Double fertilization is a complex fertilization mechanism unique to flowering plants (angiosperms). It involves the joining of a female gametophyte (megagametophyte, also called the embryo sac) with two male gametes, and it plays a crucial role in the reproduction of flowering plants. The process of double fertilization can be broken down into several key steps:
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Pollen Germination: It begins when a pollen grain adheres to the stigma of the carpel, the female reproductive structure of a flower. The pollen grain then takes in moisture and begins to germinate, forming a pollen tube that extends down toward the ovary through the style.
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Pollen Tube Entry: The tip of the pollen tube enters the ovary and penetrates through the micropyle opening in the ovule. The pollen tube proceeds to release the two sperm in the embryo sac.
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Fertilization Events:
- One sperm fertilizes the egg cell, forming a diploid zygote.
- The other sperm combines with the two polar nuclei of the large central cell of the megagametophyte, forming a triploid nucleus (triple fusion) that develops into the endosperm, a nutrient-rich tissue which provides nourishment to the developing embryo.
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Seed and Fruit Formation: The fertilized ovule forms the seed, while the tissues of the ovary become the fruit, usually enveloping the seed.
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Embryonic Development: After fertilization, the zygote divides to form two cells: the upper cell, or terminal cell, and the lower, or basal, cell. The division of the basal cell gives rise to the suspensor, which eventually makes connection with the maternal tissue. The terminal cell also divides, giving rise to a globular-shaped proembryo. Ultimately, the embryo and cotyledons fill the seed, and the seed is ready for dispersal.
Double fertilization is essential for the formation of both the embryo and its potential food source in the seed, known as the endosperm. This process ensures the successful reproduction and development of flowering plants/32%3A_Plant_Reproductive_Development_and_Structure/32.07%3A_Pollination_and_Fertilization_-_Double_Fertilization_in_Plants).
