Asexual reproduction is only part of a plant’s life cycle. The complete reproductive life cycle of a plant alternates between the diploid sporophyte phase and the haploid gametophyte phase. Diploid cells contain two copies of each chromosome, whereas haploid cells contain only one copy of each chromosome.

In vascular plants, the diploid sporophyte phase is the prevailing phase. In other plants, such as mosses, the haploid gametophyte phase is the major phase. The fact that plants have a multicellular haploid phase after meiosis and before the production of gametes makes their life cycle different from the life cycle of animals.

Sexual reproduction varies among plant groups. In flowering plants, the angiosperms, the diploid sporophyte produces haploid spores by meiosis. Meiosis marks the transition from the diploid sporophyte generation to the haploid gametophyte generation. The haploid spores divide by mitosis to form male and female gametophytes that then produce the egg and sperm cells.

Fertilization occurs when an egg and a sperm cell fuse to form a diploid zygote. Fertilization returns the haploid gametophyte generation to the diploid sporophyte generation. The zygote develops into a multicellular seed that contains a diploid sporophyte embryo. The seed is enclosed within an ovary, which matures into a fruit. The seed germinates and a new sporophyte plant develops. The sporophyte plant matures, forms flowers, and the process repeats.

The flower is the reproductive structure of angiosperms. The male reproductive organ is the stamen. Each stamen is made up of the anther and the filament. The anther contains the cells that develop into the male gametophytes, or pollen. Pollen is formed when cells within the anther undergo meiosis to form haploid microspores. Each microspore divides by mitosis to form two cell types: a generative cell and a tube cell. A wall forms around these two cells, completing the pollen grain. Sperm cells form from the generative cell.

The female reproductive organ is the carpel. Some flowers have a single carpel; others have many fused together, called the pistil. The carpel is made up of the stigma, which is joined to the ovary by the style. The sticky stigma receives pollen for fertilization. The ovary produces the female reproductive cells, the eggs. The ovary contains one or more ovules, in which the female gametophytes develop. Cells within the ovule undergo meiosis to form four haploid megaspores, only one of which typically survives. The surviving megaspore undergoes three rounds of mitosis followed by cell division to form a multicellular embryo sac with eight nuclei—the female gametophyte. Six cells in the embryo sac have a single nucleus and one cell has two nuclei. At one end, there are three antipodal cells and at the other end are three cells, two called synergids and the egg. The cell with two nuclei is called the mother cell. After fertilization, the ovary develops into a fruit and the ovules develop into seeds. The seeds contain the new plant embryos.

Pollen is released from the plant and finds its way to the stigma. Insects, birds, and the wind facilitate this process, called pollination. Plants have evolved colorful petals and scents to attract the animals that pollinate them, and sugary nectar to reward them. Animals have evolved specialized structures that allow them to extract the precious nectar from flowers. The pollination of plants by animals is an example of coevolution.

When the male and female reproductive structures are found on the same plant, the plant species is said to be monoecious. Examples of monoecious species are corn plants, apple trees, and rose bushes. When the male and female reproductive structures are found on different plants, the species is said to be dioecious. Examples of dioecious plants include date palms, persimmon trees, and holly plants.

If you have a date palm in your backyard, would you expect it to produce lots of dates? Select your answer by clicking in the box.

Yes, that’s correct.

No, that’s incorrect.

The correct answer is it depends. Because the date palm is dioecious, the male and female structures are found on separate trees. Production of dates depends on the palm being female, because it’s the female’s ovary that develops into fruit. However, a male tree must be nearby to provide pollen for fertilization–the event that leads to the development of fruit. The evolution of separate male and female plants is an example of an adaptation that prevents self-pollination. Self-pollination is the transfer of pollen from the anther to a stigma on the same plant. Plants have evolved many different strategies to prevent self-pollination to preserve the genetic diversity associated with sexual reproduction.

When a pollen grain lands on the stigma of a compatible plant, it germinates and forms a tubular structure that leads to the ovule. The generative cell divides, forming two sperm nuclei. A double fertilization then takes place. One sperm nucleus fuses with an egg nucleus to form a diploid zygote; the other sperm nucleus fuses with the nuclei of the mother cell to form the endosperm, a nutritive tissue.

Now that we’ve seen how fertilization occurs, lets see how the zygote develops into a new plant.

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