What Is an Embryo Sac?
The embryo sac is a crucial structure in the reproductive process of flowering plants. It functions as the female gametophyte, the stage in a plant’s life cycle that produces gametes. This microscopic entity is where fertilization takes place, leading to the formation of a new plant. It is fundamental for the continuation of angiosperm species through sexual reproduction.
Where the Embryo Sac Resides
The embryo sac is found deep within the ovule, a small, egg-shaped structure located inside the ovary of a flowering plant. The ovary, a part of the flower’s pistil, encloses and protects these ovules. Each ovule typically contains only one embryo sac, though some plant species may possess multiple.
The ovule is attached to the ovary wall by a stalk-like structure called the funiculus. This connection allows the ovule to receive nutrients necessary for the development of the embryo sac and, later, the embryo. After fertilization, the entire ovule transforms into a seed, with the ovary maturing into the fruit.
Inside the Embryo Sac
A mature embryo sac typically contains seven cells and eight nuclei, arranged in a specific configuration. At one end, known as the micropylar end, are three cells: one large egg cell flanked by two smaller synergid cells. The egg cell contains the female gamete, ready for fusion with a sperm cell.
The two synergid cells play a role in guiding the pollen tube towards the egg cell, often by secreting chemical signals. In the center of the embryo sac lies a large central cell, which contains two polar nuclei. At the opposite end, the chalazal end, are three antipodal cells. These cells provide nutrients or support to the developing embryo sac.
The Embryo Sac’s Role in Plant Reproduction
The embryo sac is the site for double fertilization in flowering plants. After a pollen grain lands on the stigma and germinates, it forms a pollen tube that grows down through the style and into the ovary. This pollen tube then penetrates an ovule and enters the embryo sac, delivering two sperm cells.
One of these sperm cells fuses with the egg cell, forming a diploid zygote. This zygote will then develop into the embryo, the rudimentary plant within the seed. Simultaneously, the second sperm cell fuses with the two polar nuclei within the central cell. This fusion forms a triploid primary endosperm nucleus. The endosperm then develops into a nutritive tissue that provides sustenance for the growing embryo inside the seed.