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Generalized flower structure.
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Description
Flowers are produced at stem tips and contain four types of organs: sepals, petals, pollen-producing stamens, and ovule-producing carpels. These flower organs are supported by tissue known as a receptacle, located at the tip of a flower stalk: a peduncle. The functioning of several genes that control flower organ development explains why carpels are the central-most flower organs, why stamens surround carpels, and why petals and sepals are the outermost flower organs.

Many flowers produce attractive petals that play a role in pollination: the transfer of pollen among flowers. Sepals of many flowers are green and form the outer, protective layer of flower buds. By contrast, the sepals of other flowers resemble attractive petals. All of a flower's petals and sepals are collectively known as the perianth. Most flowers produce one or more stamens, the structures that produce and disperse pollen. Most flowers also contain carpels, structures that produce ovules. Some flowers lack perianths, stamens, or carpels. Flowers that possess all four types of flower organs are known as complete flowers, while flowers lacking one or more organ types are known as incomplete flowers. Flowers that contain both stamens and carpels are described as perfect flowers, while flowers lacking either stamens or carpels are imperfect flowers.

Flowers also differ in the numbers of organs they produce. Some flowers produce only a single carpel, others display several separate carpels, and many possess several carpels that are fused together into a compound structure. Both a single carpel and compound carpels are referred to as a pistil (from the Latin word pistillum, pestle), because it resembles the device people use to grind materials to powder in a mortar. Only one pistil is present in flowers that have only one carpel and in flowers with fused carpels. By contrast, flowers possessing several separate carpels display multiple pistils.

Pistils are usually differentiated into three regions having distinct functions. A topmost portion of the pistil, known as the stigma, receives and recognizes pollen of the appropriate species or genotype. The stigma allows pollen of appropriate genetic type to germinate, producing a long pollen tube that grows through the elongate style. The pollen tube thereby delivers nonflagellate sperm cells to ovules and the eggs inside, allowing fertilization. If fertilization occurs, the ovule develops into a seed. The lowermost portion of the pistil is the ovary, which encloses and protects ovules. Ovaries (and sometimes additional flower parts) develop into fruits.
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