A plant ovary (much like an animal ovary) is the part of the pistil which contains ovules. The style is generally referred to as stalklike, without ovules located between the ovary (at the bottom of the pistil) and the stigma (located at the top portion of the pistil). In some plant species styles are not found in the pistils. Stigma is the pollen receptor within the pistil at the top of the pistil. Stigmas may be discretely defined structures or they may be within a region referred to as the stigmatic region.
Pistils or ovaries can be either simple meaning only one carpel or compound meaning two or more carpels.
A carpel is the basic unit of the female reproductive organ of a flower, the gynoecium. A flower may have zero, one, or more carpels. Multiple carpels may combine into a single pistil, or into multiple pistils.
The parts of the carpel are:
A pistil (from Latin pistillum "pestle") is made up of a carpel (if single) or carpels (if fused). A gynoecium that consists of a single free carpel is termed monocarpous. That with two or more fused carpels (called a compound ovary or compound pistil) is termed syncarpous. However, if the gynoecium consists of one or more free, simple, and distinct carpels, each carpel makes an individual pistil and the gynoecium is termed apocarpous. Fertilization of the ovule or ovules results in development of the carpel(s) into a fruit.
When two or more carpels are fused or joined together its called syncarpy. In a compound pistil, the carpels are fused together in one of two basic ways:
A complicating factor in all of this is the fact that in some species syncarpy is present only at the base of the carpels, the pistil being apocarpous in the upper part. The manner of fusing of the carpels can also vary from one part of the pistil to another.
The gynoecium, the collective term for all the carpels, is the innermost whorl of the parts of a flower, and in many flowers the other parts (sepals, petals, and stamens) are attached to the receptacle beneath the gynoecium. In such cases, where the ovary lies above the attachments of the other distinct floral parts, the flower is described as hypogynous or as having a superior ovary. In some species (examples include plum, cherry, and blackberry), the other (noncarpellary) floral parts are fused to form a cup called a floral tube or hypanthium. In these flowers, the ovary lies physically lower than the lobes of the sepals and petals and below the point of attachment of the stamen filaments — the ovary is still considered to be superior but the flower is termed perigynous.
In those flowers in which the floral tube is fused with the ovary, the sepals, petals, and stamens appear to grow out from the top of the ovary, and the flower is said to be epigynous and have an inferior ovary. Examples of plant families with inferior ovaries include orchid, sunflower, and cactus. The position of the ovary is an important consideration in the identification and classification of plant species, as well as the kind of fruit that develops after fertilization.
The ovule (from Latin ovulum "small egg"), which represents the megasporangium, when mature, consists of one or two coats surrounding the central nucellus, except at the apex where an opening, the micropyle, is left. The nucellus is a cellular tissue enveloping one large cell, the embryo-sac or megaspore. The germination of the megaspore consists in the repeated division of its nucleus to form two groups of four, one group at each end of the embryo-sac. One nucleus from each group, the polar nucleus, passes to the centre of the sac, where the two fuse to form the so-called definitive nucleus. Of the three cells at the micropylar end of the sac, all naked cells (the so-called egg-apparatus), one is the egg-cell or oosphere, the other two, which may be regarded as representing abortive egg-cells (in rare cases capable of fertilization), are known as synergidae. The three cells at the opposite end are known as antipodal cells and become invested with a cell-wall.
The carpel of a simple apocarpous gynoecium appears as a folded structure, differentiated into a basal fertile part (ovary) and an upper sterile part (style). Various interpretations of the origin from a leaf-like structure have been made (Esau, 1965), but the important anatomical description is that of a variously folded tissue surrounding a cavity (called a locule) within which projects one or more ovules, attached by or along a placenta. Typically, a carpel has two placentae. An example of a simple carpel is that of a pea, bean or Arabidopsis: the fruit develops from the single carpel consisting of two rows of ovules aligned beside one another along the placental margin.