Floral symmetry

Floral symmetry describes whether, and how, a flower, in particular its perianth, can be divided into two or more identical or mirror-image parts.

Uncommonly, flowers may have no axis of symmetry at all, typically because their parts are spirally arranged.

Peloria in Streptocarpus
[Left] Normal Streptocarpus flower (zygomorphic or mirror-symmetric), and [right] peloric (radially symmetric) flower on the same plant

Actinomorphic

Wurmbea stricta 0153
Wurmbea stricta, its tepals in actinomorphic arrangement

Most flowers are actinomorphic ("star shaped", "radial"), meaning they can be divided into 3 or more identical sectors which are related to each other by rotation about the centre of the flower. Typically, each sector might contain one tepal or one petal and one sepal and so on. It may or may not be possible to divide the flower into symmetrical halves by the same number of longitudinal planes passing through the axis: Oleander is an example of a flower without such mirror planes. Actinomorphic flowers are also called radially symmetrical or regular flowers. Other examples of actinomorphic flowers are the lily (Lilium, Liliaceae) and the buttercup (Ranunculus, Ranunculaceae).

Zygomorphic

Satyrium erectum detail Ground orchid Pienktrewwa Near Ceres, Western Cape 0414
Satyrium carneum. Ground orchid with typical zygomorphic floral anatomy

Zygomorphic ("yoke shaped", "bilateral" – from the Greek ζυγόν, zygon, yoke, and μορφή, morphe, shape) flowers can be divided by only a single plane into two mirror-image halves, much like a yoke or a person's face. Examples are orchids and the flowers of most members of the Lamiales (e.g., Scrophulariaceae and Gesneriaceae). Some authors prefer the term monosymmetry or bilateral symmetry.[1]

Asymmetry

A few plant species have flowers lacking any symmetry, and therefore having a "handedness". Examples: Valeriana officinalis and Canna indica.[2]

Differences

Actinomorphic flowers are a basal angiosperm character; zygomorphic flowers are a derived character that has evolved many times.[3]

Some familiar and seemingly actinomorphic so-called flowers, such as those of daisies and dandelions (Asteraceae), and most species of Protea, are actually clusters of tiny (not necessarily actinomorphic) flowers arranged into a roughly radially symmetric inflorescence of the form known as a head, capitulum, or pseudanthium.

Peloria

PeloricFoxglove
Digitalis purpurea (common foxglove) displaying an aberrant peloric terminal flower and normal zygomorphic flowers

Peloria or a peloric flower is the aberration in which a plant that normally produces zygomorphic flowers produces actinomorphic flowers instead. This aberration can be developmental, or it can have a genetic basis: the CYCLOIDEA gene controls floral symmetry. Peloric Antirrhinum plants have been produced by knocking out this gene.[3] Many modern cultivars of Sinningia speciosa ("gloxinia") have been bred to have peloric flowers as they are larger and showier than the normally zygomorphic flowers of this species.

Charles Darwin explored peloria in Antirrhinum (snapdragon) while researching the inheritance of floral characteristics for his The Variation of Animals and Plants under Domestication.[4] Later research, using Digitalis purpurea, showed that his results[5] were largely in line with Mendelian theory.[6]

Symmetry groups

If we consider only those flowers which consist in a single flower, rather than a flower head or inflorescence, we can group the flowers into a relatively small number of 2D symmetry groups. Monocots are identifiable by their trimerous petals, thus monocots often have rotational symmetry of order 3. If the flower also has 3 lines of mirror symmetry the group it belongs to is the dihedral group D3. If not, then it belongs to the cyclic group C3. Eudicots with tetramerous or pentamerous petals may have rotational symmetry of order 4 or 5. Again, whether they also have mirror planes decides whether they belong to dihedral (D4 and D5) or cyclic groups (C4 or C5). The sepals of some monocot flowers develop to replicate the petals, thus, superficially, certain monocots can appear to have rotational symmetry of order 6 and belong to either symmetry group D6 or C6. However, flower symmetry is rarely perfect.

See also

References

  1. ^ Craene 2010, p. 25.
  2. ^ Weberling, Focko (1992). Morphology of Flowers and Inflorescences. Cambridge University Press. p. 19. ISBN 0-521-25134-6.
  3. ^ a b Losos, J.B.; Mason, K.A; Singer, S.R. Biology (8th ed.). New York: McGraw Hill.
  4. ^ Darwin 1868, pp. 33–34
  5. ^ Darwin 1868, p. 46
  6. ^ Keeble, Frederick; Pellew, C; Jones, WN (1910). "The Inheritance of Peloria and Flower-Colour in Foxgloves (Digitalis purpurea)". New Phytologist. 9 (1–2): 68–77. doi:10.1111/j.1469-8137.1910.tb05554.x. JSTOR 2427515.

Bibliography

Acosmium

Acosmium Schott is a South America genus of Leguminosae (Fabaceae). Three species are currently recognized. Most Acosmium species have been recently transferred to Leptolobium and one species to the South American Guianodendron while the genus Acosmium itself has been transferred from the tribe Sophoreae to the tribe Dalbergieae in a monophyletic clade informally known as the Pterocarpus clade.

Allioideae

Allioideae is a subfamily of monocot flowering plants in the family Amaryllidaceae, order Asparagales. It was formerly treated as a separate family, Alliaceae. The subfamily name is derived from the generic name of the type genus, Allium. It is composed of about 18 genera.

August W. Eichler

August Wilhelm Eichler, also known under his Latinized name, Augustus Guilielmus Eichler (22 April 1839 – 2 March 1887), was a German botanist who developed a new system of classification of plants to reflect the concept of evolution.

His author abbreviation in botany is Eichler.

Author citation (botany)

In botanical nomenclature, author citation refers to citing the person or group of people who validly published a botanical name, i.e. who first published the name while fulfilling the formal requirements as specified by the International Code of Nomenclature for algae, fungi, and plants (ICN). In cases where a species is no longer in its original generic placement (i.e. a new combination of genus and specific epithet), both the author(s) of the original genus placement and those of the new combination are given (the former in parentheses).

In botany, it is customary (though not obligatory) to abbreviate author names according to a recognised list of standard abbreviations.

There are differences between the botanical code and the normal practice in zoology. In zoology, the publication year is given following the author name(s) and the authorship of a new combination is normally omitted. A small number of more specialized practices also vary between the recommendations of the botanical and zoological codes.

Cadia (plant)

Cadia is a genus of flowering plants in the legume family, Fabaceae. It belongs to the subfamily Faboideae.

Unlike most plants in the Faboideae, it has radially symmetrical flowers. In related species with bilateral symmetry, such as those of Lupinus, the dorsal (upper or adaxial) part of the flower expresses one or more genes in the Cycloidea (CYC)/Dichotoma (DICH) family. In Cadia, these genes are expressed throughout the flower. Thus, from a molecular point of view, Cadia is not reversing the ancestral evolution from radial symmetry to bilateral symmetry, but obtaining radial symmetry from a new mechanism.

Dalbergieae

The tribe Dalbergieae is an early-branching clade within the flowering plant subfamily Faboideae (or Papilionaceae). Within that subfamily, it belongs to an unranked clade called the Dalbergioids. It was recently revised to include many genera formerly placed in tribes Adesmieae and Aeschynomeneae and to be included in a monophyletic group informally known as the dalbergioids sensu lato. The members of this tribe have a distinctive root nodule morphology, often referred to as an "aeschynomenoid" or "dalbergioid" nodule.

Gilliesieae

Gilliesieae is a tribe of herbaceous geophyte plants belonging to the subfamily Allioideae of the Amaryllis family (Amaryllidaceae). Described in 1826, it contains fifteen genera and about eighty species. It has been variously treated as a subfamily or tribe. It is native to the Southern United States, Central and South America, predominantly Chile. Of the three tribes of genera that make up the subfamily Allioideae, Gilliesieae is the largest and most variable. The tribe was divided into two tribes in 2014, Gilliesiae s.s. and Leucocoryneae, based on differences in floral symmetry and septal nectaries.

Grex (horticulture)

The term grex (pl. greges or grexes; abbreviation gx), derived from the Latin noun grex, gregis meaning 'flock', has been coined to expand botanical nomenclature to describe hybrids of orchids, based solely on their parentage. Grex names are one of the three categories of plant names governed by the International Code of Nomenclature for Cultivated Plants; within a grex the cultivar group category can be used to refer to plants by their shared characteristics (rather than by their parentage), and individual orchid plants can be selected (and propagated) and named as cultivars.

International Association for Plant Taxonomy

The International Association for Plant Taxonomy (IAPT) promotes an understanding of plant biodiversity, facilitates international communication of research between botanists, and oversees matters of uniformity and stability in plant names. The IAPT was founded on July 18, 1950 at the Seventh International Botanical Congress in Stockholm, Sweden. Currently, the IAPT headquarters is located in Bratislava, Slovakia. Its current president, since 2017, is Patrick S. Herendeen (Chicago Botanic Garden); vice-president is Gonzalo Nieto Feliner (Real Jardín Botánico, Madrid); and secretary-general is Karol Marhold (Plant Science and Biodiversity Centre, Slovak Academy of Sciences, Bratislava).

Both the taxonomic journal Taxon and the series Regnum Vegetabile are published by the IAPT. The latter series includes the International Code of Nomenclature for algae, fungi, and plants, Index Nominum Genericorum, and Index Herbariorum.

International Code of Nomenclature for Cultivated Plants

The International Code of Nomenclature for Cultivated Plants (ICNCP), also known as the Cultivated Plant Code, is a guide to the rules and regulations for naming cultigens, plants whose origin or selection is primarily due to intentional human activity. Cultigens under the purview of the ICNCP include cultivars, Groups (cultivar groups), and grexes. All organisms traditionally considered to be plants (including algae and fungi) are included. Taxa that receive a name under the ICNCP will also be included within taxa named under the International Code of Nomenclature for algae, fungi, and plants, for example, a cultivar is a member of a species.

Ochnaceae

Ochnaceae is a family of flowering plants in the order Malpighiales. In the APG III system of classification of flowering plants, Ochnaceae is defined broadly, to include about 550 species, and encompasses what some taxonomists have treated as the separate families Medusagynaceae and Quiinaceae. In a phylogenetic study that was published in 2014, Ochnaceae was recognized in the broad sense, but two works published after APG III have accepted the small families Medusagynaceae and Quiinaceae. These have not been accepted by APG IV (2016).

In this article, "Ochnaceae" will refer to the larger circumscription of the family, which is otherwise known as Ochnaceae sensu lato or as the ochnoids. In this sense the family includes 32 genera with about 550 species.Ochnaceae, defined broadly or narrowly, is pantropical in distribution, with a few species cultivated outside of this range. Ochnaceae is most diverse in the neotropics, with a second center of diversity in tropical Africa. It consists mostly of shrubs and small trees, and, in Sauvagesia, a few herbaceous species. Many are treelets, with a single, erect trunk, but low in height. The Ochnaceae are notable for their unusual leaves. These are usually shiny, with closely spaced, parallel veins, toothed margins, and conspicuous stipules. Most of the species are buzz pollinated. In eight of the genera in tribe Sauvagesieae, the flower changes form after opening, by continued growth of tissue within the flower.A few species of Ochna are cultivated as ornamentals. Ochna thomasiana is probably the most commonly planted, but it is often misidentified in the horticultural literature.The leaves of Cespedesia are sometimes to 1 m (3.3 ft) in length and are used for roofing. An herbal tea is made from the pantropical weed Sauvagesia erecta.

In its evolution, Ochnaceae has been unusual, in "reverting" to character states that are regarded as ancestral or primitive. For example, an actinomorphic floral symmetry has appeared twice in the subfamily Ochnoideae. Also, two clades of Ochnaceae, one in Ochnoideae and another in Quiinoideae have a derived condition very close to apocarpy. The complete separation of the carpels (apocarpy) is thought to be the ancestral state for angiosperms.Fossils attributed to Ochnaceae are known from the early Eocene of Mississippi. The age of the family is very roughly estimated at 100 million years.A great many genus names have been published in Ochnaceae. In a taxonomic revision of Ochnaceae, as three families, in 2014, only 32 of these genera were accepted; one in Medusagynaceae, four in Quiinaceae, and 27 in Ochnaceae s.s.. In that same year, a 33rd genus, Neckia, was reestablished in order to preserve the monophyly of another genus, Sauvagesia.The largest genera in Ochnaceae are: Ouratea (200 species), Ochna (85), Campylospermum (65), Sauvagesia (39), and Quiina (34). None of the larger genera has been the subject of a phylogenetic analysis of DNA sequences of selected genes. In one study of the subfamily Quiinoideae, based on the trn L-F intergenic spacer, only nine species were sampled from this subfamily.

Pelorism

Pelorism is the term, said to be first used by Charles Darwin, for the formation of 'peloric flowers' which botanically is the abnormal production of actinomorphic flowers in a species that usually produces zygomorphic flowers. These flowers are spontaneous floral symmetry mutants. The term epanody is also applied to this phenomenon. Bilaterally symmetrical (zygomorphic) flowers are known to have evolved several times from radially symmetrical (actinomorphic) flowers, these changes being linked to increasing specialisation in pollinators.

Petal

Petals are modified leaves that surround the reproductive parts of flowers. They are often brightly colored or unusually shaped to attract pollinators. Together, all of the petals of a flower are called a corolla. Petals are usually accompanied by another set of special leaves called sepals, that collectively form the calyx and lie just beneath the corolla. The calyx and the corolla together make up the perianth. When the petals and sepals of a flower are difficult to distinguish, they are collectively called tepals. Examples of plants in which the term tepal is appropriate include genera such as Aloe and Tulipa. Conversely, genera such as Rosa and Phaseolus have well-distinguished sepals and petals. When the undifferentiated tepals resemble petals, they are referred to as "petaloid", as in petaloid monocots, orders of monocots with brightly coloured tepals. Since they include Liliales, an alternative name is lilioid monocots.

Although petals are usually the most conspicuous parts of animal-pollinated flowers, wind-pollinated species, such as the grasses, either have very small petals or lack them entirely.

Pteridophyte

A pteridophyte is a vascular plant (with xylem and phloem) that reproduces using spores. Because pteridophytes produce neither flowers nor seeds, they are also referred to as "cryptogams", meaning that their means of reproduction is hidden. The pteridophytes include the ferns, horsetails, and the lycophytes (clubmosses, spikemosses, and quillworts). These are not a monophyletic group because ferns and horsetails are more closely related to seed plants than to the lycophytes. Therefore, "Pteridophyta" is no longer a widely accepted taxon, although the term pteridophyte remains in common parlance, as do pteridology and pteridologist as a science and its practitioner, to indicate lycophytes and ferns as an informal grouping, such as the International Association of Pteridologists and the Pteridophyte Phylogeny Group.

Raceme

A raceme ( or ) is an unbranched, indeterminate type of inflorescence bearing pedicellate flowers (flowers having short floral stalks called pedicels) along its axis. In botany, an axis means a shoot, in this case one bearing the flowers. In indeterminate inflorescence-like racemes, the oldest flowers are borne towards the base and new flowers are produced as the shoot grows, with no predetermined growth limit. A plant that flowers on a showy raceme may have this reflected in its scientific name, e.g. Cimicifuga racemosa. A compound raceme, also called a panicle, has a branching main axis. Examples of racemes occur on mustard (genus Brassica) and radish (genus Raphanus) plants.

Sophoreae

The tribe Sophoreae is one of the subdivisions of the plant family Fabaceae. Traditionally this tribe has been used as a wastebasket taxon to accommodate genera of Faboideae which exhibit actinomorphic, rather than zygomorphic floral symmetry and/or incompletely differentiated petals and free stamens. Various morphological and molecular analyses indicated that Sophoreae as traditionally circumscribed was polyphyletic. This led to a re-circumscription of Sophoreae, which resulted in the transfer of many genera to other tribes (Amburaneae, Angylocalyceae, Baphieae, Camoensieae, the Cladrastis clade, Exostyleae, Leptolobieae, Ormosieae, Podalyrieae, and the Vataireoids). This also necessitated the inclusion of two former tribes, Euchresteae and Thermopsideae, in the new definition of Sophoreae. Tribe Sophoreae, as currently circumscribed, consistently forms a monophyletic clade in molecular phylogenetic analyses. The Sophoreae arose 40.8 ± 2.4 million years ago (in the Eocene).Description of morphological synapomorphies has yet to be undertaken, but members of this tribe can be distinguished by the relatively simple flowers, unspecialized pinnate leaves, accumulation of quinolizidine alkaloids, and the presence of free stamens. The tribe does have a node-based definition: the crown clade originating from the most recent common ancestor of Bolusanthus speciosus (Bolus) Harms and Sophora davidii (Franch.) Pavol..

Swartzieae

The tribe Swartzieae is an early-branching monophyletic clade of the flowering plant subfamily Faboideae or Papilionaceae. Traditionally this tribe has been used as a wastebasket taxon to accommodate genera of Faboideae which exhibit actinomorphic, rather than zygomorphic floral symmetry and/or incompletely differentiated petals and free stamens. It was recently revised and most of its genera were redistributed to other tribes (Amburaneae, Baphieae, and Exostyleae). Under its new circumscription, this clade is consistently resolved in molecular phylogenies. Members of this tribe possess "non-papilionate swartzioid flowers[…]largely characterized by a tendency to lack petals combined with a profusion and elaboration of free stamens" and a "lack of unidirectional order in the initiation of the stamens". They also have "complete or near complete fusion of sepals resulting from intercalary growth early in development, relatively numerous stamens, and a single or no petal, with other petals not at all apparent in development." The tribe is predicted to have diverged from the other legume lineages 48.9±2.8 million years ago (in the Eocene).

Symmetry in biology

Symmetry in biology is the balanced distribution of duplicate body parts or shapes within the body of an organism. In nature and biology, symmetry is always approximate. For example, plant leaves – while considered symmetrical – rarely match up exactly when folded in half. Symmetry creates a class of patterns in nature, where the near-repetition of the pattern element is by reflection or rotation.

The body plans of most multicellular organisms exhibit some form of symmetry, whether radial, bilateral, or spherical. A small minority, notably among the sponges, exhibit no symmetry (i.e., are asymmetric). Symmetry was once important in animal taxonomy; the Radiata, animals with radial symmetry, formed one of the four branches of Georges Cuvier's classification of the animal kingdom.

Valerian (herb)

Valerian (Valeriana officinalis, Caprifoliaceae) is a perennial flowering plant native to Europe and Asia. In the summer when the mature plant may have a height of 1.5 metres (5 ft), it bears sweetly scented pink or white flowers that attract many fly species, especially hoverflies of the genus Eristalis. It is consumed as food by the larvae of some Lepidoptera (butterfly and moth) species, including the grey pug.

Crude extract of valerian root may have sedative and anxiolytic effects, and is commonly sold in dietary supplement capsules to promote sleep.

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