After fertilization, the embryo sac undergoes a series of remarkable transformations, evolving into the foundational components of a seed, which houses the developing embryo and its nutrient supply.
Key Transformations within the Embryo Sac after Fertilization
The post-fertilization changes within the embryo sac are crucial for the development of a new plant. Here's a breakdown of what happens:
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Embryo Development:
- The zygote, formed from the fusion of the egg cell and one male gamete, begins to divide mitotically. This process leads to the formation of a multicellular embryo, which is the rudimentary plant within the seed.
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Endosperm Formation:
- The primary endosperm nucleus, which results from the fusion of the central cell's polar nuclei with the second male gamete, divides repeatedly and gives rise to the endosperm.
- The endosperm is a vital nutritive tissue that supplies nourishment to the developing embryo. It provides carbohydrates, proteins, and lipids essential for the embryo's growth and eventual germination. In some plants (like corn), endosperm is abundant in the mature seed, while in others (like beans), it's largely absorbed by the cotyledons of the embryo.
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Embryo Sac Expansion:
- To accommodate the growing structures within, the embryo sac expands to accommodate the developing embryo and the growing endosperm. This physical enlargement facilitates the increase in tissue mass and ensures adequate space for development.
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Fate of the Nucellus and Perisperm:
- The nucleus gets mostly used up by the embryo, and its remnant forms the perisperm. In botanical terms, the nucellus, which is the nutritive tissue surrounding the embryo sac within the ovule, is largely consumed by the developing embryo for its growth. In certain seeds, a persistent portion of this nucellar tissue remains and forms the perisperm, serving as an additional storage tissue (e.g., in black pepper and beetroot).
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Degeneration of Other Cells:
- The other accessory cells within the embryo sac typically degenerate after fertilization:
- Synergids: These cells, located adjacent to the egg cell, play a role in guiding the pollen tube. They usually degenerate soon after fertilization.
- Antipodal cells: Situated at the chalazal end of the embryo sac, these cells also typically undergo degeneration.
- The other accessory cells within the embryo sac typically degenerate after fertilization:
Summary of Post-Fertilization Events in the Ovule
The entire ovule, encompassing the transformed embryo sac, develops into the seed. The outer protective layers of the ovule, known as the integuments, harden to form the seed coat. Concurrently, the ovary surrounding the ovules ripens to become the fruit.
| Ovule/Embryo Sac Component | Post-Fertilization Development | Role in Seed/Fruit |
|---|---|---|
| Egg Cell (Zygote) | Transforms into Embryo | Future Plant |
| Central Cell (PEN) | Develops into Endosperm | Nutrient storage for embryo |
| Embryo Sac | Expands, becomes part of the seed | Houses embryo & endosperm |
| Nucellus | Largely consumed; remnant forms Perisperm | Additional nutrient storage (if present) |
| Ovule Integuments | Develop into Seed Coat | Protection of the embryo |
| Ovary | Develops into Fruit | Protection & dispersal of seeds |
This complex sequence of events ensures the formation of a viable seed, which is crucial for the dispersal and perpetuation of plant species.
For more detailed insights into plant reproduction and seed development, you can refer to academic resources such as Khan Academy's overview of flowering plant reproduction or university botanical guides.
