Fertilization: discharge and movement of sperms, syngamy and triple fusion, post fertilization changes in embryo sac
Fertilization:- Fertilisation is angiosperms, is unique. It is called double fertilisation. This is because there are two sperms per pollen tube. One of the sperms fertilises the egg cell to form zygote, while remaining sperm fertilises two polar nuclei, resulting in the formation of a triploid endosperm cell. The phenomenon of fertilization was first reported by Strasburger (1884) in Monotrapa. The male gametes are brought to the egg present in female gametophyte by a pollen tube. This phenomenon is called siphonogamy which was discovered by G. B. Amici in Protulaca plant.
Important events in fertilization are following:
1. Entry of pollen tube into ovule and embryo sac:-
a. After arriving in the ovary, the pollen tube finds its way through style and enters the ovule.
b. Depending on the place, the retry of pollen tube into the ovule, can be of following three type:
i. Porogamy:- It is the most common type. Here the pollen tube enters the ovule from the micropylar end. According to some workers, the porogamy is directed by filiform apparatus (of synergid).
ii. Chalazogamy (Basigamy):- In this case the pollen tube enters the ovule from chalazal end; however the entry into the embryo sac occurs through micropyle. Ex- Casuarina.
iii. Mesogamy:- In this case the pollen tube enters the ovule through the funiculus or integuments. However the final entry into the embryo sac occurs through micropyle. But the pollen tube with two mole gametes enters into the embryosac only through the micropyle.
Obturator:- It is a special structure which facilitates the entry of pollen tube into the ovule, by forming a sort of bridge. Soon alter fertilization the obturator shrinks and disappears.
2. Discharge of male gametes (sperms):-
a. The pollen tube contains two sperms (each is a haploid male gamete).
b. When pollen tube enters the embryo-sac (inside the ovule), it bursts to release its contents i.e., two sperms along with certain amount of protoplasm .The first male gamete discharge in one of the synergid.
3. Double fertilization:-
> Both the male garnet present in pollen tube utilises in fertilisation process of angiosperm is known as double fertilisation. It is the characteristic feature of angiosperms except Family Orchidaceae, Podostemaceae and Trapaceae.
> It was first observed by Nawaschin (1898) in Fritilaria and Lilium. It was supported by Guignard (1899).
> It involves two types of fusion –
a. Syngamy (fusion of egg cell and one male gamete):-
- One of the two sperms goes to fertilize the egg cell. This fusion is called syngamy.
- It results in the formation of zygote, which gives rise to proper embryo.
b. Triple fusion (fusion of remaining male garnet and two proper nuclei):-
- The remaining sperm now fuses with the two haploid polar nuclei (present in the centre of embryo sac).
- This fusion is called as triple fusion (as three nuclei i.e., one male garnet and 2 polar nuclei, are fused).
- It results in the formation of triploid endosperm nucleus, which on development (Repeatd mitosis) form the endosperm.
- Endosperm is therefore triploid in angiosperms (It is a characteristic feature of angiosperms)
- Endosperm serves to provide nutrition to the developing embryo.
Post-fertilization changes in the flower:- After the process of fertilization, the embryo undergoes a number of mitotic divisions to form a multicellular embryo. The endosperm nucleus also goes through a series of divisions to form a mass of endosperm cells. These endosperm cells provide nutrition to the developing embryo.
After fertilization, the following changes are observed in a flower:
i. There is the formation of a diploid zygote and it develops into an embryo, which forms the future plant.
ii. The endosperm cells serve as a source of nutrition for the developing embryo.
iii. The ovule becomes the seed.
iv. The ovary becomes the fruit.
v. In most plants, the antipodals and synergies disintegrate before, during, or immediately after fertilization.
vi. The outer and inner integuments of the ovule become the testa or the seed coat of the seed.
Petals and sepals fall off.
