SECTION B Answer the following questions in about 150 words each : (a) Give an account of post-fertilization changes leading to formation of seeds in angiosperms.

Post-Fertilization Changes Leading to Seed Formation

The transformation of the fertilized ovule into a seed is a coordinated developmental process involving several key changes:

1. Endosperm Development

The primary endosperm nucleus (PEN), formed by the fusion of a male gamete and two polar nuclei, undergoes repeated divisions to form the endosperm. This nutritive tissue is typically triploid (3n) and serves as the primary food source for the developing embryo.

• Nuclear Endosperm: In this common type, free nuclear divisions occur without immediate cell wall formation, resulting in a multinucleate cytoplasm (e.g., coconut water).
• Cellular Endosperm: Cell wall formation follows each nuclear division, leading to a cellular tissue from the start (e.g., coconut meat).
• Helobial Endosperm: An intermediate type, where the first division is followed by cell wall formation, but subsequent divisions in the micropylar chamber are free-nuclear.

The endosperm provides essential nutrients like starch, proteins, and lipids for the embryo's growth. In some seeds (e.g., pea, bean), the endosperm is completely consumed by the developing embryo, and food is stored in cotyledons. In others (e.g., wheat, castor bean), it persists until germination.

2. Embryo Development (Embryogeny)

The diploid zygote, after a period of rest, undergoes a series of mitotic divisions and differentiations to form the embryo.

• The zygote first divides transversely into a smaller terminal cell (towards the chalazal end) and a larger basal cell (towards the micropylar end).
• The terminal cell develops into the embryo proper, undergoing globular, heart-shaped, and torpedo stages before forming a mature embryo.
• The basal cell typically forms a suspensor, a filamentous structure that pushes the embryo deeper into the endosperm and facilitates nutrient transfer from the endosperm to the developing embryo.
• The mature embryo consists of an embryonic axis (comprising plumule, radicle, and hypocotyl) and one or two cotyledons, which serve as food storage or absorptive organs.

3. Ovule Maturation into Seed

Concurrently with endosperm and embryo development, the entire ovule undergoes significant structural transformations to become a seed.

• Integuments to Seed Coat: The protective outer layers of the ovule, called integuments, harden and differentiate into the seed coat. This tough protective layer consists of an outer testa and an inner tegmen, shielding the embryo from mechanical injury and desiccation.
• Micropyle: The small opening in the integuments, the micropyle, persists in the seed coat, allowing for the absorption of water and oxygen during germination.
• Nucellus: The nucellus, the central tissue of the ovule, usually degenerates. However, in some plants (e.g., black pepper, beet), it persists as a nutritive tissue called perisperm.
• Funiculus: The stalk that attaches the ovule to the placenta (funiculus) develops into the stalk of the seed.

The ovary, which encloses the ovules, simultaneously matures into the fruit, providing further protection and aiding in seed dispersal.