FERTILIZATION - DOUBLE FERTILIZATION
Fertilization or syngamy is a biological process in which two dissimilar sexual
reproductive units or gametes are fused to produce the first sporophytic stage
of the angiospermic plant, known as zygote.
The
process of fertilization in angiosperms involves the following steps –
1. Germination of the pollen grain:- As
soon as the pollen grain reaches the surface of the stigma, it absorbs the
liquid from the surface of the stigma and swell up. The intine protrudes out
through the germ pore and forms a long tube like structure called pollen tube.
The pollen tube continues to elongate and passes through the style to reach the
ovule. All the contents of the pollen are transferred to the pollen tube.
The pollen tube enters the ovule in different
directions and accordingly fertilization may be of three types – (a) Porogamy – enters through the micropyle
(e.g., Datura, Mustard), (b) Chalazogamy
– enters through the chalazal region (e.g., Casuarina) and (c) Mesogamy – enters through the
integuments (e.g., Gourd, Cucumber, etc.).
2. Entry of the pollen tube into
embryo sac:- Just
before the entry of the pollen tube into embryo sac, the degeneration of
synergids take place. Thereafter the pollen tube enters through the
degenerating synergids through the filiform apparatus. After the pollen tube
enters into the embryo sac, the tip of the tube dissolves and both the male
gametes are set free inside the embryo sac.
3. Fusion of the gametes:- In normal
fertilization, one male gamete (n) fuses with the egg cell (n), while the other
male gamete (n) moves further into the embryo sac and unites with the secondary
nucleus or definitive nucleus (2n). Thus in angiosperms fertilization occur
twice and this phenomenon is known as double
fertilization.
The
fusion of one male gamete with the egg cell and the other male gamete with the
secondary nucleus is often termed as triple
fusion, which results in the formation of triploid endosperm nucleus (3n). On the other hand, the fusion of one male
gamete and the egg cell forms a diploid structure, known as zygote or oospore (2n).
SIGNIFICANCE OF
FERTILIZATION
1.
As a result of fertilization the oospore forms which gradually transform into
an embryo which on germination gives rise to seedlings.
2.
The ovule is transformed into seed which protects embryo and the endosperm.
3.
Fertilization stimulates the developmet of ovary and leads to the formation of
the fruit.
4.
Fertilization helps mingling of parental characters and establishment of
diploid nature of the plant.
5. Fertilization is a link between
gametophytic and sporophytic generations of the plant. The pollen grain and the
embryo sac represent the gametophytic (n) phase and the embryo represents the sporophytic
(2n) phase of the life cycle of the plant.
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