A flower is a plant's reproductive structure. It attracts pollinators that carry pollen to other flowers. When pollen merges with an ovule of a flower, a seed is formed. The flower then forms a fruit to protect the seed.
Complete and Incomplete
Besides petals, most flowers have a stamen, a pistil and sepals. If a flower has all four of these parts, botanists consider it "complete." A rose, for example, is a complete flower. If one of these parts is missing, the flower is called "incomplete." A begonia is an incomplete flower, because its flowers have either a stamen or a pistil, but not both.
Stamen and Pistil
The stamen is the flower's male reproductive organ; the pistil is the female reproductive organ. Botanists also divide flowers into perfect or imperfect. Perfect flowers have functioning male and female organs, even if they are missing petals and sepals. Imperfect flowers lack either a stamen or a pistil, and so are always incomplete.
Sepals and Petals
Sepals are green, leaf-like structures that enclose a developing flower and protect it until the flower is ready to open. The calyx is what the sepals together are called. Petals are designed to attract pollinators. They are generally highly colored and perfumed portions of a flower, and often have markings to attract insects.