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Read the below given notes to learn about the process of fertilisation in higher plants with the help of diagram.
Fertilisation is the process of the union of two dissimilar gametes, male and female. Pollen grains are carried to the stigma by pollination. It is a necessary preliminary to the more important process, fertilisation.
The pollen grains rest on the stigma of the carpels. Uninucleate pollen grains generally become bi-nucleate before liberation from the anthers. Of the two, the larger one is the tube nucleus and the smaller one is the generative nucleus.
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The stigmatic fluid stimulates germination of the pollen grain, as a result of which the exine is stretched and intine with the protoplasmic contents comes out through a weak spot, germ pore, to form a tube, called the pollen tube.
The tube nucleus is located at the tip of the pollen tube and is followed by the generative nucleus. Now the pollen tube elongates and pushes its way through the style ultimately reaching the ovary. During the growth of the pollen tube, the tube nucleus gradually disintegrates and the generative nucleus divides into two nuclei, called sperm nuclei. They are the male gametes (Fig. 103).
The pollen tube moves on through the micropyle to the nucellus and embryo sac when its tip swells and bursts liberating the two male gametes (Fig. 104).
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One of the synergids is usually crushed during the process. One male gamete fuses with the egg, i.e. female gamete, and that results in actual fertilisation. The fertilised egg soon secretes a wall around itself and forms the zygote, called oospore, which in course of time, develops into the embryo.
The second male gamete moves on to the definitive nucleus located at the centre of the embryo-sac and fuses with it, forming the endosperm nucleus. Though the second method is subsidiary to fertilisation it is nevertheless important, because the endosperm nucleus ultimately produces the nutritive tissue endosperm by method of free cell formation. This behaviour of the male gametes is called double fertilisation.
Fertilisation is a very important process which restores the original or diploid number of chromosomes. The pollen grains are formed from pollen-mother cells by meiosis or reduction division. Hence they have haploid or ‘n’ chromosomes.
Megaspores also have ‘n’ chromosomes. Only when the two gametes meet the number becomes ‘2n’ or diploid, and that is first noticed in the oospore. The endosperm has ‘3n’ chromosomes, as it is formed by ‘triple nuclear fusion’, since the endosperm nucleus is the product of fusion of 3’n’ nuclei—two already present in the definitive nucleus and the third coming from the male gamete.
Normally pollen tube reaches the embryo-sac through the micropyle, whatever may be the position of the ovule. This method is called porogamy. In some plants like Casuarina (B. Jhau), the pollen tube moves through the base piercing the chalaza. This is chalazogamy.
Changes in the Ovule due to Fertilisation:
The stimulus of fertilisation brings about a series of changes in the ovule. Oospore develops into embryo, endosperm nucleus into endosperm. Synergids and antipodal cells generally disappear. Nucellus is usually exhausted during the formation of embryo.
In some seeds like castor, four O’clock remnants of nucellus persists even in the seed. This is called perisperm. Endosperm is also used up in many seeds, so that the kernel corresponds to the embryo; they are exalbuminous or non-endospermic seeds. In some seeds, however, endosperm remains present.
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They are albuminous or endospermic seeds. The two integuments develop into two seed coats, testa and tegmen. Often outgrowths like caruncle are formed on the testa. The simple soluble food matters of the ovule are converted into their complex insoluble forms in the seeds. The whole ovule is transformed into seed. Calyx, corolla, etc., usually wither and fall off and the ovary usually develops into the fruit.
