Organogénèse de la fleur pistillée du Begonia dregei et de l’Hillebrandia sandwicensis (Begoniaceae)

Abstract

The pistillate flower organogenesis of Begonia dregei Otto Dietr. (section Augustia) and Hillebrandia sandwicensis Oliver is described. Vascular anatomy and scanning electron microscopy show that the inferior ovary of B. dregei develops below the perianth, whereas the semi-inferior ovary of H. sandwicensis grows both above and below the insertion level of the perianth. A developmental study of both species showed that placental tissues have a mixed nature: parietal placentae develop from the ovary wall, whereas axile placentae are derived from the floral apex, with a gradual transition zone between the two tissues. Depending on the rate of growth of the two placental regions, the ovary becomes mostly axile or parietal, all intermediates being possible. Our study does not allow us to determine unambiguously which of the two types of placentation is the most primitive. Also, it is impossible to clearly ascertain the evolutionary status of the semi-inferior ovary of H. sandwicensis. However, ontogenetic results allowed us to shed new light on the nature of the ovary wall in Begonia. One may hypothesize that it is an intermediate structure between axial and appendicular organs, difficult to compare directly with a set of fused carpels and perianth parts, as postulated by the classical theory of the flower.