Double fertilization
This is
characteristic of angiosperms. It was discovered by S.G. Nawaschin in 1898.
Both the sperms present in the pollen tube fertilize to the different cells of
the embryo-sac one of the sperms fuses with the egg and develops into the
zygote, while the other one fuses with the secondary nucleus (result of the fusion of two polar nuclei) to
form PEN (Primary Endosperm Nucleus) . The latter fusion is known as triple
fusion and it leads to the formation of the nutritive tissue called endosperm.
The method of
transfer of the sperm into the egg or to the secondary nucleus is a matter of
conjecture. According to Jensen (1973), each of these sperms makes contact with
membrane of the egg cell and the central cell (secondary nucleus). The
membranes at the point of contact dissolve and the nuclei are released to the
female cells the male nucleus is carried by cytoplasmic streams to the female
nucleus , there each of them fuses with their counterparts the triple fusion
takes place at a higher pace in comparison to the syngamy.
SYNGAMY-
The fusion of
the sperm with the egg nucleus is called syngamy. It is of three types
(G.Navashina)-
Premitotic- Sperm nucleus fuses with egg nucleus
as soon as it comes into contact with it. E.g. Poaceae
Postmitotic- Sperm and egg nuclei remain together
up to the time they enter in mitotic phase. E.g. Lillium.
Intermediate- The sperm nucleus fuses with egg nucleus
when the egg nucleus is completing its previous mitosis.
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