zygomorphic

Zygomorphic is a term used in biology to describe organisms or structures that display bilateral symmetry, meaning they can be divided into mirror-image halves by a single plane. The word derives from Greek zygos, meaning “yoked” or “joined,” and morphe, meaning “form.”

In botany, zygomorphic flowers have a single plane of symmetry, as opposed to actinomorphic flowers, which are radially symmetric with multiple planes of symmetry. Zygomorphy is common in several plant groups. For example, many members of the Fabaceae (legumes) exhibit papilionaceous, butterfly-like corollas with a distinct upper banner, two lateral wings, and a fused keel. Other families with examples of zygomorphic flowers include some members of Rubiaceae, Scrophulariaceae, and Orchidaceae. The characteristic anatomy and floral architecture of zygomorphic flowers are often linked to specialized pollination strategies, guiding pollinators to precise contact points on the flower.

Evolutionarily, zygomorphy has arisen multiple times across different lineages and can contribute to pollinator specialization, though not all related species share the trait. In broad usage, the term can also describe bilateral symmetry in other organisms, but in practice it is most frequently applied to floral morphology in plants.

See also: symmetry, actinomorphy, floral morphology.