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1. REPRODUCTION IN LOWER & HIGHER PLANTS

-CREATED BY-

PROF. DESHMUKH A. B.

ASST. TEACHER

SEC. & HIGHER SEC. SCHOOL, SHENDI (BHANDARDARA DAM), TAL. – AKOLE, DIST. - AHMEDNAGAR

1.9 Double Fertilization :

• It is a complex fertilization mechanism in flowering (angiospermic) plants.
• It was discovered by Nawaschin in the liliaceous plants like Lilium and Fritillaria.
• After a pollen grain has reached the surface of the stigma,
• it germinates and forms a pollen tube,
• pollen tube penetrates the stigma, style, ovary chamber and then enters ovule.

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Lilium sps.

Fritillaria sps.

DOUBLE FERTILIZATION

Stigma

Style

Ovary

Ovule

Nucleus

Cytoplasm

Germ Pore

Exine

Intine

Nucleus (n) divides (Mitosis)

This nuclear division either happens

before pollination inside pollen sac or

after pollination on stigma during germination.

Vegetative Nucleus / Tube Nucleus

Generative Nucleus

Vegetative cell

Generative cell

Stigma secretes watery substance

Pollen absorbs watery substance secreted by stigma

The growth of pollen tube is guided by the chemicals secreted by the synergids.

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Pollen Tube

Tube nucleus enters pollen tube along with the cytoplasm.

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It is then followed by generative cell.

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Pollen tube grows through the style

Generative cell (n) divides mitotically to form two haploid (n) male gametes

• Pollen tube usually enters ovule through the micropyle.

It is termed as porogamy.

• But in some cases, it is found to enter through chalaza,

known as chalazogamy ,

• and in some plants by piercing the integuments,

called mesogamy.

• Finally, it penetrates embryo sac of ovule through its micropylar end.

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The pollen tube carrying male gametes penetrates in one of the synergids.

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Watery contents of synergid are absorbed by pollen tube

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Polar Nuclei (n)

Male Gametes (n)

Synergid(n)

Antipodals

Pollen tube ruptures and release the contents, including the two non-motile male gametes.

As non motile male gametes are carried through hollow pollen tube, it is known as siphonogamy.

This ensures fertilization to take place.

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Syngamy and triple fusion are two events of sexual reproduction in angiospermic flowering plants.

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Syngamy is the fusion of haploid male gamete with haploid female gamete (egg) to produce a diploid zygote

One male gamete (n) fuses with Egg cell (n)

(Syngamy)

Second male gamete (n) fuses with Secondary nucleus (2n)

(Triple Fusion)

Secondary Nucleus (2n)

In triple fusion, second haploid male gamete fuses with diploid secondary nucleus producing primary endosperm nucleus (PEN) that develops into triploid endosperm.

The zygote develops into an embryo.

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Primary Endosperm Nucleus (3n)

Zygote(2n)

Syngamy is a type of generative fertilization

whereas

triple fusion is a type of vegetative fertilization.

Here, both the male gametes participate and therefore, it is described as or called double fertilization.

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Significance of Double Fertilization :

• • It is a unique feature of angiosperms. It ensures that the parent plant invests a seed with a food store, only if the egg is fertilized.
• • The diploid zygote develops into an embryo which consequently develops into a new plant.
• • The triploid PEN develops into nutritive endosperm tissue.
• • It restores the diploid condition by fusion of haploid male gamete with haploid female gamete (i.e. through syngamy).
• • It also helps to avoid polyembryony.

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