Like almost all complex life, plants are usually male or female. Flowers are the method most plants use for reproduction. The process of how this happens will vary, depending on the plant and its characteristics. Like the animal kingdom, plants have a male sex cell and a female sex cell. Fertilization is the process of bringing these two cells together.
The structure of different flowers have evolved over time to attract birds and insects that will help with fertilization. Although some flowers can also be fertilized via wind, bird and insect fertilization is more likely to be successful. The petals of a flower attract the appropriate pollinators. Some flowers produce nectar. The scent of this nectar attracts specific insects or birds. The male organs of a flower are called stamens. The female organs of a flower are called pistils. Fertilization occurs when pollen, the male reproductive cell, is deposited on the pistil. The pistil contains the ovule that will, when fertilized, become a fruit or a seed.
Flowers can be male, female, or can have both male and female structures. If a flower is male only, it will not have a stamen. Female flowers of these types of species generally don't have pistils. If a plant has sex specific flowers, the entire plant is often sex specific. Some flowers can have both male and female structures.
Flowers reproduce via pollination. The pollen produced on the stamen contains half of a the genetic material needed to produce a particular species of plant. The ovum contains the other half. The process of pollination and fertilization combines these two partial strands of genetic material into a complete strand. Once fertilized, a flower will begin to change into a seed. That seed has the potential to eventually grow into a new plant of the same species as the parent plants.
Fertilization vectors for plants can vary, depending on the plant. Some flowers attract birds, like humming birds or other small birds. Other flowers attract bees or other insects. The pollen sticks to the legs, feet, or bodies of these insects and birds as they take nectar from male flowers. The pollen falls off as the birds or insects move to take nectar from female flowers. In some cases, especially with flowers that contain both male and female structures, wind can cause pollen to fall from the stamen into the pistil. However, insects and birds can also play an important role in pollinating these types of flowers.
Although many flowers can be fertilized by flowers from the same species, some female flowers can only be pollinated by flowers from slightly different species. For example, some apples and cherries require pollen from different cultivars to produce fruit. Most sweet cherries are self-unfruitful, and require pollen from a different variety to produce fruit. Some varieties, like Bing cherries, require pollen from a tree of the same variety.