Although people plant flowers for their fragrance and beauty, flowers serve a reproductive function for plants, producing male and female sex cells and attracting pollinators, such as honey bees, that pass pollen from plant to plant. Usually located inside of flowers, they produce seeds, each of which contains an embryonic plant. The structure and function of flowers all share the aim of producing that seed and, through it, ensuring the survival of the species.
All parts of the flower contribute to the production of seeds, some more directly than others. The male parts of the flower are the stamens, and these produce male sex cells, or pollen, needed to fertilize the seed. The female parts of the flower include the pistil, a visible structure on which pollen is collected, and the ovary, a structure inside of the flower that contains the female sex cells and where seeds are actually produced. The brightly colored petals of the flower attract insects and birds that feed on the flower's nectar and pollen and aid in delivering pollen between different plants.
The ovary is a swelling at the base of the pistil, the female structure that is generally visible as a stalk protruding from the center of the flower. Sometimes the ovary is visible but, more often, it is inside of the flower. Inside of the ovary are one to hundreds of further subdivisions called ovules. Each ovule has the potential to develop into a seed. The ovules are attached to the wall of the ovary in an area called the placenta.
Ovules are the structures that develop into seeds. Each ovule is enclosed inside two sacs called integuments. As the seed matures, the integuments will harden and produce the tough seed coat that protects the tender embryonic plant inside from harm. Inside of the integument is the megasporangium. This is the part of the seed that is actually fertilized, and it contains an egg cell. Part of it will eventually develop into the embryonic seedling while the other part develops into endosperm, a substance inside of the seed that nourishes the embryo.
Seeds are not produced until the ovules are fertilized. When pollen lands on the pistil, it forms a filament called a pollen tube that grows down into the flower, where it enters the ovule. The tip of the tube bursts, releasing sperm that fertilize the female egg cells inside of the ovule.
Flowering plants demonstrate a process called double fertilization that provides an advantage to the seeds. While one sperm cell fuses with the egg to produce the embryo, another sperm cell joins with other cells inside of the ovule, producing the nutrient-rich endosperm that will feed the embryonic plant inside the seed.
Seed Distribution and Fruit
Once the seeds are mature, they need to find their way to soil, where they can germinate and grow into mature plants. Seeds themselves make this happen. Some, like dandelions, develop structures that allow them to be carried by the wind. Others are eaten by animals and deposited in the animals droppings. However, some plants provide even further help to the young seeds by developing fruit. When the ovules are fertilized, the ovary that contains them swells, developing a pericarp, or fruit coating. This is often sweet and edible, encouraging animals to ingest the fruit and, therefore, the seeds, distributing them more widely than if the plant had to depend on mechanical means of dispersal.