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Postfertilization Events
Post-fertilisation refers to the biological processes that occur after the fusion of male and female gametes, known as fertilisation. These events mark the beginning of a new generation and lead to the zygote's transformation into a mature seed capable of germination. In flowering plants, post-fertilisation involves a series of developmental stages including endosperm formation, embryo development, seed formation, and fruit maturation. Each step ensures the continuity of the plant species through successful reproduction.
1.0Definition
Postfertilization events are the sequential developmental changes that take place in the ovule and ovary after fertilisation, leading to the formation of the seed and fruit.
2.0Major Postfertilization Events
Once fertilisation occurs, several structural and physiological changes occur in the reproductive organs. The major events include:
Endosperm Formation
- Endosperm is a nutritive tissue that supports the developing embryo.
- It develops from the primary endosperm nucleus (PEN) formed after triple fusion.
- Depending on how it develops, endosperm can be of three types:
- Nuclear type: Free nuclei divide without cell wall formation (e.g., coconut).
- Cellular type: Cell wall formation occurs immediately after nuclear division (e.g., Petunia).
- Helobial type: A combination of both nuclear and cellular types (e.g., Monocots like Asphodelus).
- Embryo Development (Embryogenesis)
- The zygote undergoes mitotic divisions and differentiates into an embryo.
- Early embryonic development involves the formation of the suspensor and embryo proper.
- Stages of embryo development include:
- Zygote → 2-celled stage → 4-celled stage → Globular stage → Heart-shaped stage → Torpedo stage → Mature embryo.
- The mature embryo consists of:
- Embryonal axis with radicle (root end) and plumule (shoot end).
- Cotyledons (seed leaves) — one in monocots, two in dicots.
- Development of the Seed
After embryogenesis, the ovule transforms into a seed.
Key changes during seed formation:
- The integuments of the ovule form the seed coat.
- The zygote develops into an embryo.
- Endosperm provides nourishment for the growing embryo.
- Seeds can be of two main types:
- Endospermic (albuminous): Retains endosperm (e.g., maize, castor).
- Non-endospermic (exalbuminous): Endosperm consumed during development (e.g., pea, groundnut).
- Formation of Fruit
- The ovary matures into a fruit after fertilisation.
- The ovary wall (pericarp) develops into the fruit wall.
- Types of fruits:
- True fruit: Develops only from the ovary (e.g., mango, tomato).
- False fruit: Develops from the ovary and other floral parts (e.g., apple).
- Changes in Ovule and Ovary
- Ovule → Seed
- Ovary → Fruit
- Funiculus (stalk of ovule) → Seed stalk
- Ovule integuments → Seed coat (testa and tegmen)
- Zygote → Embryo
- Endosperm nucleus → Endosperm tissue
- Perisperm and Pericarp Development
Sometimes, parts of the nucellus persist in the mature seed as perisperm (e.g., black pepper, beet).
The pericarp may become fleshy or dry depending on fruit type.
- Dormancy and Germination
Mature seeds often enter a period of dormancy — a temporary suspension of growth.
Under favourable conditions (water, oxygen, temperature), dormancy ends and germination begins.
3.0Significance of Postfertilization Events
- This process ensures the formation of viable seeds for propagation.
- Protects and nourishes the developing embryo.
- Facilitates dispersal through fruit formation.
- Maintains species continuity across generations.