DOUBLE FERTILIZATION

CHAPTER 2                                    SEXUAL REPRODUCTION IN FLOWERING PLANTS

DOUBLE FERTILISATION

[1]  After entering one of the synergids, the pollen tube releases the two male gametes into the cytoplasm of the synergid.

[2]  One of the male gametes moves towards the egg cell and fuses with its nucleus thus completing the syngamy. This results in the formation of a diploid cell, the zygote.

[3]  The other male gamete moves towards the two polar nuclei located in the central cell and fuses with them to produce a triploid primary endosperm nucleus (PEN)

[4]  As this involves the fusion of three haploid nuclei it is termed triple fusion.

[5]  Since two types of fusions, syngamy and triple fusion take place in an embryo sac the phenomenon is termed double fertilisation, an event unique to flowering plants.

[6]  The central cell after triple fusion becomes the primary endosperm cell (PEC) and develops into the endosperm while the zygote develops into an embryo.

POST-FERTILISATION : STRUCTURES AND EVENTS

[1]  Following double fertilization , events of endosperm and embryo development , maturation of ovule(s) into seed(s) and ovary into fruit are

collectively termed post-fertilisation events.

Endosperm

[1]  Endosperm development precedes embryo development.

[2]  The primary endosperm cell divides repeatedly and forms a triploid endosperm tissue.

[3]  The cells of this tissue are filled with reserve food materials and are used for the nutrition of the developing embryo.

[4]  In the most common type of endosperm development, the PEN undergoes successive nuclear divisions to give rise to free nuclei.

[5]  This stage of endosperm development is called free-nuclear endosperm.

[6]  Subsequently cell wall formation occurs and the endosperm becomes cellular.

[7]  The number of free nuclei formed before cellularisation varies greatly.

[8]  The coconut water from tender coconut that you are familiar with, is nothing but free-nuclear endosperm (made up of thousands of nuclei) and the surrounding white kernel is the cellular endosperm.

[9]  Endosperm may either be completely consumed by the developing embryo (e.g., pea, groundnut, beans) before seed maturation or it may persist in the mature seed (e.g. castor and coconut) and be used up during seed germination.