zygomorphism

Zygomorphism, also zygomorphy, is a morphological condition in which an organism or structure has bilateral symmetry with a single plane of symmetry. The term derives from Greek zygon "yoke" and morphe "form." In botany, zygomorphy describes flowers that can be divided into mirror-image halves by only one vertical plane, in contrast with actinomorphy, where the flower has radial symmetry with multiple planes of symmetry. Zygomorphic flowers are widespread among angiosperms and are especially common in the Fabaceae (legumes), where many species exhibit papilionaceous flowers, and in many orchids and related groups. The one-plane symmetry is often associated with pollination by specialized visitors, guiding their approach along a defined path and potentially increasing pollination efficiency. Developmentally, zygomorphy arises from asymmetric growth controlled by genetic networks that pattern dorsal and lateral organs, although the specific mechanisms vary among lineages.

In zoology, bilateral symmetry is the predominant condition; the term zygomorphy is mainly used in botanical contexts, whereas zoological descriptions typically refer to bilateral symmetry rather than zygomorphy. The presence or absence of zygomorphy can have phylogenetic and ecological significance, reflecting evolutionary relationships and pollination strategies.

See also: bilateral symmetry, actinomorphy, floral symmetry, plant morphology.