Floral development in Polygala myrtifolia (Polygalaceae) and its similarities with Leguminosae

Abstract

Floral development of Polygala myrtifolia was studied by the use of SEM, and compared with floral development in Leguminosae, with which Polygalaceae is grouped in Fabales. Five sepals and petals are formed in a helix, the direction of which is predetermined by the sequentially formed bracteoles. The two lateral sepals are delayed in their development, and the two lateral petals are suppressed early. For stamen initiation a hypothetical pathway is presented, in which the initiation is in two helical whorls. The abaxial, antesepalous stamen and the adaxial, antepetalous stamen are lost. In the gynoecium the late closure is distinct. Although the floral morphology of Polygalaceae and Leguminosae differs clearly, the two families do share some developmental features: (1) The formation of a racemose inflorescence, (2) the acropetal and helical initiation of floral organs, (3) the morphogenetic function of bracteoles, (4) the suppression and loss of organs, (5) the synchronous stamen differentiation (after asynchronous initiation) and late fusion of filaments, (6) the late closure of the gynoecium, and (7) the late expression of floral asymmetry. On the basis of these developmental similarities, the placement of Polygalaceae in Fabales becomes more plausible. Further insights in floral evolution of Fabales will be possible by means of a detailed phylogenetic analysis of developmental characters, based on a broadened data matrix.