Floral symmetry describes whether, and how, a

flower Flowers, also known as blooms and blossoms, are the reproductive structures of flowering plants ( angiosperms). Typically, they are structured in four circular levels, called whorls, around the end of a stalk. These whorls include: calyx, m ...

, in particular its

perianth The perianth (perigonium, perigon or perigone in monocots) is the non-reproductive part of the flower. It is a structure that forms an envelope surrounding the sexual organs, consisting of the calyx (sepals) and the corolla (petals) or tepal ...

, can be divided into two or more identical or mirror-image parts. Uncommonly, flowers may have no axis of

symmetry Symmetry () in everyday life refers to a sense of harmonious and beautiful proportion and balance. In mathematics, the term has a more precise definition and is usually used to refer to an object that is Invariant (mathematics), invariant und ...

at all, typically because their parts are spirally arranged.

Actinomorphic

Most flowers are actinomorphic ("star shaped", "radial"), meaning they can be divided into three or more identical sectors which are related to each other by rotation about the center of the flower. Typically, each sector might contain one

tepal A tepal is one of the outer parts of a flower (collectively the perianth). The term is used when these parts cannot easily be classified as either sepals or petals. This may be because the parts of the perianth are undifferentiated (i.e. of very ...

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'' ( ) is a genus of herbaceous flowering plants growing from bulbs, all with large and often prominent flowers. Lilies are a group of flowering plants which are important in culture and literature in much of the world. Most species are ...

(''

Lilium ''Lilium'' ( ) is a genus of Herbaceous plant, herbaceous flowering plants growing from bulbs, all with large and often prominent flowers. Lilies are a group of flowering plants which are important in culture and literature in much of the world ...

'',

Liliaceae The lily family, Liliaceae, consists of about 15 genera and 610 species of flowering plants within the order Liliales. They are monocotyledonous, perennial, herbaceous, often bulbous geophytes. Plants in this family have evolved with a fai ...

) and the buttercup ('' Ranunculus'',

Ranunculaceae Ranunculaceae (, buttercup or crowfoot family; Latin "little frog", from "frog") is a family (biology), family of over 2,000 known species of flowering plants in 43 genera, distributed worldwide. The largest genera are ''Ranunculus'' (600 spec ...

Zygomorphic

Zygomorphic ("

yoke A yoke is a wooden beam used between a pair of oxen or other animals to enable them to pull together on a load when working in pairs, as oxen usually do; some yokes are fitted to individual animals. There are several types of yoke, used in dif ...

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

orchid Orchids are plants that belong to the family Orchidaceae (), a diverse and widespread group of flowering plants with blooms that are often colourful and fragrant. Orchids are cosmopolitan plants that are found in almost every habitat on Eart ...

s and the flowers of most members of the Lamiales (e.g., Scrophulariaceae and

Gesneriaceae Gesneriaceae, the gesneriad family, is a family (biology), family of flowering plants consisting of about 152 genera and ca. 3,540 species in the tropics and subtropics of the Old World (almost all Didymocarpoideae) and the New World (most Ges ...

). Some authors prefer the term monosymmetry or bilateral symmetry. The asymmetry allows pollen to be deposited in specific locations on pollinating insects and this specificity can result in evolution of new species. Globally and within individual networks, zygomorphic flowers are a minority. Plants with zygomorphic flowers have smaller number of visitor species compared to those with actinomorphic flowers. Sub-networks of plants with zygomorphic flowers share greater connectance, greater asymmetry and lower coextinction robustness for both the plants and the visitor species. Plant taxa with zygomorphic flowers can have a greater risk of

extinction Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...

due to

pollinator decline Pollinator decline is the reduction in abundance of insect and other animal pollinators in many ecosystems worldwide that began being recorded at the end of the 20th century. Multiple lines of evidence exist for the reduction of wild pollinator p ...

Asymmetry

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

Differences

Actinomorphic flowers are a basal angiosperm character; zygomorphic flowers are a derived character that has evolved many times. Some familiar and seemingly actinomorphic so-called flowers, such as those of daisies and dandelions (

Asteraceae Asteraceae () is a large family (biology), family of flowering plants that consists of over 32,000 known species in over 1,900 genera within the Order (biology), order Asterales. The number of species in Asteraceae is rivaled only by the Orchi ...

), and most species of '' Protea'', are actually clusters of tiny (not necessarily actinomorphic) flowers arranged into a roughly radially symmetric

inflorescence In botany, an inflorescence is a group or cluster of flowers arranged on a plant's Plant stem, stem that is composed of a main branch or a system of branches. An inflorescence is categorized on the basis of the arrangement of flowers on a mai ...

of the form known as a head, capitulum, or pseudanthium.

Peloria

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 ''Antirrhinum'' is a genus of plants in the Plantaginaceae family, commonly known as dragon flowers or snapdragons because of the flowers' fancied resemblance to the face of a dragon that opens and closes its mouth when laterally squeezed. They ...

'' plants have been produced by knocking out this gene. 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 Charles Robert Darwin ( ; 12 February 1809 – 19 April 1882) was an English Natural history#Before 1900, naturalist, geologist, and biologist, widely known for his contributions to evolutionary biology. His proposition that all speci ...

explored peloria in ''Antirrhinum'' (snapdragon) while researching the inheritance of floral characteristics for his ''

The Variation of Animals and Plants Under Domestication ''The Variation of Animals and Plants Under Domestication'' is a book by Charles Darwin that was first published in January 1868. A large proportion of the book contains detailed information on the domestication of animals and plants but it al ...

''. Later research, using ''

Digitalis purpurea ''Digitalis purpurea'', the foxglove or common foxglove, is a toxic species of flowering plant in the plantain family Plantaginaceae, native to and widespread throughout most of temperate Europe. It has also naturalized in parts of North Ame ...

'', showed that his results were largely in line with Mendelian theory.

References

Bibliography

* * * * {{Patterns in nature Plant morphology