Page 6 - LN-CH-2[2]
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Double fertilization:
After entering one of the synergids, the pollen tube releases two male gametes into the cytoplasm
of the synergids.
Syngamy: one of the male gamete fused with egg cell, to form a diploid zygote.
Two polar nuclei of central cell fused to form a diploid secondary nucleus.
Triple fusion: The second male gamete fused with the secondary nucleus to form a
triploid primary endosperm nucleus.
Since two type of fusion, syngamy and triple fusion take place in the embryo sac the phenomenon
is termed asdouble fertilization.
The central cell after triple fusion becomes primary endosperm cell and developed into
the endosperm.
The zygote developed into an embryo.
POST- FERTILIZATION : STRUCTURE AND EVENTS
Events of endosperm and embryo development, maturation of ovule into seed and ovary into fruit, are collectively
termed aspost-fertilization events.
Endosperm:
Development of endosperm takes place before the embryo development.
Primary endosperm cell divides repeatedly to form a triploid endosperm.
Cells are filled with reserve food material and are used for the nutrition of the developing embryo.
PEN undergoes successive nuclear division to give rise to free nuclei. This is called free-nuclear
endosperm.
Subsequently cell wall formation takes place and become cellular endosperm.
The coconut water is free nuclear endosperm and the white kernel is the cellular endosperm.
Endosperm may be consumed completely during embryo developed or it may be consumed during
germination of seed.
Embryo:
Zygote formed and placed at the micropylar end of the embryo sac.
Zygote starts its development only after some amount of endosperm formed.
Embryo development takes place in following stages:
o Proembryo
o Globular stage
o Heart shaped
o Matured embryo.
Dicot embryo:
A typical dicotyledonous embryo consists of an embryonal axis and two cotyledons.
Embryonal axis above the cotyledon is the epicotyls.
Terminal part of the epicotyls is the plumule (gives rise to the shoot).
