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Floral symmetry describes whether, and how, a
flower A flower, sometimes known as a bloom or blossom Image:Cerisier du Japon Prunus serrulata.jpg, Cherry blossoms in Paris in full bloom. In botany, blossoms are the flowers of stone fruit fruit tree, trees (genus ''Prunus'') and of some other plan ...

flower
, in particular its
perianth The perianth (perigonium, perigon or perigone in monocots) is the non-reproductive part of the flower, and structure that forms an envelope surrounding the sexual organs, consisting of the calyx (botany), calyx (sepals) and the corolla (flower), ...
, can be divided into two or more identical or mirror-image parts. Uncommonly, flowers may have no axis of
symmetry Symmetry (from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is appro ...

symmetry
at all, typically because their parts are spirally arranged.


Actinomorphic

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 A tepal is one of the outer parts of a flower A flower, sometimes known as a bloom or blossom, is the reproduction, reproductive structure found in flowering plants (plants of the division Magnoliophyta, also called angiosperms). The biological ...
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 ''Nerium oleander'' , most commonly known as oleander or nerium, is a shrub or small tree cultivated worldwide in temperate and subtropical areas as an ornamental and landscaping plant. It is the only species currently classified in the genus ...

Oleander
is an example of a flower without such mirror planes. Actinomorphic flowers are also called
radially symmetrical Symmetry in biology refers to the symmetry observed in organisms, including plants, animals, fungi, and bacteria. External symmetry can be easily seen by just looking at an organism. For example, take the face of a human being which has a plan ...
or regular flowers. Other examples of actinomorphic flowers are the
lily ''Lilium'' is a genus of Herbaceous plant, herbaceous flowering plants growing from bulbs, all with large prominent flowers. They are the true lilies. Lilies are a group of flowering plants which are important in culture and literature in much ...

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

Lilium
'',
Liliaceae The lily family In human society A society is a Social group, group of individuals involved in persistent Social relation, social interaction, or a large social group sharing the same spatial or social territory, typically subject ...

Liliaceae
) and the
buttercup ''Ranunculus'' is a large genus of about 600 species of flowering plants in the family Ranunculaceae. Members of the genus are known as buttercups, spearworts and water crowfoots. The familiar and widespread buttercup of gardens throughout N ...

buttercup
(''
Ranunculus ''Ranunculus'' is a large genus of about 600 species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined a ...

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 specie ...
).


Zygomorphic

Zygomorphic ("
yoke A yoke is a wooden Beam (structure), beam normally 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 ...
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 Orchidaceae ( ), commonly called the orchid family, is a diverse and widespread family In human society A society is a Social group, group of individuals involved in persistent Social relation, social interaction, or a large social ...

orchid
s and the flowers of most members of the
Lamiales The Lamiales are an order in the asterid group of dicotyledonous flowering plants. It includes about 23,810 species, 1,059 genera, and is divided into about 24 family (biology), families. Being one of the largest orders of flowering plants, Lam ...
(e.g.,
Scrophulariaceae The Scrophulariaceae are a family In human society, family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of families is to maint ...
and
Gesneriaceae Gesneriaceae, the gesneriad family, is a family In human society A society is a Social group, group of individuals involved in persistent Social relation, social interaction, or a large social group sharing the same spatial or soci ...
). 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 Floral symmetry describes whether, and how, a flower A flower, sometimes known as a bloom or blossom, is the reproduction, reproductive structure found in flowering plants (plants of the division Magnoliophyta, also called angiosperms). T ...

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 a kind of organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by ...

extinction
due to
pollinator decline Pollinator decline is the reduction in abundance of insect Insects or Insecta (from Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken ...
.


Asymmetry

A few plant species have flowers lacking any symmetry, and therefore having a "handedness". Examples: ''Valeriana officinalis'' and ''
Canna indica ''Canna indica'', commonly known as Indian shot, African arrowroot, edible canna, purple arrowroot, Sierra Leone arrowroot, is a plant species in the family Cannaceae. It is native to much of South America, Central America, the West Indies, 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
daisies
and
dandelions ''Taraxacum'' () is a large genus Genus (plural genera) is a taxonomic rank Taxonomy (general) is the practice and science of classification of things or concepts, including the principles that underlie such classification. The term may al ...

dandelions
(
Asteraceae The family (biology), family Asteraceae, alternatively Compositae, consists of over 32,000 known species of flowering plants in over 1,900 genera within the Order (biology), order Asterales. Commonly referred to as the aster, daisy, composite, o ...

Asteraceae
), and most species of ''
Protea ''Protea'' () is a genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses ...

Protea
'', are actually clusters of tiny (not necessarily actinomorphic) flowers arranged into a roughly radially symmetric
inflorescence An inflorescence is a group or cluster of flower A flower, sometimes known as a bloom or blossom Cherry blossoms in Paris in full bloom. In botany, blossoms are the flowers of stone fruit fruit tree, trees (genus ''Prunus'') and of some ...
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 commonly known as dragon flowers or snapdragons because of the flowers' fancied resemblance to the face of a European dragon, dragon that opens and closes its mouth when laterally squeezed. They are native ...

Antirrhinum
'' 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 naturalist Natural history is a domain of inquiry involving organism In biology, an organism () is any organic, life, living system that fu ...

Charles Darwin
explored peloria in ''Antirrhinum'' (snapdragon) while researching the inheritance of floral characteristics for his ''
The Variation of Animals and Plants Under Domestication#REDIRECT The Variation of Animals and Plants Under Domestication {{Redirect category shell, {{R from move {{R from miscapitalization {{R unprintworthy ...
''. Later research, using ''
Digitalis purpurea ''Digitalis purpurea'', the foxglove or common foxglove, is a poisonous species of flowering plant in the Plantago, plantain family (biology), family Plantaginaceae, native plant, native to and widespread throughout most of temperateness, tempera ...

Digitalis purpurea
'', showed that his results were largely in line with
Mendelian Mendelian inheritance is a type of biological inheritance Inheritance is the practice of passing on private property, titles A title is one or more words used before or after a person's name, in certain contexts. It may signify either ...

Mendelian
theory.


Symmetry groups

If we consider only those flowers which consist of a single flower, rather than a
flower head A pseudanthium (Greek for "false flower") is an inflorescence that resembles a flower. The word is sometimes used for other structures that are neither a true flower nor a true inflorescence. Examples of pseudanthia include flower head, composite ...
or other form of
inflorescence An inflorescence is a group or cluster of flower A flower, sometimes known as a bloom or blossom Cherry blossoms in Paris in full bloom. In botany, blossoms are the flowers of stone fruit fruit tree, trees (genus ''Prunus'') and of some ...
, we can categorize their symmetries into a relatively small number of two-dimensional symmetry groups. These groups are characterized by two types of symmetries: reflection (or mirror) symmetries, and rotational symmetries. Figures that are left invariant under reflections about a single axis have reflection symmetry, which is described by the
cyclic group In group theory The popular puzzle Rubik's cube invented in 1974 by Ernő Rubik has been used as an illustration of permutation group">Ernő_Rubik.html" ;"title="Rubik's cube invented in 1974 by Ernő Rubik">Rubik's cube invented in 1974 by Er ...

cyclic group
of
order Order or ORDER or Orders may refer to: * Orderliness Orderliness is associated with other qualities such as cleanliness Cleanliness is both the abstract state of being clean and free from germs, dirt, trash, or waste, and the habit of achieving a ...
2, C_2 (sometimes denoted Z_2). Figures that are left invariant under rotations by 2\pi/n have a rotational symmetry belonging to the
cyclic group In group theory The popular puzzle Rubik's cube invented in 1974 by Ernő Rubik has been used as an illustration of permutation group">Ernő_Rubik.html" ;"title="Rubik's cube invented in 1974 by Ernő Rubik">Rubik's cube invented in 1974 by Er ...

cyclic group
of order n, C_n (or Z_n). Many flowers that are invariant under rotations by 2\pi/n are also invariant under reflections about n distinct axes, the combination of these two symmetries forms the larger
dihedral group In mathematics, a dihedral group is the group (mathematics), group of symmetry, symmetries of a regular polygon, which includes rotational symmetry, rotations and reflection symmetry, reflections. Dihedral groups are among the simplest example ...
of dimension n, D_n (which has order 2n). Flowers with
bilateral symmetry Symmetry in biology refers to the symmetry observed in organisms, including plants, animals, fungi, and bacteria. External symmetry can be easily seen by just looking at an organism. For example, take the face of a human being which has a pla ...
, such as
orchids Orchidaceae ( ), commonly called the orchid family, is a diverse and widespread family In human society A society is a Social group, group of individuals involved in persistent Social relation, social interaction, or a large social ...

orchids
have reflection symmetry about a single axis and no rotational symmetry, meaning that they are described simply by the reflection group C_2.
Monocots Monocotyledons (), commonly referred to as monocots, (Lilianae ''sensu'' Chase & Reveal) are grass and grass-like flowering plants (angiosperms), the seeds of which typically contain only one Embryo#Plant embryos, embryonic leaf, or cotyledon. The ...

Monocots
are identifiable by their
trimerousMerosity (from the greek "méros," which means "having parts") refers to the number of component parts in a distinct whorl of a plant structure. It is most commonly used in the context of flowers where it refers to the number of sepals in a whorl o ...
petals upright=1.4, Diagram showing the parts of a mature flower. In this example the perianth is separated into a calyx (sepals) and corolla (petals) Petals are modified leaves A leaf (plural leaves) is the principal lateral appendage of the ...

petals
, meaning that they are often invariant under rotations by 2\pi/3 and thus have
rotational symmetry Rotational symmetry, also known as radial symmetry in geometry, is the property a shape has when it looks the same after some rotation by a partial turn. An object's degree of rotational symmetry is the number of distinct orientations in which it ...
. Monocots that exhibit rotational symmetry but not mirror symmetry (for instance, if their petals exhibit
chirality Chirality is a property of important in several branches of science. The word ''chirality'' is derived from the (''kheir''), "hand", a familiar chiral object. An object or a system is ''chiral'' if it is distinguishable from its ; that is, i ...

chirality
) are described by the cyclic group of order 3, C_3, and monocots with both rotational symmetry and reflection symmetry about 3 axes are described by the dihedral group of dimension 3, D_3.
Eudicots The eudicots, Eudicotidae or eudicotyledons are a clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyly, monophyletic – that is, composed of a common ancestor and all its lineage ...
with tetramerous or pentamerous petals are often invariant under rotations by \pi/2 or 2\pi/5. Again, whether they also have mirror planes decides whether they belong to dihedral (D_4 and D_5) or cyclic groups (C_4 or C_5). We can see the trend forming that, in general, the order of the cyclic group or dimension of the dihedral group that describes a flower's symmetry will correspond to the
merosityMerosity (from the greek "méros," which means "having parts") refers to the number of component parts in a distinct whorl of a plant structure. It is most commonly used in the context of flowers where it refers to the number of sepal Image:Mature ...
of its petals. However, the
sepals upright=1.4, Diagram showing the parts of a mature flower. In this example the perianth is separated into a calyx (sepals) and corolla (petals) A sepal ( or ) is a part of the flower A flower, sometimes known as a bloom or blossom, is the ...
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 D_6 or C_6. Some composite flowers may also have at least a superficial cyclical or dihedral symmetry. How exact this symmetry is depends on the structure of the head of the flower. Even in monocots and eudicots, flower symmetries are rarely perfect, as any imperfections in the petals will result in imperfect invariance under rotations or reflections.


See also

*
Patterns in nature Patterns in nature are visible regularities of form found in the natural world. These patterns A pattern is a regularity in the world, in human-made design, or in abstract ideas. As such, the elements of a pattern repeat in a predictable man ...
*
Phyllotaxis In botany Botany, also called , plant biology or phytology, is the science of plant life and a branch of biology. A botanist, plant scientist or phytologist is a scientist who specialises in this field. The term "botany" comes from the Ancie ...
*
Symmetry in biology Symmetry in biology refers to the symmetry observed in organisms, including plants, animals, fungi, and bacteria. External symmetry can be easily seen by just looking at an organism. For example, take the face of a human being which has a plan ...
*
Whorl (botany) In botany, a whorl or verticil is an arrangement of leaves A leaf (plural leaves) is the principal lateral appendage of the vascular plant plant stem, stem, usually borne above ground and specialized for photosynthesis. The leaves, stem ...


References


Bibliography

* * * * {{Patterns in nature
Plant morphology ''Plant morphology'' is the field in botany that studies the diversity in forms, with the naked eye or slight optical magnification. This is opposed to plant anatomy (see :Plant anatomy) that needs to cut into plants to be able to study its subject, ...