The formation of the embryo and endosperm in angiosperms (flowering plants) is a complex process that begins with fertilization and involves several key steps:
1. Double Fertilization:
* Pollen grain lands on the stigma and germinates, forming a pollen tube that grows down the style towards the ovary.
* The pollen tube carries two sperm nuclei.
* One sperm nucleus fuses with the egg cell inside the ovule, forming the zygote (2n). This is syngamy or fertilization which marks the beginning of the embryo.
* The other sperm nucleus fuses with the central cell of the ovule, which contains two polar nuclei, forming the primary endosperm nucleus (3n). This is triple fusion.
2. Embryo Development:
* The zygote undergoes repeated mitotic divisions to form a multicellular embryo.
* The embryo develops a radicle (root), plumule (shoot), and cotyledons (seed leaves).
* Dicotyledonous plants (e.g., beans, peas) have two cotyledons, while monocotyledonous plants (e.g., corn, rice) have one cotyledon.
3. Endosperm Development:
* The primary endosperm nucleus divides mitotically to form the endosperm, a nutrient-rich tissue that nourishes the developing embryo.
* Endosperm can be cellular (with cell walls) or nuclear (with free nuclei).
* In some plants, the endosperm is completely absorbed by the developing embryo before seed maturity. This is the case in beans and peas.
* In other plants, like corn and rice, the endosperm remains as a significant part of the seed, providing nourishment to the germinating seedling.
4. Seed Formation:
* The fertilized ovule develops into a seed, which contains the embryo, endosperm (if present), and a seed coat derived from the integuments of the ovule.
* The seed remains dormant until favorable conditions for germination are present.
Here is a simplified diagram:
```
|
| Double Fertilization
|
/\
/ \
/ \
/______\
/ \
/ \
/ Zygote \
/ (2n) \
/ \
/ \
/_________ __________\
| Embryo | Primary Endosperm Nucleus |
| (2n) | (3n) |
|_____ |______ |
| |
| |
/ \ / \
/ \ / \
/ \ / \
/ \ / \
/_________ \ /_________ \
| Endosperm | | Seed |
| (3n) | | |
|_____ |___|______|
```
Key Points:
* Double fertilization is unique to angiosperms and contributes to the efficient nourishment of the developing embryo.
* The endosperm provides nutrients to the embryo and is essential for seed development.
* The embryo develops from the zygote, while the endosperm develops from the primary endosperm nucleus.
This detailed explanation should help you understand the intricate process of embryo and endosperm formation in flowering plants.
