Double Fertilization in Angiosperms

Before fertilization, the ovule undergoes changes. It starts with a diploid megaspore mother cell, which undergoes meiosis to produce four haploid megaspores. Three degenerate, leaving one that expands and undergoes three rounds of mitosis, resulting in eight haploid nuclei within the embryo sac.

Within the embryo sac, cell walls form. Three antipodal cells form opposite the micropyle, and above the micropyle, two synergids and one egg cell form. Two central nuclei remain in one large cell. The egg cell and this central nucleate cell are crucial for double fertilization.

For double fertilization, male gametes (sperm) travel from the anther to the embryo sac. A pollen grain contains a tube cell and a generative cell. When a pollen grain lands on the stigma, it germinates. The tube cell forms a pollen tube down the style into the ovary, with the generative cell following.

Near the ovary, the generative cell divides by mitosis to produce two haploid sperm cells. The pollen tube reaches the micropyle and releases the sperm into the embryo sac. One sperm fertilizes the egg cell, forming a diploid zygote (which becomes the embryo). The other sperm fuses with the two central nuclei, forming a triploid cell that develops into an endosperm, providing food for the embryo.

This double fertilization, resulting in both a diploid zygote and a triploid endosperm, is a characteristic unique to angiosperms (flowering plants). Other plant groups like gymnosperms, tracheophytes, and non-tracheophytes lack this specific reproductive feature.