Pollination, fertilization and fruit formation processes in flowering plants
The acts of pollination and fertilization processes lead to fruit formation in flowering plants. This article gives an account of pollination, pre-fertilization changes in pollen grains and ovaries, post-fertilization changes in flowers to transform themselves into fruits. Some frequently asked questions were also presented along with this article.
Introduction
Flower is the sexual reproductive organ of a plant. Flowers bear the stamens, the male reproductive organs, and ovaries, the female reproductive organs in the same flower, or separately in different flowers. The fertile part of the stamen is the anther and it produces pollen grains. The pollen grain is a structure that produces male gametes (male sex cells). Similarly within the ovary ovules are present, and each ovule produces one egg cell or female sex cell. Fertilization occurs when a male and a female gamete cell fuse together within the ovary. The fertilization process in the ovary leads to various changes in flower and that ultimately leads to the formation of fruit.Structure of Pollen grain and its development
Pollen grains are the male reproductive structures formed inside the anthers of stamens. Each pollen grain can develop into a male gametophyte. In Angiospermic plants, the male gametophyte is a characteristic 3-celled structure.
Structure and development of Pollen grain (Diagram by the Author)
Each pollen grain usually measures around 10 to 200 microns. Each pollen grain is protected by a two-layered covering around it. The outer covering layer of the pollen grain is known as exine. The exine of the pollen grain is very tough, cutinized, and possess spiny ornamentation or reticulations. The exine also includes delicate thin specific slits or pores on it called germ pores. Next to exine, a smooth thin cellulose layered intine is present. During germination of pollen grain on stigma, the thin intine layer projects out in the form of a pollen tube through the germ pores on exine.
Each pollen grain at the beginning contains one nucleus inside it. During maturation of the pollen grain, this single nucleus divides into two unequal nuclei. The larger nucleus of it is known as vegetative nucleus or tube nucleus and the smaller one is known as generative nucleus. The generative nucleus then divides into two bits and these two bits transform into two male gametes. Thus the male gametophyte is a 3-celled reduced structure in Angiosperms is a distinct characteristic feature of that group. During germination of pollen grain on the stigma of the female reproductive part, a pollen tube grows out through the germ pore of the pollen grain. The tube nucleus is located towards the apex of the pollen tube and it guides the pollen tube to reach the embryo sac for the fertilization of the egg. As the pollen tube enters the embryo sac through the micropyle, the tube nucleus disintegrates and it has no role in fertilization.Structure ovary and female gametophyte at the time of fertilization
Ovary, style, and stigma are the female reproductive parts of the flowers. All these parts aid in the process of fertilization. Stigma is the receptive part of pollen grains and thus aids in the pollination process. The stylar canal of style allows the pollen tube from germinating pollen grain to travel and reach the embryo sac to complete the act of fertilization. The ovary is the part where the ovule is located and in which the egg develops. 
Germination of the pollen grain and fertilization (Diagram by the Author)Structure of ovule
Ovule is present in the ovary attached to it by a connection called the placenta. The ovule is surrounded by two protective coverings called integuments. These integuments leave a small opening at one end called micropyle. Inside the ovule, a large sac-like structure present called embryo sac. The embryo sac is surrounded by a nutritive tissue around it called nucellus. The end of the embryo sac facing towards the micropyle is known as a micropylar end and the opposite end of it is known as the chalazal end. In the embryo sac, a total of 8 cells present at the maturity of the female gametophyte. An 8 celled reduced female gametophyte in Angiosperms is considered to be one of the highly advanced features of this group of plants. Out of the 8 cells in the embryo sac, 3 cells are situated at the micropylar end of the embryo sac. At the micropylar end, one enlarged egg cell surrounded by two synergid cells are present. The 3 cells together at the micropylar end are known as egg apparatus. The egg cell involves the fertilization process while the synergids facilitate easy entry of the pollen tube and also help to protect the egg cell. The three cells that are present at the chalazal end are known as antipodals. These cells have no role in the process of fertilization and sooner or later they get disintegrated. In the middle of the embryo sac, two polar nuclei are present and they fuse together to form a secondary nucleus just before the time of fertilization making the gametophyte to be a 7 celled structure.Pollination and fertilization
Pollination is the transfer of pollen grains from the anther to the stigma of a flower. This pollination occurs with the aid of various agents like insects, wind, water, animals, etc. This first step is essential for the second step of fertilization to occur. The process of fertilization in flowering plants was first studied by Strasburger in 1884.
Once the pollen grains get deposited on the stigma, the pollen grains start germinating by absorbing moisture and sugary secretions of the stigma. Pollen grains can germinate only if they are deposited on the stigmas of flowers of the same species only or otherwise they get disintegrated. During germination of the pollen grain, the intine layer of pollen grain comes out as a pollen tube carrying a tube nucleus at it its tip and two sperm nuclei within it. This pollen tube travels down through the stylar canal and reaches the embryo sac. The pollen tube after reaching the embryo sac enters into the embryo sac by piercing through the micropyle. Such an entry of pollen tube into the embryo sac through micropyle is known as porogamy. In some cases, the pollen tube may even enter through the chalazal end and such a condition are known as chalazogamy.
Once the pollen tube makes an entry into the embryo sac, the tip of the pollen tube bursts and releases the two sperm nuclei into the embryo sac. Once the sperm nuclei are released by bursting the tip of the pollen tube, the tube nucleus degenerate. One of the sperm nuclei of the two now fuses with the egg nucleus resulting in a zygote cell or Oospore with diploid ( 2 N) condition. This first step of fusion of one sperm nucleus with an egg nucleus is known as the first fertilization. Now the second sperm nucleus fuses with the secondary nucleus (formed by the fusion of two polar nuclei) resulting in Primary Endosperm Nucleus (PEN). This fertilization is known as second fertilization and the resulting nucleus is known as triploid (3 N). The second fertilization involves the fusion of three nuclei (one sperm nucleus and two polar nuclei) is also known as triple fusion. As two fertilizations are occurring inside the embryo sac, the type of fertilization is known as double fertilization.Double fertilization is a characteristic feature of all Angiosperm plants.Post-fertilization changes in ovule
Most frequently asked questions:-
Ans. The two fertilizations that occur in the ovaries of flowering plants of Angiosperms are known as double fertilization. The fusion of one sperm nucleus with the egg nucleus and the fusion of another sperm nucleus with two polar nuclei together known as double fertilization.
Ans. The fusion of the second sperm nucleus with two polar nuclei to form the Primary Endosperm Nucleus (PEN) is known as triple fusion and since three nuclei are fused in this process it is known as triple fusion.
Ans. The fusion of the nucleus of the male gamete with the nucleus of the female gamete is known as fertilization.
Ans. The generative nucleus of the pollen grain divides into two bits and transform into two sperm nuclei. The germ pore present on the exine layer of pollen grain will allow the pollen tube to come out.
Ans. The nutritive tissue present around the embryo sac of the ovule is known nucellus.
Ans. The zygote resulted in the fertilization process will transform into an embryo by mitotic cell divisions. The developed embryo include radicle and plumule and in the future, it develops into a young baby plant.
Ans. The combined nucleus formed by the fusion of two polar nuclei present at the center of the embryo sac is known as secondary nucleus.
Ans. After fertilization, the ovule changes into seed, ovary into fruit, stamens fall off, and calyx may fall off or persist.
Ans. The tube nucleus directs the pollen tube to grow towards the micropyle of the embryo sac.
Ans. Stigma of the flower that receives pollen grains.