameteMales spores called

microspores germinate

to form the male gametophyte or

microgametophyte.

Female spores called megaspores

germinate to formthe female

gametophyte or megagametophyte.

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Female gamete

• Flowers, the reproductive organs of the angiosperm sporophyte.

• Typically, it composed of four whorls of highly modified leaves called floral organs, which are separated by very short internodes.

• Flowers cease growing once the fruit are formed.

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9.2 : Sexual reproduction in flowering plants (b) Describe general structures & functions of reproductive organs in flowering plants.

Structures & functions of reproductive organs in flowering plants

The four kinds of floral organs are the :

1. Sepals2. Petals3. Stamens4. Carpels

Their site ofattachment to the

stem is the receptacle 11


• Sepals are non-reproductive organs.

• Sepals (collectively called the calyx), which enclose & protect the floral bud before it opens, are usually green & more leaf like in appearance. 12


sepal

• Petals are non-reproductive organs (collectively called corolla) & are brightly colored.

• Petals function to attract pollinators & play an important role in ensuring that sexual reproduction will occur.

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petals

Source of picture: http://www.flmnh.ufl.edu/

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• Stamen is the male reproductive organs.

• A stamen consists of a stalk (the filament) & a terminal anther.

• Pollen sacs locate within the anther & functioning produce the pollen. stalk

anther

• Carpel is female reproductive organs.

• Carpel has an ovary at the base & a slender neck, the style.

• At the top the style is a sticky structure called stigma that serves as a landing platform for pollen.

• Within the ovary are one or more ovules.

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• The stamens & carpel of flowers contain sporangia (sing, sporangium), within which the spores & then gametophytes developed.

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The development of angiosperm gametophytes involve meiosis & mitosis.

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9.2 : Sexual reproduction in flowering plants (c) Describe the development of a pollen grain and formation of male gamete.

Gamete Formation

• The male gametophyte begins its development within the sporangia /pollen sacs of the anther.

• The male gametophytes are sperm-producing structures called pollen grain.

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Development of the pollen grains &the formation of the male gametes

1. Pollen grains are produced in anthers.2. Each anther consist of 4 pollen sacs or microsporangia.3. Each microsporangia contains numerous numbers of diploid

microspore mother cells or microsporocyte.4. Each microspore mother cell (microsporocyte) undergoes

meiosis.5. Producing 4 haploid microspores/ tetrad.6. Each microspores undergo mitosis.7. Producing a male gametophyte/microgametophyte that

consisting of only two cells (the generative cell & the tube cell/nucleus).

8. Together, these two cells & the spore wall constitute a pollen grain. 20

Step of development of a pollen grain & formation of male gamete.

Location : Anther

4 microsporangia/ pollen sac

consist of

Many microsporocytes (2n)/ microspore mother

cells

each contain

4 microspores (n)

each undergo meiosis

Male gametophyte (pollen grain, n) – consist of

generative cell & tube cell surrounded by spore wall

each undergo mitosis, produced

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During pollination, pollen grain will be transferred from anther to the stigma.

The pollen grain will germinate on stigma.

And tube cell /nucleus will produce pollen tube which will elongate down the style.

As the pollen tube elongates through the style, the generative cell/ nucleus will divide by mitosis.

To produce two male gametes (2 sperm cells) which fertilize egg and polar nuclei.

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Pollinator agent

26Pollen grain & stigma - electron

microscope

• The development of immature male gametophytes /microgametophytes / pollen grain occurs in female reproductive organ with get sufficient nutrient & protection from the female reproductive organ / tissue.

• The pollen grain become mature gametophyte when the generative cell divides by mitosis to form 2 male gametes /2 sperm cells.

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Mature vs. immature pollen grain

• Ovule located within the ovary.

• The structure of ovule:

i. The main tissue in ovule is the nucellus, which supported by the short stalk called funicle.

ii. The ovule is protected by integuments.iii. There is a tiny opening called micropyle at the apex

of the ovule.iv. And chalaza at the opposite end.

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Development of the ovule, embryo sac & the formation of the female gamete

9.2 : Sexual reproduction in flowering plants (d) Describe the development of ovule, embryo sac and formation of female gamete.

1. The female gametophyte begins to develop within the ovules of the ovary.

2. Each ovule contains megasporangium which produces one megasporocytes (megaspore mother cell, 2n).

3. Megaspore mother cell (megasporocyte) enlarges & undergo meiosis, producing four haploid megaspores.

4. In many species, three of these megaspores degenerate, only one megaspore survives.

5. The remaining megaspore enlarges to form the embryo sac (female gametophyte).

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Step of development of a ovule, embryo sac & formation of female gamete.

6. The nucleus of surviving megaspore divides by mitosis three times without cytokinesis to form eight haploid nuclei of cell which are arranged in groups of four nuclei at the two poles the embryo sac formed.

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Megaspore formation

Development of embryo sac

7. One of the nucleus from each pole migrate to the centre of the embryo sac.

8. These two nuclei are called polar nuclei (n+n). They are not partitioned into separate cells but share the cytoplasm of the large central cell of the embryo sac.

9. The three cells/nuclei at the opposite end to the micropyle form three antipodal cells with unknown function.

10.The three cells at the micropyle end consist of two synergid cells that flank the egg cell.

11.The synergids function in attract & guide the pollen tube to the embryo sac.

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Location : Within Ovule

Megasporangium

contain

One megasporocyte/ megaspore mother

cell (2n)

produced

4 megaspores (n)

enlarge & meiosis

3 megasporeDegenerate

1 megaspore

survivedHaploid (n) Embryo

sac, (female gametophyte) contain

8 haploid nucleus

enlarge, mitosis 3

times without

cytokinesis

4 Haploid nucleus arrange at each poles.

1 nucleus from each pole migrate to the centre, forming polar nuclei

(2 haploid nuclei)

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The arrangementsof cells / nuclei

aredetermined by agradient of thehormone auxin originating near

themicropyle

1. Three antipodal cells 2. Two polar nuclei 3. Two synergid cells & the egg cell

COMPARISON BETWEEN POLLEN GRAIN AND EMBRYO SAC OF ANGIOSPERM

S IMILAR IT IES Both are haploid structure // both are gametophytes Both are produced through mitosis of haploid spore

D IFFERENCES:

Embryo sac develop in ovule / megasporangium while pollen grain develop in pollen sac / microsporangium.

Megasporocyte / megaspore mother cell undergo meiosis to produce megaspore during formation of embryo sac while microsporocyte / microspore mother cell undergo meiosis to produce microspore during formation of pollen grain.

Eight nuclei are formed through three times mitosis without cytokinesis in embryo sac formation while only 2 nuclei/cell are formed through mitosis with cytokinesis in pollen grain formation.

Seven haploid cells produced after cytokinesis in embryo sac formation while only two haploid cells produced in pollen grain formation.

Embryo sac surround by integuments while pollen grain surround by exine and intine.

Embryo sac retained within flower while pollen grain released from the flower.

• Pollination is a process of transfer of pollen grain from an anther to a stigma.

• It occurs when pollen released from anthers is carried by wind or animals (pollination agents/pollinators) to land on a stigma.

• A pollinator is any agent that transfers pollen from male to female reproductive parts of flowers of the same plant species.

• E.g.: insects, birds, other animals, water currents, air currents & wind. 43

9.2 : Sexual reproduction in flowering plants (e) Describe the process of pollination.

Pollination

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Double Fertilization

Fertilization generally means fusion of a sperm nucleus with an egg nucleus.

1. After landing on receptive stigma, the pollen grain absorbs moisture & germinates.

2. Tube nucleus producing a pollen tube that extends down the style toward the ovary.

3. The generative cell divides by mitosis to produce two male gametes.

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9.2 : Sexual reproduction in flowering plants (f) Explain double fertilization in the formation of a seed.

4. The tip of the pollen tube enters the ovary through the micropyle & discharges two male gametes within the embryo sac.

5. Both male gametes fuse with nuclei in the embryo sac.

6. One male gamete (n) fertilizes the egg (n) to form the zygote (2n).

7. The other male gamete combines with the two polar nuclei to form a triploid (3n) nucleus in the central cell of embryo sac.

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8. This large cell will give rise to the endosperm, a food-storing tissue of the seed.

9. The union of two male gametes cells with different nuclei of the embryo sac is termed double fertilization.

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• Double fertilization ensures that the endosperm will develop only in the ovules where the egg has been fertilized.

• The endosperm is rich in nutrients, which it provides to the developing embryo.

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Important of Double Fertilization

embryoendosperm

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Structure of the mature seed(Monocots & Dicots)

• After double fertilization, the ovule develops into a seed & the ovary develops into a fruit enclosing the seed.

• It remains attached to the parent plant & continues to receive nutrients from it as it develops.

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FORMATION & SEEDS DEVELOPMENT


• Initially, these nutrients are stored in the endosperm, but later in seed development in many species, the storage function is taken over by the swelling storage leaves (cotyledons) of the embryo itself.

• The cotyledons store food absorbed from the endosperm before the seed germinate.

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• A seed contains the embryo from which a new plant will grow under proper conditions.

• Seeds also contain a supply of stored food (endosperm) & is wrapped in the seed coat or testa.

• The embryo has one cotyledon or seed leaf in monocotyledons, two cotyledons in dicotyledons.

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Structure of the seed(Monocots & Dicots)


• The radicle is the embryonic root. Radicles develops into the root system.

• The plumule is the embryonic shoot. Plumule consists of the stem and first leaves.

• The embryonic stem above the point of attachment of the cotyledon(s) is the epicotyl.

• The embryonic stem below the point of attachment is the hypocotyl.

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• The embryo of a monocot has a single cotyledon

• Members of grass family (eg: maize, wheat) have a specialized cotyledon called scutellum

• Scutellum is pressed against by endosperm, from which it absorbs nutrients during germination.

• The embryo of grass seed is also enclosed by two protective sheathes; a coleoptile (which covers the young shoot) and coleorhiza (which covers the young root).

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