Sexual reproduction in flowering plants (New syllabus chapter 1 / old syllabus Chapter 2)
Endosperm developement| Embryo development|reproduction in flowering plants|short notes| NCERT| standard 12| Biology
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Note 12
POST-FERTILISATION : STRUCTURES AND EVENTS
Following double fertilisation, many events occur
- endosperm development
- embryo development
- maturation of ovule(s) into seed(s)
- Maturation of ovary into fruit
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Endosperm developement
The primary endosperm cell divides many times - forms a triploid endosperm tissue.
The cells of this tissue are filled with reserve food materials.
They are used for the nutrition of the developing embryo.
This way Endosperm development precedes embryo development.
Commonly, in endosperm development, the PEN (Primary Endosperm Nucleus) undergoes successive nuclear divisions -- give rise to free nuclei.
This stage of endosperm development is called free-nuclear endosperm.
After that cell wall formation occurs - the endosperm becomes cellular.
The number of free nuclei formed before cellularisation can be different in different plants.
The coconut example:
Coconut water from tender coconut is free-nuclear endosperm (made up of thousands of nuclei)
The surrounding white kernel is the cellular endosperm.
Endosperm can be completely consumed by the developing embryo before seed maturation - example: pea, groundnut, beans
Endosperm may persist in the mature seed and be used up during seed germination - example: castor and coconut
Embryo development
Embryo development is also termed as embryogeny
Embryo develops at the micropylar end of the embryo sac where the zygote is situated.
Most zygotes divide only after certain amount of endosperm is formed. - adaptation to provide assured nutrition to the developing embryo.
The seeds differ greatly but the early stages of embryo development (embryogeny) are similar in both monocotyledons and dicotyledons.
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| the stages of embryogeny in a dicotyledonous embryo |
The zygote gives rise to the proembryo and subsequently to the globular, heart-shaped and mature embryo.
A typical dicotyledonous embryo
It has an embryonal axis and two cotyledons.
The portion of embryonal axis above the level of cotyledons is the epicotyl,
Epicotyl terminates with the plumule or stem tip.
The cylindrical portion below the level of cotyledons is hypocotyl.
Hypocotyl terminates at its lower end in the radicle or root tip.
The root tip is covered with a root cap.
Embryos of monocotyledons
It has only one cotyledon.
In the grass family the cotyledon is called scutellum
Scutellum is situated towards one side (lateral) of the embryonal axis.
At its lower end, the embryonal axis has the radical and root cap enclosed in an undifferentiated sheath called coleorrhiza.
The portion of the embryonal axis above the level of attachment of scutellum is the epicotyl.
Epicotyl has a shoot apex and a few leaf primordia enclosed in a hollow foliar structure, the coleoptile.
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