Flowering plants (angiosperms) rely on sexual reproduction to create the next generation. This cycle requires the precise union of specialized male and female gametes to form a genetically unique individual. The entire reproductive system is contained within the flower, which serves as the plant’s breeding apparatus. To ensure propagation, the flower must develop a single, haploid female gamete, the egg cell, ready to combine with the male gamete delivered by pollen.
Identifying the Female Reproductive Structure
The structure containing the female reproductive parts is collectively known as the pistil. This centrally located organ is composed of three distinct sections that facilitate plant reproduction. At the top is the stigma, a sticky surface designed to receive and capture pollen grains.
Below the stigma is the style, a stalk-like column that acts as a pathway for the male gametes. The style connects the stigma to the lower, swollen chamber, which is the ovary. The ovary is a hollow structure that provides a safe environment for the internal processes.
The ovary encloses the ovules, which are specialized bodies where the female gametes develop. The ovary wall provides physical protection for these reproductive structures. The actual egg-forming activity happens inside these enclosed ovules.
The Ovule and Egg Cell Development
The ovule is the specific structure inside the ovary where the female gamete is manufactured. Each ovule begins with a specialized, diploid cell called the megaspore mother cell (megasporocyte), housed in the central tissue known as the nucellus. This cell undergoes megasporogenesis, a type of meiosis.
Meiosis results in the formation of four haploid megaspores, each containing half the genetic material of the mother plant. This reduction ensures the resulting zygote will have the correct chromosome number after fertilization. In most flowering plants, three of these megaspores degenerate.
Only one functional megaspore remains to continue the developmental process. This single surviving cell begins megagametogenesis, a series of rapid mitotic divisions that increase cell number. The nucleus divides three times in succession, resulting in a total of eight haploid nuclei.
Cell walls form around these eight nuclei to create the female gametophyte, commonly called the embryo sac. This mature embryo sac is a seven-celled structure. It consists of three antipodal cells, a large central cell, and three cells at the micropylar end.
The three cells at the micropylar end include two synergid cells and the single egg cell. The egg cell is the fully formed female gamete, positioned near the ovule opening called the micropyle. Synergids release chemical signals to guide the incoming pollen tube, ensuring precise delivery of the male gametes.
Role of the Egg Cell in Seed Creation
Once mature, the egg cell participates in double fertilization, a characteristic of all flowering plants. The pollen grain lands on the stigma and grows a pollen tube down the style into the ovule. This tube delivers two sperm cells directly into the embryo sac, facilitated by the synergid cells.
The first sperm cell fuses with the haploid egg cell, creating a diploid zygote. This newly formed zygote combines genetic material from both parents. It then develops into the embryo, the miniature plant inside the seed.
The second sperm cell joins the two polar nuclei within the central cell of the embryo sac. This fusion creates a triploid cell, specific to angiosperms. This triploid cell develops into the endosperm, a nutrient-rich tissue that serves as the primary food source for the embryo.
After double fertilization, the ovule matures and transforms into the seed. The protective layers (integuments) harden to form the seed coat. Concurrently, the ovary wall matures to become the fruit, ensuring seed dispersal.