XII BIOLOGY
CH-1 Reproduction in lower and higher plants
PART-PART-Double
Fertilization
It is a unique process in flowering plants
where, after pollination, the pollen tube enters the ovule and releases two
sperm cells.
One sperm fuses with the egg cell to form a
diploid zygote and the other sperm fuses
with the two polar nuclei in the
central cell to form a triploid endosperm.
This dual fertilization ensures that the developing
embryo has a reliable food source.
Here's
a breakdown of the process:
Pollen Tube Formation: After
pollination, a pollen grain on the stigma grows a pollen tube that carries the
male gametes (sperm cells) to the ovule.
Entry into Ovule: The
pollen tube enters the ovule, a structure containing the female
gametophyte.
Sperm Release: The
pollen tube bursts, releasing two sperm cells into the embryo sac.
First Fertilization (Syngamy): One
sperm cell fuses with the egg cell, forming a diploid zygote (2n).
Second Fertilization (Triple Fusion): The
second sperm cell fuses with the two polar nuclei (each haploid) in the central
cell, resulting in a triploid primary endosperm nucleus (3n).
Result: The
zygote will develop into the embryo, and the primary endosperm nucleus will
divide to form the endosperm, which serves as the nutrient supply for the
growing embryo.
Double
Fertilization –
The process of fusion of male gamete with the female
gamete to form the diploid zygote is
called as fertilization.
The fusion of one male gamete with egg and another
male gamete with secondary nucleus is called as double fertilization.
Thus fertilization occurs two times and both the
male gametes participate so called as double fertilization.
Process
of double fertilization –
1) After pollination pollen grain forms pollen tube,
which grows towards the ovule.
2)Pollen tube carries two male gametes, thin
cytoplasm and tube nucleus at tip.
3)Non motile male gametes transfers through tube
called as siphonogamy.
4)Pollen tube enters in ovule through different ways
-
a)Porogamy - Entry of
pollen tube inside the ovule through micropyle.
b)Chalazogamy - Entry of
pollen tube inside the ovule through chalaza.
c)Mesogamy- Entry of
pollen tube inside the ovule through integuments i.e. body of ovule.
5)Pollen tube always enters the embryo sac near the
egg apparatus.
6)Filiform apparatus of synergids attract the pollen
tube towards the egg appratus.
7)The tip of pollen tube absorb water from one of
the synergids and burst open to release two male gametes.
8)The tube nucleus degenerates completely.
9)In Syngamy or first fertilization - one of the
male gametes get fuses with the egg to form diploid zygote or oospore
10)In Triple fusion or second fertilization -The other male gamete fuses with the diploid
secondary nucleus to form triploid primary endosperm nucleus (PEN).
Significance
of double fertilization-
i) It is a unique feature of angiosperms. It ensures that the parent plant invests a seed with a food store, only if the
egg is fertilized.
ii)The diploid zygote which develop in to embryo and
then in to new plant.
iii)Syngamy maintain diploidy in the life cycle.
iv) It restores the diploid condition by fusion of
haploid male gamete with haploid female gamete
v) Triple fusion produces triploid PEN which
develops in to endosperm.
vi) Endosperm provide nutrition for developing
embryo.
vii) Because of double fertilization endosperm and embryo
get produced together which increases viability of seeds.
viii)It also helps to avoid polyembryony.. It uses both the male gametes.