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.
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 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
Zygomorphic (" 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
and the flowers of most members of the
Lamiales (e.g.,
Scrophulariaceae and
Gesneriaceae). 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 due to pollinator decline.
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 Bellis perennis 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 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 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 explored peloria in Antirrhinum (snapdragon) while researching the inheritance of floral characteristics for his The Variation of Animals and Plants Under Domestication.
See also
-
Floral morphology
-
Patterns in nature
-
Phyllotaxis
-
Symmetry in biology
-
Whorl (botany)
Bibliography
