ORIGIN OF SYMMETRY IN FLOWERS

Abstract

Phylogenetically significant floral features include number, order of initiation and position of appendages, symmetry, aestivation, degree of fusion, and loss of parts. These features can be studied ontogenetically, and the ontogenies compared among related taxa to yield information on the relative time during ontogeny when the feature is determined or manifested. The family Leguminosae has three subfamilies which differ in floral symmetry, aestivation, and in location and degree of fusion of organs. It is proposed that these kinds of features arise early in ontogeny (during organogeny) and that as a consequence they are considerably more stable than those features which arise at mid-stage (during form change and enlargement) or at late stages of ontogeny (those which involve differentiation of tissues and cells). The mid-stage-determined features include many which separate related genera, while those determined late in ontogeny tend to characterize and separate related species. For any one feature, advanced taxa tend to express the feature precociously. For example, zygomorphy can be expressed even before organ initiation as an oval rather than circular shape of the floral apex. In other flowers, zygomorphy may be expressed first at sepal, petal, or stamen initiation, or it may be delayed until enlargement of petals or stamens. The order of organ initiation also is an expression of symmetry; in many legumes, members of each whorl are initiated sequentially from one side of the floral apex to the opposite side rather than simultaneously or helically. Elucidation of developmental differences between similar or related taxa can provide a basis for understanding how subfamilial and tribal differences could have evolved in the Leguminosae.