Pollination and fertilization are two distinct, yet essential, processes in the sexual reproduction of flowering plants (angiosperms). Pollination precedes fertilization.
Pollination
Pollination is the transfer of pollen grains from the anther (male part) of a flower to the stigma (female part) of a flower. This process doesn't directly involve the fusion of gametes; it's simply the delivery mechanism for the male gametophyte (pollen grain). Pollination can occur via different agents:
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Wind: Some plants release copious amounts of lightweight pollen that is carried by the wind to other flowers. These plants often have inconspicuous flowers.
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Water: Some aquatic plants use water to transport pollen.
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Animals: Many plants rely on animals, such as insects (bees, butterflies, flies), birds, and mammals (bats), to transfer pollen. These plants often have brightly colored and fragrant flowers to attract pollinators.
- Self-pollination: Occurs when pollen is transferred from the anther to the stigma of the same flower or another flower on the same plant.
- Cross-pollination: Occurs when pollen is transferred from the anther of a flower on one plant to the stigma of a flower on a different plant of the same species.
Fertilization
Fertilization occurs after successful pollination. It is the fusion of the male gamete (sperm) from the pollen grain with the female gamete (egg) inside the ovule, leading to the formation of a zygote. Flowering plants exhibit a unique process called double fertilization:
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Pollen Tube Growth: After a pollen grain lands on the stigma, it germinates and grows a pollen tube down through the style, a structure connecting the stigma to the ovary. This growth is guided by chemical signals from the ovule.
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Delivery of Sperm Cells: The pollen tube carries two sperm cells into the ovule.
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Double Fertilization:
- One sperm cell fuses with the egg cell to form a diploid (2n) zygote. This zygote will develop into the embryo of the seed.
- The other sperm cell fuses with the two polar nuclei in the central cell of the ovule to form a triploid (3n) endosperm. The endosperm serves as a nutritive tissue for the developing embryo.
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Seed Development: After fertilization, the ovule develops into a seed. The seed contains the embryo and the endosperm, enclosed within a protective seed coat.
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Ovary Development: The ovary, which surrounds the ovule(s), develops into a fruit. The fruit protects the seed(s) and aids in their dispersal.
In summary, pollination is the act of transferring pollen, while fertilization is the fusion of male and female gametes, which is unique in flowering plants as it's double fertilization, creating both the embryo and its food source.
