Gynoecium
The gynoecium (from Ancient Greek γυνή, gyne, meaning woman, and οἶκος, oikos, meaning 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.
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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 the term pistil (from Latin pistillum meaning pestle) to describe the visible structure(s) in a gynoecium. 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 composition | Carpel terminology | Pistil terminology | 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 |
* Also called '''choricarpous'''
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 multiple fused carpels, usually consists of:
- An enlarged basal portion called the ovary (from Latin ovum meaning 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 meaning stake, or 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 στίγμα, stigma meaning mark, or 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 primordia on a floral apical meristem, forming later than, and closer to the (floral) 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 example 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, like leaves, 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 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, gynophores, 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 placentatilon). 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 meaning 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
The style and stigma of the flower are involved in most types of self incompatibility reactions. Self incompatibility, if present, prevents fertilization by pollen from the same plant or from genetically similar plants, and ensures outcrossing.
Stigmas can vary from long and slender to globe shaped to feathery. The stigma is the receptive tip of the carpel, which receives pollen at pollination and on which the pollen grain germinates. The stigma is adapted to catch and trap pollen, either by combining pollen of visiting insects or by various hairs, flaps, or sculpturings.[3] Stigmas must distinguish and reject the pollen of other species, and in some cases are responsible for self incompatibility.
The style of a pistil is the tube-like portion between the stigma and the ovary. It can be either long or short. In some cases the style is responsible for self incompatibility, causing pollen tubes to fail.
References
- Esau, K. 1965. Plant Anatomy, 2nd Edition. John Wiley & Sons. 767 pp.
This article incorporates text from a publication now in the public domain: Chisholm, Hugh, ed (1911). Encyclopædia Britannica (Eleventh ed.). Cambridge University Press.
- ↑ 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
- ↑ The Penguin Dictionary of Botany, edited by Elizabeth Toothill, Penguin Books 1984 ISBN 0-14-051126-1