describe the post fertilization changes in embryo sac.(marks=4)

Fertilisation leads to formation of zygote which forms embryo.
Embryo is the progenitor of the next generation.
Double fertilisation in Angiosperms ensures that proper nutrition is provided to the embryo. (Formation of endosperm due to triple fusion).

Post fertilisation changes taking place are:
1. Embryo development. (Zygote gets converted to embryo)
2. Endosperm development. (PEN gets converted to endosperm)
3. Formation of seeds. (Ovules are converted to seeds)
4. Formation of fruits. (Ovary is converted to fruit)

Other changes are:
Style, Stigma, Anthers, Sepals, Petals wither away. Ovary wall gets changed to fruit wall (Pericarp).

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• Embryo develops at the micropylar end of the embryo sac where the zygote is situated. Most zygote divides only after certain amount of endosperm is formed. This way, it ensures that nutrition is provided to the developing embryo.
• Though there are great differences in monocot and dicot seeds, the early stages of embryo development are similar in both monocots and dicots.
• The zygote gives rise to proembryo and subsequently to globular, heart shaped and mature embryo.
• A dicotyledonous embryo consists of an embryonal axis and two cotyledons. The portion of the embryonal axis above the level of the cotyledon is epicotyls which terminates with the plumule.
• The cylindrical portion below the level of the cotyledons is the hypocotyl which terminates into the radical or root tip. The root cap covers the root tip
• Embryos of monocots have only one cotyledon
• In the poaceae , cotyledon is called scutellum
• Scutellum is situated towards the lateral side of the embryonal axis.
• At the lower end, the embryonal axis has the radical and root cap enclosed in an undifferentiated sheath coleorrhiza.
• The portion of the embryonal axis above the level of scutellum is called epicotyl.
• Epicotyl has the shoot apex and a few leaf primordial enclosed in a coleoptile.
Endosperm
• The primary Endosperm Cell divides repeatedly and forms the triploid endosperm tissue
• The cells of this tissue are filled with reserve food materials and are used for nutrition of the embryo
• Generally, PEN undergoes successive nuclear divisions to give rise to free nuclei. This stage of endosperm development is called free nuclear endosperm.
• Later cell wall formation occurs making the endosperm cellular.
• Endosperm may be completely used by the developing embryo before seed maturation as in pea, groundnut or can be used during germination as in castor and coconut.