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POLLINATION

Pollination is the transfer of pollen grains from the anther of a flower to the stigma of the same flower or another flower but of the same species.

Types of Pollination

1. Self Pollination

Transfer of pollen grains from the anther to the stigma of the same flower.

2. Cross Pollination

Transfer of pollen grains from the anther to the stigma of another flower of the same species.

NOTE: Cross pollinated flowers produce healthier seeds than self pollinated flowers.

Devices Favoring Self Pollination

1. Homogamy

When male (stamen) and female (pistil) organs mature simultaneously in the same flower, enabling self pollination.

2. Cleistogamy

In bisexual flowers where organs mature at different times but the flower remains closed, forcing self pollination as no external agents can enter.

Devices Favoring Cross Pollination

1. Unisexuality

Flowers containing only male or female organs, making cross pollination mandatory.

2. Dichogamy

When male and female organs in bisexual flowers mature at different times:

  • Protandry: Male organs mature first (protandrous flower)
  • Protogyny: Female organs mature first (protogynous flower)

Agents of Pollination

Insect (Entomorphily)

Characteristics:

  • Bright, conspicuous petals
  • Nectar present
  • Scented
  • Sticky pollen grains
Wind (Anemorphily)

Characteristics:

  • Dull, inconspicuous petals
  • No nectar
  • No scent
  • Light, numerous pollen
  • Protruding stigma
Animals (Zoophily)

Common pollinators: birds, snails

Water (Hydrophily)

Disadvantage: Many pollen grains get destroyed by water.

Fertilisation in Flowers

Fertilisation is defined as the union of male and female gametes with haploid (n) number of chromosomes to form a zygote. After pollination, the intine of the inner coat of the pollen grain will germinate to form the pollen tube. The germination is stimulated by the sugary substance produced on the stigma.

Initially, the pollen tube contains two nuclei. The larger nucleus is called the vegetative or tube nucleus while the smaller nucleus is called the generative nucleus. The vegetative nucleus eventually disintegrates while the smaller nucleus (generative nucleus) divides to form two male gametes.

The pollen tube containing the two male gametes passes through the tissues of the style to the ovary. On reaching the ovary, the pollen tube will bend toward the ovule through the micropyle, and when the pollen tube arrives at the embryo sac, one of the male gametes will fuse with the egg or ovum to form a zygote.

The other male gamete will fuse with the secondary nucleus to form the endosperm nucleus. The zygote is diploid (2n) but the endosperm nucleus is triploid (3n) because the secondary nucleus is already diploid (2n).

It is the function of the synergids to direct the male gametes to the appropriate portal. Since the pollen tube typically enters the embryo sac through the micropyle end, this type of fertilization is called porogamic fertilisation.

In some cases, the pollen tube may enter the embryo sac through the chalaza, and in this case such fertilization is described as chalazogamic fertilisation. Since during fertilisation one male gamete fuses with an egg and the other male gamete fuses with the secondary nucleus, the process is known as double fertilisation. Double fertilisation occurs in all angiosperms.

Changes That Take Place After Fertilisation

  • The calyx of the sepals withers
  • The corolla of the petals withers
  • The receptacle disappears and leaves a scar
  • The style and stigma disappear and leave a scar
  • The synergids disappear after they have finished performing their function
  • The antipodal cells wither
  • The ovary develops into a fruit
  • The ovary wall develops to form the pericarp or fruit wall. The pericarp typically has three layers: epicarp, mesocarp, and endocarp.
  • The ovule becomes the seed
  • The ovule wall or integument becomes the seed coat. The outer seed coat is known as the testa while the inner seed coat is called the tegmen
  • The endosperm nucleus will grow to form the endosperm of the seed
  • The zygote develops to form the embryo of the seed
  • The embryo later differentiates into radicle, plumule, and cotyledon. The radicle is the primordial root, the plumule is the primordial shoot, while the cotyledon serves as nourishing tissue for the developing embryo

Pollination JAMB/WAEC questions

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