In plants that reproduce by flowering (angiosperms), the pistil is the female reproductive portion (the gynoecium) of the flower. The term carpel is a more specific way of denoting one unit of a pistil, while the term pistil is a less exact way of describing the same part of the flower as the carpel, the ovule producing area.
The base of the pistil is called the ovary. The ovary contains the ovules, a stalk (or a style) and a stigma. The ovules are female sex cells which potentially form seeds. The stigma receives pollen and may be sticky in order to help catch the grains.
A flower has four basic parts: sepals, petals, stamens and carpels. Flowers may have a single pistil or many. Flowers may contain both stamens and carpels, occur without stamens or occur without carpels.
A simple pistil has one carpel. A compound pistil has more than one carpel, joined together to varying degrees. Apocarpous flowers have separate pistils and separate carpels. Syncarpous flowers have one pistil and multiple united carpels.
To fertilize, a flower may self-pollinate (supply its own pollen). This is known as autogamy. If the flower comes into contact with the pollen from another plant through a pollinator, it may cross-pollinate. This is known as heterogamy. Types of pollinators include insects, wind, birds or bats, mammals (including humans) and water.
Plants use attractants to encourage pollinators to approach and linger. Attractants include foods such as pollen or nectar and enticing aromas. Pollinators lured into visiting the flower may pick up pollen as they investigate the attractants or they may bring pollen along on their bodies from other flowers which can transfer and adhere to the stigma.
Pollen germinates on the stigma. A pollen tube forms and grows down to one of the ovules at the base of the pistil and fertilization occurs.
Some plants are capable of self pollination, such as those with hermaphroditic flowers containing both male and female reproductive parts and monoecious plants, which produce both male and female flowers.
Many plants capable of self pollination have developed an adaptation called dichogamy. The plant creates and releases pollen when the plant's own stigmas are not ready to accept pollen. This may happen through protandry, when the stamens mature first, or protogyny, when the pistils do.