Fertilization is the process of fusion of the male and female gametes to develop a new individual. In plants, fertilization is after pollination of the carpel, there is germination of the pollen grain and a pollen tube grows and travels to the ovary. The pollen grain adheres to the stigma, a pollen tube grows and penetrates the ovule and the pollen tube burst into the embryo sac.
- The pollen grain germinates after the carpel is pollinated.
- From the germinated pollen grain, a pollen tube emerges and grows.
- It travels and grows and moves towards the ovary by creating a path through the female tissue.
- Two types of nucleus, the vegetative tube and generative nuclei of the pollen grain pass into the pollen tube.
- Stigma secretes a sugary substance that stimulates the growth of the pollen tube.
- The pollen contains the vegetative and the generative nucleus and the cell ruptures the stigma and passes through the style.
- The pollen grains attaches itself to the stigma of the female reproductive structure, the pollen tube grows and enters the ovule making a tiny pore called a micropyle.
- The pollen tube does not reach the ovary in a straight line. The pollen tube grows near the style and curls to the bottom of the ovary and then near the receptacle.
- The pollen tube then breaks into the ovule through the micropyle and then the micropyle bursts into the embryo sac.
- In the embryo sac, on the male nucleus fuses with the nucleus of the egg and forms a diploid zygote. This process is known as true fertilization or syngamy.
- The other male gamete or nucleus enters further into the embryo sac and it fuses with secondary nucleus. This gives rise to a triploid nucleus called the primary endosperm nucleus. This process of nuclear fission, where there is formation of primary endosperm nucleus is called triple fission.
- After the process of fertilization the ovary swells up and develops into a fruit. In case of fruits with multiple seeds, multiple pollen grains are necessary to fertilize with each ovule.
- The pollen tube growth is controlled vegetative cytoplasm. To digest the female tissue, the pollen tube produces hydrolytic enzymes as the tube moves down towards the stigma and style. The tissue digestive by the hydrolytic enzymes acts as a nutrient source for the pollen tube.
Depending upon the place of entry of the pollen tube into the ovule, three types of fertilization have been recognized. They are Porogamy, Chalazogamy and Mesogamy.
Porogamy - It the most common type of fertilization seen in angiosperm plants. In porogamy, the entry of the pollen tube into the ovule takes place through the micropyle.
Chalazogamy - In chalazogmay, the pollem tube enters thorugh the chalaza. It can be seen in Casuarina plants.
Mesogamy - Mesogamy is seen in Cucurbita plants. Here, the pollen tube enters through the integuments of the ovule.
Double fertilization is complex mechanism of fertilization in flowering plants or angiosperms. Double fertilization is joining of a female gametophyte with two male gametes. One sperm nucleus fertilizes the egg cell and the other sperm combines with the two polar nuclei of the megagametophyte. The haploid sperm and haploid egg combine to form a diploid zygote. The other sperm nucleus fuse with the other two haploid polar nuclei of the megagametophyte to form a triploid nucleus, this develops into a endosperm and is called the primary endosperm nucleus. This entire phenomenon of fertilization that involves the fusion of the egg and one male gamete and the fusion of the other male gamete with the secondary or the polar nuclei is called double fertilization.
In gymnosperms, the gametophyte stage is short-lived. The male gametes are microgametophytes and develop from microspores producing sperm cells. Megagametophytes or the female gametes develop from the megaspores and are present in the ovule. The female gametophyte produces multiple archegonia.
Pollen grains are transferred between plants from pollen cone to the ovule through pollinators like wind or insects. Pollen grains enter into ovule through micropyle. The pollen grains mature inside the female gametophyte and produce sperm cells.
Two modes of fertilization is seen in gymnosperms. In plants like Cycads and Ginkgo, sperms are motile and they swim directly into the egg inside the ovule. In plants like conifers and gnetophytes, the sperms have no flagella and they are passed on to the egg through the pollen tube. After fertilization, the resulting embryo develops in the female gametophyte and the ovule forms into a seed and a seed coat. This will become are new sporophyte which consists of two embryonic leaves.
In angiosperms, the female reproductive organ is the pistil and is present in middle of the flower. The male gametophyte is the pollen grains as in gymnosperms. Pollinators like insects other animals help in fertilization of flowering plants.
Double fertilization takes place in angiosperms. The pollen tube penetrating into the ovule releases two sperm cells. One sperm cell fuses with the egg to form a diploid zygote. The other sperm cells fuses two polar nuclei to form a triploid nucleus. The triploid nucleus forms the endosperm and nourishes the developing embryo. The ovary containing the ovules develops into a fruit after fertilization.
After the process of fertilization, the embryo undergoes a number of mitotic divisions to form multicellular embryo. The endopserm 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:
- There is formation of a diploid zygote and it develops into an embryo, which forms the future plant.
- The endosperm cells serve as a source of nutrition for the developing embryo.
- The ovule becomes the seed.
- The ovary becomes the fruit.
- In most of the plants the antipodals and synnergids disintegrate before, during or immediately after fertilization.
- The outer and inner integuments of the ovule become the testa or the seed coat of the seed.
- Petals and sepals fall off.
Based on development, the endosperm is of 3 types nuclear endosperm, cellular endopserm and helobial endosperm.
Nuclear type is the most common type of endopserm development. In this type of development the nucleus of the endoperm undergoes repeated divisions. Each nuclei formed is arranged towards the periphery, enclosing a large central vacuole. Cytokinesis begins from the periphery towards the center making it cellular at maturity.
Example: Wheat, maize, rice, sunflower.
In the cellular type of endosperm formation, every nuclear division is followed by cytokinesis, making it cellular from the beginning.
In the helobial type of endosperm formation, the first mitotic division is followed by the process of cytokinesis resulting in the formation of two unequal cells. Subsequent divisions are of the nuclear free type making the endosperm cellular after cytokinesis or at maturity.
The fertilized zygote undergoes a regular sequence of divisions to form the embryo. The oospore divides to form a suspensor or basal cell and a terminal embryo cell. The basal is present towards the micropyle and the terminal cells towards the chalaza. The basal cells divide repeatedly to produce a row of 4 to 8 cells. These cells constitute the suspensor. The proembryo is formed as the terminal cell divides in various planes to form a cluster of cells. The suspensor pushes the proembryo into the endosperm, to enable the developing embryo to obtain nourishment. The embryo and the endosperm mature, the integuments of the ovule becomes hard to form the seed coat, which protects the seed.