6.3 - Pollination and Fertilisation

Pollination

Definition and Pollinating Agents

• Pollination: Transfer of pollen grains from the anther to the stigma.
• Pollinating agents include insects, mammals, birds, water, and wind.
• The presence of pollen grains on the stigma initiates fertilisation.

Formation of Pollen Tube and Male Gametes

Formation Sequence

1. The anther wall of mature pollen grains dries, shrinks, and splits; pollen grains are released from the pollen sac.
2. Pollinating agents transfer the pollen grains to the stigma of the same flower or another flower.
3. Pollen grains on the stigma germinate and form pollen tubes, which grow down the style towards the ovule.
4. The generative nucleus moves along the pollen tube towards the ovule and divides by mitosis to form two haploid ($n$) male gametes.
5. The end of the pollen tube secretes an enzyme to digest the tissues of the style.
6. The pollen tube penetrates the ovule through the micropyle. The tube nucleus degenerates, and both male gametes enter the embryo sac.

Double Fertilisation

Formation of the Diploid Zygote and Triploid Endosperm Nucleus

1. One haploid male gamete ($n$) fertilises the haploid egg cell ($n$), producing a diploid zygote ($2n$).

$\text{Male gamete }(n)+\text{Egg cell }(n)\to \text{Diploid zygote }(2n)$

2. The second haploid male gamete ($n$) fuses with the two polar nuclei ($n+n$), forming a triploid endosperm nucleus ($3n$).

$\text{Male gamete }(n)+\text{Two polar nuclei }(n+n)\to \text{Triploid endosperm nucleus }(3n)$

Importance of Double Fertilisation for the Survival of Flowering Plants

• Formation of the zygote:
  • Passes genetic information from one generation to the next.
  • Restores the haploid condition in the gametes through formation of a diploid zygote.
• Formation of endosperm tissue:
  • Supports embryo development and the survival of the plant species.
  • In eudicots such as legumes, mangoes, and mustard, the embryo fully uses the endosperm before the seed matures.
  • In most monocots such as coconut, wheat, barley, and corn, the embryo uses only part of the endosperm; some is stored in the cotyledon for seed germination.
  • Enables the embryo to survive for a long time in the seed when conditions are unsuitable for germination.