Gynoecium or pistil is the fourth whorl of the flower. It is the female reproductive part of flower. The individual members of the pistil are called carpels. When the pistil consists of one carpel, it is called monocarpellary or simple pistil, example Dolichous, Pisium, phaseolus etc. When the number of carpel is more than one, it is said to be polycarpellary or compound pistil.
Polycarpellary pistil may be bicarpellary, tricarpellary, tetracarpellary, and pentacarpellary depending on the number of carpels. But when the number of carpel exceeds five, it is described as a typical polycarpellary pistil. The carpel in a pentacarpellary pistil may be either free or fused together.
When the carpels are free, the pistil is said to be apocarpous as in Michelia, Nelumbium etc. When the carpels are fused together, the pistil is called syncarpous as in Solanum melongena, Abelmoschus, Datura. In some cases, the pistil is sterile and functionless and is called pistillode.
Each pistil or gynoecium is made of three parts-the enlarged basal region called the ovary, a stalk-like style and a terminal expanded portion, the stigma.
The ovary is the swollen basal portion of pistil or gynoecium. It is formed due to the fusion of the margins of one or more carpels. In an apocarpous pistil, each carpel is separate and has separate ovary, style and stigma. In such pistil, an aggregation of free ovaries is seen. The ovary of a polycarpellary pistil is chambered according to the number of the constituent carpels, for example, if there is only one carpel, the ovary is one chambered and if there are two or five carpels, the ovary is two or five chambered respectively.
It is considered that each carpel is a leaf-like structure and due to infolding and fusion of their margins, the carpels are formed. The junction where the margins of the carpel fuse is known as the ventral suture and the region corresponding with the midrib is the dorsal suture. Inside the ovary and along its ventral suture is developed a mass of tissue, called the placenta, which subsequently gives rise to the ovules.
The style is the upward outgrowth of the ovary which connects it with the stigma. It is usually distinct being either long or short, but sometime it may be very much reduced or indistinct. The style is deciduous, as it falls off soon after fertilization but sometime it is persistent, as in Clematis. Usually, the style is terminal and develops on top of the ovary but sometime the style is placed on the lateral side of the ovary and is known as lateral style, as in Mangifera (mango). It some other cases, it also develops from the depressed region of a four-lobe ovary and is called gynobasic, example, Ocimum.
Stigma is the terminal portion of the pistil and lies at the tip of the style. when the style is absent, the stigma is said to be sessile as in Nelumbium (lotus). In some cases, the stigma is branched and the number of lobes corresponds to the number of carpels. The stigma may be two-lobed or bifid as in Ocimum or it may be three-lobed or trifid. When the stigma appears like a knob it is called capitate as in Brassica. It is called feathery when the stigma appears like a feather as in Oryza. The stigma is dumbbell-shaped in case of Vinca, Thevetia etc.
The portion of the ovary to which the ovules are attached is known as the placenta. The mode of arrangement of placentae inside the ovary is called placentation. The different types of placentation are as follows:
- Marginal placentation: In marginal placentation, the ovary remains one chambered and the placentae with the ovules are developed from the margin of the carpel or ventral suture. This type of placentation is found in monocarpellary pistil. The ovules are arranged in a row inside the chamber.Examples, Dolichos, Pisum, Phaseolus, Cajanus, Cassia, Caesalpinia etc.
- Axile placentation: In this type of placentation, the ovaries are many chambered and the number of chambers corresponds to the number of carpels in it. The placentae are developed at the central axis of the ovary in such a way that one placenta remains in each chamber. Each placenta bears one or more number of ovules. This type of placentation is found in syncarpous pistil. Examples, Datura, Hibiscus, Allium cepa, Solanum melongena etc.
- Central placentation: The central placentation is similar to axile placentation but in this case, the ovary is one chambered. At the early stage, the ovary remains many chambered but later on, the partition wall or septa in between the chambers disintegrate, forming one chamber. The placentae bearing the ovules developed all around the central axis which remains free at the centre of the ovary, as in Dianthus.
- Parietal placentation: In parietal placentation, the ovary is usually one chambered and the placentae bearing the ovules are developed from the inner wall of the ovary. The location of a placenta corresponds to the region where two carpel walls meet each other. For this reason, the number of placentae also corresponds to the number of carpels as in Cucurbita, Carica papaya, Momordica, Cucumis, Argemone. In case of Brassica, Raphanus etc., the placentation is parietal, but the ovary becomes two chambered due to the formation of a false partition wall called the replum.
- Superficial placentation: In this case, the ovary is multilocular, i.e., many chambered and the number of chambers is as many as the number of carpels. The placentae bearing the ovules are developed from the inner walls of the septa or partition walls between the chambers as in Nymphaea.
- Basal placentation: In this case, the ovary is unilocular (one chambered) and the placenta is developed at the basal region of the ovary. The basal placenta bears only one ovule. Examples: Helianthus, Tagetes, Tridax etc.