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Diagram of fertilization Related: NCERT Textbook Chapter 12 Reproduct...
Fertilization, union of a spermatozoal nucleus, of paternal origin, with an egg nucleus, of maternal origin, to form the primary nucleus of an embryo. In all organisms the essence of fertilization is, in fact, the fusion of the hereditary material of two different sex cells, or gametes, each of which carries half the number of chromosomes typical of the species. The most primitive form of fertilization, found in microorganisms and protozoans, consists of an exchange of genetic material between two cells.

The first significant event in fertilization is the fusion of the membranes of the two gametes, resulting in the formation of a channel that allows the passage of material from one cell to the other. Fertilization in advanced plants is preceded by pollination, during which pollen is transferred to, and establishes contact with, the female gamete or macrospore. Fusion in advanced animals is usually followed by penetration of the egg by a single spermatozoon. The result of fertilization is a cell (zygote) capable of undergoing cell division to form a new individual.

The fusion of two gametes initiates several reactions in the egg. One of these causes a change in the egg membrane(s), so that the attachment of and penetration by more than one spermatozoon cannot occur. In species in which more than one spermatozoon normally enters an egg (polyspermy), only one spermatozoal nucleus actually merges with the egg nucleus. The most important result of fertilization is egg activation, which allows the egg to undergo cell division. Activation, however, does not necessarily require the intervention of a spermatozoon; during parthenogenesis, in which fertilization does not occur, activation of an egg may be accomplished through the intervention of physical and chemical agents. Invertebrates such as aphids, bees, and rotifers normally reproduce by parthenogenesis.

In plants, certain chemicals produced by the egg may attract spermatozoa. In animals, with the possible exception of some cnidarians (coelenterates), it appears likely that contact between eggs and spermatozoa depends on random collisions. On the other hand, the gelatinous coats that surround the eggs of many animals exert a trapping action on spermatozoa, thus increasing the chances for successful sperm-egg interaction.

The eggs of marine invertebrates, especially echinoderms, are classical objects for the study of fertilization. These transparent eggs are valuable for studies observing living cells and for biochemical and molecular investigations because the time of fertilization can be accurately fixed, the development of many eggs occurs at about the same rate under suitable conditions, and large quantities of the eggs are obtainable. The eggs of some teleosts and amphibians also have been used with favourable results.

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Diagram of fertilization Related: NCERT Textbook Chapter 12 Reproduct...
Fertilization in Plants

Fertilization is the process in which male and female gametes fuse to form a zygote, which develops into a new individual. In plants, fertilization is a crucial step in the reproductive process and involves the fusion of the male gamete or pollen with the female gamete or ovule. Let's understand the process of fertilization in plants in detail.

1. Pollination:
Pollination is the transfer of pollen grains from the anther to the stigma of the same or a different flower. It can occur through various means such as wind, water, insects, birds, or other animals. Once the pollen grain reaches the stigma, it undergoes germination and develops a pollen tube.

2. Germination of Pollen Grain:
After landing on the stigma, the pollen grain absorbs moisture and germinates. The pollen tube starts growing out of the pollen grain towards the ovary. The tube nucleus and generative nucleus are present inside the pollen tube.

3. Growth of Pollen Tube:
The pollen tube grows through the style and reaches the ovary. It provides a pathway for the male gametes to reach the female gametes present in the ovule. The generative nucleus divides to form two male gametes.

4. Double Fertilization:
When the pollen tube reaches the ovule, it enters through the micropyle and releases the two male gametes. One male gamete fuses with the egg cell present in the ovule and forms a zygote. This process is called syngamy and results in the formation of a diploid zygote.

Simultaneously, the other male gamete fuses with the two polar nuclei present in the ovule's central cell. This fusion results in the formation of a triploid primary endosperm nucleus. This process is known as triple fusion.

5. Zygote and Embryo Formation:
The zygote develops into an embryo, which is the young plant in the early stage of development. The primary endosperm nucleus divides repeatedly to form the endosperm, which provides nourishment to the developing embryo.

6. Seed Formation:
The ovule matures into a seed, which contains the embryo and the endosperm. The ovary develops into a fruit, protecting the seed and aiding in its dispersal.

Conclusion:
Fertilization in plants is a complex process involving pollination, pollen tube growth, double fertilization, embryo formation, and seed development. This process ensures the continuation of the plant species and the production of new individuals.
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Diagram of fertilization Related: NCERT Textbook Chapter 12 Reproduct...
The fusion of male and female gamete is called fertilization
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Diagram of fertilization Related: NCERT Textbook Chapter 12 Reproduction in Plants , Class 7, Science
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