The gynoecium (from Ancient Greek γυνη, gyne, "woman" + οἰκος, oikos, "house") is a term with several meanings in botanical usage. In reference to mosses, liverworts and hornworts, "gynoecium" refers to a cluster of archegonia and any associated modified leaves or stems present on a gametophyte shoot. The more common use of gynoecium, however, is to refer to the ovule-producing part of a flower. The gynoecium in angiosperms is typically the innermost whorl of structures in a flower and is surrounded (in perfect flowers) by the androecium (stamens) and (in complete flowers) by the perianth (petals and sepals). The gynoecium is often inaccurately referred to as female because it gives rise to female (egg-producing) gametophytes. [1]
The structure from which the gynoecium is fashioned is the carpel. A gynoecium may consist of a single carpel, multiple distinct (unfused) carpels or multiple connate (fused) carpels. Flowers that bear a gynoecium but no androecium are called carpellate. Flowers lacking a gynoecium are called staminate flowers.
Gynoecia give rise to and protect ovules (within one or more ovaries). When mature, gynoecia may function to attract pollinators (through nectaries or visual cues). Gynoecia receive pollen on their surface (usually on a stigma), in some cases actively selecting genetically different pollen so as to promote outcrossing. Gynoecia may facilitate pollen tube growth to the ovule and delivery of the sperm to the egg. The gynoecium forms the pericarp of fruit.
Types of Gynoecia
Carpels are the building blocks of the gynoecium. If a gynoecium has a single carpel, it is called monocarpous. If a gynoecium has multiple, distinct (free, unfused) carpels, it is apocarpous. If a gynoecium has multiple carpels fused into a single structure, it is syncarpous. A syncarpous gynoecium can sometimes appear very much like a monocarpous gynoecium. Because of this difficulty in interpreting carpel number and arrangement, some people use another term to describe the visible structure(s) in a gynoecium. This term is pistil (from Latin pistillum "pestle"). The existence of two different ways of describing gynoecia has led to much confusion, even among botanists. The pistil terminology is less exact and specific, and is more cumbersome, but can be useful in keys and field descriptions where detailed studies are not possible.
| Gynoecium has: | In "Carpel" Terminology The flower has a(n): | In "Pistil" Terminology The flower has: | Examples |
|---|---|---|---|
| Single carpel | Monocarpous or Unicarpellate Gynoecium | A Pistil (simple) | Avocado (Persea sp.), most legumes (Fabaceae) |
| Multiple distinct (unfused) carpels | Apocarpous Gynoecium | Pistils (simple) | Strawberry (Fragaria sp.), buttercup (Ranunculus sp.) |
| Multiple connate (fused) carpels | Syncarpous Gynoecium | A Pistil (compound) | Tulip (Tulipa sp.), most flowers |
Gynoecium development and arrangement is important in systematic research and identification of angiosperms, but can be the most challenging of the floral parts to interpret.
Carpel Morphology
Each structure in a gynoecium, whether it consists of a lone carpel or mulitple fused carpels, usually consists of:
- An enlarged basal portion called the ovary (from Latin ovum "egg"), which contains placentas bearing one or more ovules (integumented megasporangia). A chamber inside an ovary, in which the ovules develop, is called a locule (or sometimes cell). No matter how many carpels fuse together to form a syncarpous ovary, there may be a single locule or more than one locule. If there are multiple locules the walls dividing them are called septa.
- The style (from Latin stilus "stake, stylus"), a stalk-like conduit allowing the pollen (male gametophytes) to grow to the egg cell and deliver the sperm.
- The stigma (from Ancient Greek stigma "mark, puncture"), usually found at the tip of the style, the portion of the structure that receives the pollen (male gametophytes). It is commonly sticky or feathery to capture pollen.
Carpels begin as small primorida on a floral apical meristem, forming later than and closer to the shoot apex than sepal, petal and stamen primordia. Morphological [2] and molecular studies of carpel ontogeny reveal that carpels are most likely homologous to leaves. In some basal angiosperm lineages, for ecample Degeneria, native to Fiji, carpels begin as a shallow cup and eventually form a folded, leaf-like structure, sealed at its margins by interlocking hairs. No stigma or style exists, but a broad stigmatic crest allows pollen tubes access to two rows of ovules enclosed in the carpel. Carpels generally bear three vascular traces
In a fully syncarpous gynoecium the challenge is to determine how many carpels fused to form the gynoecium. Sometimes the number of stigmas or stigma lobes is useful, as can be ridges on the ovary or locules within the ovary.
The degree of connation (fusion) in a syncarpous gynoecium can vary. Fusion can occur very early in carpel development, or later. In some gynoecia carpels are fused only at their base, but retain separate styles and stigmas. In other gynoecia, the ovaries and styles are fused but the stigmas remain separate. Sometimes (e.g. Apocynaceae) carpels are fused by their styles or stigmas but possess distinct ovaries.
In flowers with multiple distinct carpels, the carpels are nearly always single (apocarpous gynoecium).
Carpel Position
Basal angiosperm groups tend to have carpels arranged spirally around a conical or dome-shaped receptacle. In later diverging lineages, carpels tend to be in whorls.
The relationship of the other flower parts to the gynoecium can be an important systematic and taxonomic character. The ovary of the gynoecium can be above the attachment of petals and sepals to the receptacle. In this case the ovary is called superior. This corresponds to a the term hypogynous, used to describe a flower with a fully superior ovary.
In some flowers with superior ovaries, (examples include many members of the rose family, Rosaceae) the base of the stamens, petals and sepals are fused into a floral tube or hypanthium. In these flowers, the ovary can be free of or partially adnate (fused) to the hypanthium, but remains at least partially superior. These flower are called perigynous.
In some flowers, the ovary is completely fused to the hypanthium. Only the style and stigma project above the sepals, petals, and stamens, which appear to attach to the top of the ovary. This sort of ovary is called inferior ovary, and the flower is said to be epigynous. Examples of plant families with inferior ovaries include the orchids Orchidaceae, sunflower Asteraceae and the evening primrose Onagraceae.
The Ovary
Ovaries are attached directly to the receptacle, but can be sunken into it (inferior, as described above), superior with no stalk, or on stalk-like extensions of the receptacle, as in Isomeris arborea.
The ovary surrounds one or more locules in which ovules develop from placenta. In monocarpous or apocarpous gynoecia, placenta are typically arranged in longitudinal strips near the margin (before fusion) of the carpel (called marginal placentation). In syncarpous gynoecia where the fused carpels form a central column and several locules from their fused margins, placenta can occur in the center of the ovary, on the column (axile placentation). In some ovaries, the placenta are found in clumps along the wall (parietal placentation) or on a central column with no septa defining locules (free central placentation). In some cases a single ovule is attached to the bottom or top of the locule (basal or apical placentation, respectively).
The Ovule
The ovule (from Latin ovulum "small egg") is a complex structure, born inside ovaries of carpels in angiosperms. The ovule initially consists of a stalked, integumented megasporangium. Typically one cell in the megasporangium undergoes meiosis resulting in one to four megaspores. These develop into reduced megagametophytes (often called embryo sacs) within the ovule. Before fertilization, the ovule consists of one or two layers of integuments surrounding the remains of the megasporangium, called the nucellus and an embryo sac, with a small number of cells and nuclei, including one egg cell and two polar nuclei (which will form, together with a sperm cell, the primary endosperm nucleus). The gap in the integuments through which the pollen tube enters to deliver sperm to the egg is called the micropyle. The stalk attaching the ovule to the placenta is called the funiculus. Ovules are typically positioned so that the micropyle is facing the point of funiculus attachment, but other positions are found in a variety of plant groups.
The Stigma and Style
Stigmas can vary from long and slender to globe shaped to feathery. Their role in the life cycle of angiosperms is to receive and adhere to pollen from whatever vector has carried it from it flower of origin. Stigmas must distinguish and reject the pollen of other species, and in some cases the pollen from the same or genetically similar flowers. This self incompatibility function ensures outcrossing.
Pollen is typically highly dessicated when it leaves an anther. Stigmas have been shown to assist in the rehydration of pollen and in promoting germination of the pollen tube. [3] Stigmas also ensure proper adhesion of the correct species of pollen. Stigmas can play an active role in pollen discrimination and some self-compatibility systems involve interaction between the stigma and the surface of the pollen grain.
The style can be open (containing few or no cells in the central portion) or closed (densely packed with cells throughout). Pollen tubes grow the length of the style to reach the ovules. Studies have shown that in some species, at least, the pollen tube is directed to the micropyle of the ovule by the style.
References
- Esau, K. 1965. Plant Anatomy, 2nd Edition. John Wiley & Sons. 767 pp.
- This article incorporates text from the Encyclopædia Britannica, Eleventh Edition, a publication now in the public domain.
- ^ Judd, Campbell, Kellogg, Stevens, Donoghue, 2007 Plant Systematics: A Phylogenetic Approach, 3rd Edition Sinauer Associates, Inc. Sunderland, MA
- ^ Gifford, E. and Foster A. 1989Morphology and Evolution of Flowering Plants 3rd EditionW. H. Freeman and Company, New York
- ^ Edulnd, Swanson and Preuss. 2004. Pollen and stigma structure and function: the role of diversity in pollination. Plant Cell 16:Supplement 84-97.
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