‘The process of Fertilization in Plants’, if you have any doubt related to this topic, that is going to be clear in this article.
But, before knowing more about Fertilization, you should know ‘what is fertilization in plant ?’ first.
So let get start…
What is Fertilization ?
Successful pollination is followed by fertilization.
Fertilization is the fusion of the nuclei of male and female gametes leading to the formation of zygote or oospore.
According to Wikipedia, Fertilization is the fusion of gametes to give rise to a new individual organism or offspring and initiate its development.
Processes such as insemination or pollination which happen before the fusion of gametes are also sometimes informally called fertilization.
It was all about the definition of fertilization, lets discuss about the process of fertilization in plants.
Process of Fertilization in Plants
Structure of pollen grain
The pollen grain is a microscopic and unicellular structure. It is covered by two layers. The outer layer is very hard and thick called exine. The inner layer called intine is thin walled.
The exine at certain places has pores called germ pores through which pollen tube emerges out during its germination. The pollen grain has two nuclei called tube nucleus and generative nucleus.
Structure of ovule
The ovule is a small structure present in the ovary. It is attached to the placenta by a stalk called funicle. The funicle provides nourishment to the ovule.
The ovule is covered by two protective layers called integuments. It bears a small opening called micropyle at one end. The ovule contains a structure called embryo sac.
In the embryo sac the following structures are present :
- A female gamete called egg towards the micropyle.
- Two polar nuclei which fuse to form the secondary nucleus in the centre.
- Two synergids
- Three antipodal cells.
The embryo sac is surrounded by a nutritive tissue called the nucellus.
How the process of Fertilization takes place ?
During fertilization a number of events take place which are summarized as follows :
- After the pollen grains are deposited on the stigma, they absorb nutrients (water and sugar) secreted by the stigma and swell up Growth of pollen tube is stimulated.
- From one of the germ pores in exine, the intine of pollen grain grows out into a fine tube called pollen tube.
- The pollen tube grows and passes through the solid style. The tissues of the style are dissolved with the help of enzymes, secreted at the tip of pollen tube. Eventually, the pollen tube travels down the whole length of the style into the ovary.
- Meanwhile, the generative nucleus divides by mitosis into two male gametes or nuclei. The male gametes move to the tip of the pollen tube and lie in the mass of cytoplasm. The tube nucleus degenerates. The pollen grain with a pollen tube containing two male gametes constitutes the mature male gametophyte.
- The pollen tube carrying the two male gametes now turns towards the micropyle and enters the embryo sac. Now tip of the pollen tube dissolves.
- The two male gametes are released into the embryo sac.
- One of the two male gametes fuses with the egg cell and forms the fertilized egg or zygote.
- The second male gamete fuses with the secondary nucleus (definitive nucleus). It forms the endosperm nucleus. The secondary nucleus is formed by the fusion of polar nuclei.
- The zygote develops into an embryo. Whereas, the endosperm nucleus develops into endosperm. The endosperm provides nourishment to the developing embryo.
Double fertilization and triple fusion
In angiosperms, the two different types of nuclei in the embryo sac (egg cell and secondary nucleus) are fertilized by two male gametes released by the pollen tube in the embryo sac. This is called double fertilization.
The fusion of second male gamete with the two polar nuclei is often referred to as triple fusion.
Double fertilization Egg cell (n) + Male gamete (n) =Zygote (2n)
Secondary nucleus (2n) + Male gamete (n) = Endosperm (3n) (Triple fusion)
Synergids do not seem to be essential for the process of fertilization. They get degenerated after fertilization or even before it.
In case the egg cell fails to be fertilized due to any reason, the synergids, also called help cells, assume the role of egg cell. Antipodal cells also do not have any positive role to play and may disappear even before fertilization.
Post-fertilization changes in the ovule
After fertilization, a large number of changes take place in the ovule which can be summarized as follows :
- The egg cell is surrounded by a cell wall and is called oospore.
- The oospore develops into an embryo.
- The definitive nucleus, now called endosperm nucleus, develops into endosperm.
- The synergids and antipodal cells disappear.
- The two integuments develop into the seed coats – the outer testa and inner tegmen.
- Complex and insoluble food is formed from the simple and soluble food. It is stored for the future use in cotyledons or endosperm.
- The ovule is converted into seed and the ovary matures into fruit.
- In some cultivated varieties of banana, papaya, grape, orange, etc., the ovary may develop into the fruit without fertilization. The development of fruit from ovary without fertilization is called parthenocarpy. The parthenocarpic fruits never contain seeds.
- The non-essential whorls of flower i.e. calyx and corolla fall off. The calyx persists in some cases e.g. brinjal, tomato, etc.
After the process of fertilization, the whole ovary changes into a fruit. The fruit is defined as a ripened or matured ovary.
The wall of ovary may harden or may become fleshy and succulent fruit wall called pericarp. The fruit contains one to many seeds.
Biological significance of fruits
The formation of fruits has its own biological significance for the plants, such as:
- Fruits protect the immature seeds from unfavourable climatic conditions. The seeds remain enclosed in the fruit until they are ready for germination.
- When mature, the fruit is brightly coloured to attract animals that help in the dispersal of seeds.
- Immature fruits contain unpalatable and repelling substances, such as tannin, bitter alkaloid, sour acids etc. They provide chemical defence against the animals.
Economic importance of fruits
- Many fruits form a part of our food. Moreover, Many birds and other animals eat fruits as their food.
- Staple food of human beings consists of cereals which are seeded dry fruits.
- A large number of fruits, such as lady’s-finger tomato, egg plant, pumpkin, cucumber, etc., ar used as vegetables.
- Fruits are a good source of vitamins, minerals organic acids, pectin and sugars.
- Some fruits, such as the unripe fruits of Papaver are used as medicines.
The ovule develops rapidly after fertilization, using the food reserves. The mature ovule after fertilization is known as seed. The seed is covered from outside by hard and protective seed coat.
The seed coat develops from the integuments of ovule. The seeds contain a potential plant or embryo. The embryo consists of tiny root and a small shoot.
There are one or two food laden cotyledons that provide nourishment to the developing embryo.
Seeds are very significant in the life of flowering plants, such as,
- Seeds help in the development of new flowering plants.
- They help in the dispersal of fixed plants.
- They help in overcoming unfavourable conditions.
Question & Answer related to Fertilization
Q. What is Fertilization ?
Ans. Fusion of male and female gametes is called fertilization.
Q. How Pollen Tubes are produced ?
Ans. Pollen grains after their deposition on stigma, produce pollen tube that extends up to embryo sac. Two functional male gametes are released in the embryo sac
Q. What is Double Fertilization ?
Ans. One male gamete fertilizes egg cell and the other fertilizes the definitive or secondary nucleus. This is called double fertilization and it is characteristic of angiosperms.
Q. What is Triple Fusion ?
Ans. Fusion of two polar nuclei with one male gamete is called triple fusion.
Q. What happens to Ovary and Ovule after Fertilization ?
Ans. After fertilization, the ovule develops into seed and the ovary matures into fruit.
Q. What is called Parthenocarpy ?
Ans. In some cultivated varieties (such as banana, papaya etc.) the ovary may develop into fruit without fertilization. It is called parthenocarpy. Parthenocarpic fruits do not contain seeds.
Title : Describe the process of Fertilization in Plants with Diagram.
In this article, we have mainly discuss about the process of fertilization in plants. the following topics are covered briefly in this article.
- What is Fertilization ?
- Process of Fertilization in Plants.
- How the process of Fertilization take place ?