Question

Describe the development of the female gametophyte in a flowering plant. How does double fertilization occur?

Short answer

The development of the female gametophyte in a flowering plant begins with the differentiation of a haploid megaspore, which undergoes mitotic divisions to form an eight-nucleate, seven-celled embryo sac. Double fertilization is a unique feature of flowering plants where one sperm nucleus fuses with the egg cell to form a diploid zygote, and another sperm nucleus fuses with the two polar nuclei to form a triploid primary endosperm nucleus. This process contributes to the formation of zygote, endosperm, seed development, and genetic diversity of the species.

Step-by-step solution

Development of the Female Gametophyte in Flowering Plants

The development of the female gametophyte, also known as the embryo sac or megagametophyte, begins with the differentiation of a haploid cell called the megaspore. The megaspore undergoes three mitotic divisions, resulting in an eight-nucleate, seven-celled structure. One large central cell with two polar nuclei and six other smaller cells, which include the egg cell and two synergids (the egg apparatus) at the micropylar end, and three antipodal cells at the chalazal end, are present inside the embryo sac.

Process of Double Fertilization

Double fertilization is a unique feature of flowering plants. It refers to the fusion of one sperm nucleus with the egg cell to form a diploid zygote (2n) and another sperm nucleus with the two polar nuclei in the central cell to form a triploid (3n) primary endosperm nucleus. The process involves the following steps: 1. Pollination: Pollen grains from an anther (male reproductive organ) are transferred to the stigma of the pistil (female reproductive organ) in a process called pollination. 2. Pollen Germination: The pollen grain germinates and forms a pollen tube that grows through the style and passes through the micropyle to reach the embryo sac. 3. Release of Sperm Cells: Once the pollen tube reaches the embryo sac, the tube bursts and releases the two sperm cells inside the embryo sac. 4. Fertilization: One sperm cell fuses with the egg cell to form the diploid zygote (2n), and the other sperm cell fuses with the two polar nuclei to form the triploid primary endosperm nucleus (3n).

Significance of Double Fertilization

The double fertilization process has several advantages and significant roles in flowering plants: 1. Formation of Zygote: Fusion of a sperm nucleus with the egg cell forms a zygote, which eventually develops into an embryo and gives rise to a new plant. 2. Formation of Endosperm: Fusion of a sperm nucleus with the polar nuclei forms the primary endosperm nucleus, which later develops into the endosperm to nourish the growing embryo. 3. Seed Development: After double fertilization, the ovule develops into a seed, and the surrounding ovary develops into a fruit, providing protection and aiding in seed dispersal. 4. Genetic Diversity: Double fertilization enables genetic recombination, which contributes to the genetic diversity of the species and enhances the adaptation ability of plants in their environment.