When a pollen tube enters the embryo sac during the process of fertilization in angiosperms (flowering plants), several critical events occur.
- Pollination: Pollination is the first step in fertilization. It involves the transfer of pollen from the anther (male reproductive part) of one flower to the stigma (female reproductive part) of another flower of the same species. This can occur through various agents, including wind, insects, birds, or other animals.
- Pollen Germination: Once pollen grains land on the stigma, they absorb moisture and germinate. A pollen grain consists of two cells: the generative cell and the tube cell. The generative cell will eventually give rise to two sperm cells, while the tube cell will form the pollen tube.
- Pollen Tube Growth: The tube cell elongates and forms a pollen tube. The pollen tube grows down through the style (the stalk connecting the stigma and ovary) and eventually reaches the ovule-containing ovary. The tube cell's nucleus is located at the tip of the pollen tube, guiding its growth.
- Penetration of the Micropyle: The pollen tube continues to grow until it reaches the micropyle, a small opening in the ovule's integuments (protective layers). The micropyle is the gateway to the embryo sac.
- Entry into the Embryo Sac: The tip of the pollen tube, called the vegetative nucleus, releases enzymes that help digest the integuments surrounding the micropyle. Once these barriers are breached, the pollen tube enters the embryo sac.
- Double Fertilization: Inside the embryo sac, two important fertilization events occur:
- Syngamy: One of the sperm cells released from the generative cell fuses with the egg cell (female gamete) to form a diploid zygote. This zygote is the future embryo of the plant.
- Triple Fusion: The second sperm cell fuses with two polar nuclei in the central cell of the embryo sac. This process results in the formation of a triploid cell, which will develop into the endosperm. The endosperm is a nutrient-rich tissue that nourishes the developing embryo.
- Development of the Embryo: The zygote divides and undergoes multiple rounds of cell division to form an embryo. The embryo will eventually give rise to the various parts of the mature plant, including the shoot, root, and leaves.
- Development of the Endosperm: The triploid cell formed by triple fusion also divides and develops into the endosperm, which provides nutrients to the growing embryo.
- Maturation of the Seed: As the embryo and endosperm continue to develop, other changes occur within the ovule, including the formation of the seed coat. The seed coat surrounds and protects the embryo and endosperm.
- Seed Dispersal: Once the seed is mature, the ovary develops into a fruit, which can aid in the dispersal of the seeds to new locations. This ensures the propagation of the plant species.