Gynoecium

A "gynoecium" (from Ancient Greek "gyne", "woman") is the female reproductive part of a flower. The male counterpart is called an "androecium". A gynoecium is composed of one or more pistils. A pistil may consist of a single free carpel, or be formed from a number of carpels that are fused. The pistil itself is formed from the stigma, style, and ovary.

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". Simpson, M.G.: "Plant Systematics", pp. 374-375. Elsevier Academic Press, 2006 ]

Pistils or ovaries can be either simple meaning only one carpel or compound meaning two or more carpels.

Carpel anatomy

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:
*the stigma (from Ancient Greek "stigma" "mark, puncture"), usually the terminal (end) portion that receives the pollen (male gametophytes); it is commonly somewhat glutinous or viscid;
*the style (from Latin "stilus" "stake, stylus"), a stalk connecting the stigma with the ovary below containing the transmitting tract, which facilitates the growth of the pollen tube and hence the movement of the male gamete to the ovule; and
*the ovary (from Latin "ovum" "egg") or megasporophyll (see sporophyll) containing the female reproductive cell or ovule.

The pistil

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:
* the carpels are fused at or near their margins (parietal placentation), usually forming a single large cavity — an example would be the violet.
* the folded carpels extend in towards the center, being fused along their outer faces (laterally concrescent), with the placentae arranged around a central column of tissue (axile placentation). There may be as many locules as there are carpels; and tissue of the receptacle may be involved in forming the axillary column. An example of axile placentation would be the lily.

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.

Inferior vs. superior ovaries

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

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.

References

* Esau, K. 1965. "Plant Anatomy", 2nd Edition. John Wiley & Sons. 767 pp.
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