This structure was formerly regarded as pointing to the fusion of two organs, and the pale was considered by Robert Brown to represent two portions soldered together of a trimerous perianth - whorl, the third portion being the " lower pale."
A flower in which the parts are arranged in twos is called dimerous; when the parts of the whorls are three, four or five, the flower is trimerous, tetramerous or pentamerous, respectively.
The symmetry which is most commonly met with is trimerous and pentamerous - the former occurring generally among monocotyledons, the latter among dicotyledons.
- Diagram of the trimerous symmetrical flower of Iris.
- Diagram of the symmetrical trimerous flower of Fritillary (Fritillaria).
In the Euphorbiaceae we have an excellent example of the gradual suppression of parts, where from an apetalous, trimerous, staminal flower we pass to one where one of the stamens is suppressed, and then to forms where two of them are wanting.