actinomorphy

Actinomorphy, or actinomorphic symmetry, is a property of flowers in which the floral parts are arranged around a central axis so that the flower can be divided into mirror-image halves by multiple planes of symmetry. The corolla is radially symmetric, allowing more than one plane of reflection through the axis. The term derives from Greek roots meaning “ray” and “form.”

In contrast, zygomorphy refers to bilateral symmetry, where the flower has a single plane of symmetry and a distinct left-right orientation. Zygomorphic flowers typically exhibit specialized floral forms that restrict the directions from which they can be pollinated.

Actinomorphy is widespread among flowering plants and is considered the ancestral state for many lineages. It occurs in a variety of dicot and monocot groups, including several basal angiosperms and numerous herbaceous taxa. While actinomorphic flowers are common in families such as Rosaceae and Caryophyllaceae, other lineages have evolved zygomorphy in particular genera or clades, illustrating the diversity of floral symmetry.

From an evo-devo perspective, the evolution of bilateral symmetry often involves changes in regulatory gene networks that alter petal and organ development; actinomorphy represents the default radial condition, with zygomorphy arising as a derived state in certain lineages, frequently associated with shifts in pollination strategies.

Ecologically, the radial form of actinomorphic flowers can support visitation by a broad range of pollinators, whereas bilateral, zygomorphic flowers are commonly linked to more specialized pollination relationships.