The ovule in plants is a structure that encloses the reproductive cells in females which contains – the nucleus, the integument and the female gametophyte. The embryo sac or female gametophyte is an oval structure present in the ovule of flowering plants. It is formed when haploid megaspore nucleus divides. It possesses 2 haploid nuclei and 6 haploid cells which do not have cell walls. In some cases, the haploid nuclei combine forming a single cell(endosperm motor cell). At the event of fertilization, one male nucleus and egg nucleus combine for the formation of zygote leading to the development of the embryo. Endosperm nucleus is formed with the fusion of the primary endosperm nucleus and second male nucleus.
The embryo sac is formed in two phases. In the first stage, megasporogenesis occurs, where the haploid megaspore tetrad is formed due to meiosis of single diploid mother cell. Out of this, all three disintegrates and one sustains which later develops into the embryo sac.
The second stage is the megagametogenesis, where the embryo sac is formed due to mitosis of the functional haploid megaspore to produce 8-nucleate, 7-celled gametophyte. The polar nuclei move to the centre out of all the eight nuclei, to combine and generate a single diploid cell at that location(centre). This single diploid cell then fuses with the sperm to generate the triploid endosperm. The other three nuclei grow into antipodal cells while two modify into synergid cells which are degenerated gradually.
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