radial symmetry

Floral symmetry

Floral symmetry refers to whether, and how, a flower can be divided into two or more identical or mirror-image parts.

Most flowers are actinomorphic ("star shaped"), meaning they can be divided into symmetrical halves by more than one longitudinal plane passing through the axis, much as a pie can be cut into several equal and identical pieces. In these flowers, the petals are usually similar in shape, size, and color. Such flowers are also called radially symmetrical or regular flowers. Examples of actinomorphic flowers are the lily (Lilium, Liliaceae) and the buttercup (Ranunculus, Ranunculaceae).

Zygomorphic ("yoke shaped") 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). Zygomorphic flowers generally have petals of two more different shapes, sizes, and colors. Least commonly, flowers may be asymmetrical; they cannot be divided into two identical or mirror-image halves on any plane. Such flowers are typical of most members of the Zingiberales, such as cannas and various gingers. In most cases, different kinds of floral symmetry are linked to particular pollinators.

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

Some familiar and seemingly actinomorphic flowers, such as those of daisies and dandelions (Asteraceae), are actually clusters of tiny zygomorphic flowers arranged into a radially symmetric inflorescence.

Peloria or a peloric flower refers to an 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.

References

  • P.K. Endress. 2001. Evolution of floral symmetry. Current Opinions in Plant Biology 4: 86-91 (abstract here)
  • P.R. Neal, A. Dafni and M. Giurfa. 1998. Floral symmetry and its role in plant-pollinator systems: terminology, distribution, and hypotheses. Annual Review of Ecology and Systematics 29: 345-373 (abstract here).

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