|The purpose of sexual reproduction in nature is to combine
the genetic material from different individuals. In order to carry out
sexual reproduction, an organism must produce gametes. Gametes are haploid
cells that are produced through meiosis. In animals, the diploid cells
of the body produce haploid gametes (eggs and sperms) through meiosis.
Then an individual sperm and an individual egg combine through fertilization
to produce a diploid zygote that will develop into a new and unique individual.
In plants however, the process is a bit more complex .
Flowers contain the sexual organs of plants, most plants have organs to produce both the male and the female gametes inside the flower, whereas most animals produce only male or only female gametes.
First, let's examine the formation of the female gametes. Gametes are formed inside the ovary (found at the base of the flower) in structures called ovules. An ovary may have one to many hundred ovules. The ovary will become the fruit that you eat, and each ovule will become the seed inside the fruit. Like in animals, sexual reproduction begins with meiosis where a single diploid cell divides to produce 4 haploid cells. Only one of these cells continues to develop and the other 3 haploid cells disintegrate. The one surviving haploid cell undergoes 3 cycles of mitosis but no cell division to give rise to one very large cell with 8 haploid nuclei. Together these 8 haploid nuclei form a structure called an embryo sac. Each of these nuclei has its own role in fertilization and moves to its place inside the embryo sac to do its job. Most importantly one of the nuclei is the egg and another two found in the center of the embryo sac are called the polar nuclei.
Meanwhile in a part of the flower called the anther, the male gametes are being produced. Individual gametes are carried inside of small particles called pollen grains. Like female gamete formation, the process begins when a diploid cell of the anther undergoes one cycle of meiosis to produce 4 haploid cells. However, unlike female gamete formation, all 4 of the haploid cells survive. Each of these cells begins to form a thick cell wall that will become the outside of the pollen grain. Inside the thick cell wall, the nucleus undergoes one cycle of mitosis to produce two nuclei, but the nuclei do not separate into daughter cells. The nuclei are called the vegetative nucleus and the generative nucleus. The generative nucleus will undergo one more cycle of mitosis without daughter cell formation to give a total of three nuclei in one cell.
The pollen grains with their three nuclei are released from the anther and are carried by wind or by an insect or bird to the stigma. The stigma is part of the female sexual organ and has a sticky surface that can catch and hold pollen grains. The pollen that lands on the stigma can be from a different or the same from flower.
When a pollen grain lands on the stigma, it will begin to grow a pollen tube down to the ovule, which is found inside the ovary. The two generative nuclei will travel down the pollen tube to the ovule. Once inside the ovule, one of the generative nuclei fertilizes one of the egg nucleus in the embryo sac. This fertilization is very similar to fertilization in animals. In most seed plants however, there is a special phenomenon called double-fertilization in which the second generative nucleus combines with the 2 polar nuclei of the embryo sac to form a triploid nucleus.
The diploid zygote will continually divide by mitosis to grow and form the plant embryo, and the triploid cell will also divide by mitosis to form what is called the endosperm. The endosperm of the seed serves as a food storage site to provide nourishment to the growing embryo. Additionally, the endosperm comprises the majority of the seed and the bulk of food for humankind. Together the embryo and endosperm will grow and eventually the seed is released from the plant. Given the correct conditions, this seed will use the energy stored in the endosperm to germinate and grow into what will become a new and unique plant.