Development Of New Seeds - SS3 Biology Past Questions and Answers - page 2

Pollen grains are the male gametophytes in flowering plants. They consist of a tough outer layer called the exine and a delicate inner layer known as the intine. The exine is composed of sporopollenin, making it resistant to environmental factors. Within the pollen grain, there are two haploid cells: the generative cell and the tube cell. The generative cell divides to form two sperm cells.

Fertilization in flowering plants involves the fusion of male and female gametes to form a zygote. When a pollen grain reaches the stigma, it germinates and forms a pollen tube that grows down the style. The two sperm cells are released into the embryo sac through the pollen tube. One sperm cell fertilizes the egg cell, forming the zygote, while the other combines with the two polar nuclei to create the triploid endosperm, providing nourishment to the developing embryo.

Seeds are the mature ovules of flowering plants. They consist of three main parts: the seed coat, endosperm, and embryo. The seed coat, derived from the integuments of the ovule, provides protection to the inner parts of the seed. The endosperm is a nutritive tissue that stores reserve food materials, such as starch and proteins, to support the initial growth of the germinating seedling. The embryo contains the embryonic root (radicle), embryonic shoot (plumule), and one or two cotyledons, which eventually develop into the first leaves of the seedling.

Germination is the process by which a seed develops into a new plant under suitable conditions. There are three main types of germination: epigeal, and hypogeal germination.

Epigeal germination: In this type, the cotyledons emerge above the soil level, and the epicotyl elongates, lifting the cotyledons and the plumule into the air.

Hypogeal germination: Here, the cotyledons remain below the soil surface, and the epicotyl doesn't elongate significantly.

Germination is influenced by various environmental factors such as water, temperature, oxygen availability, and light. Adequate water supply is essential to activate enzymes and metabolic processes. Suitable temperatures trigger biochemical reactions, and oxygen is required for cellular respiration.