La placentation axillaire chez Ochna atropurpurea

Abstract

In Ochna atropurpurea DC., five to seven crescent-shaped gynoecial primordia are formed. They become continuous with each other soon after their inception and thus form a syncarpous gynoecium whose final appearance is pseudoapocarpous (ecological apocarpy). There are as many ovules as gynoecial primordia. Each ovule arises from the biseriate tunica and the corpus on the periphery of the floral apex in the axil of a gynoecial primordium. The presence of across zone (Querzone) could not be clearly demonstrated. Since the position of ovule inception is equivalent (homotopous) to that of axillary branches, we propose to term this configuration axillary placentation. If it is agreed upon that axillary branches arise from the stem (caulome), then the axillary placentation of this species is also of cauline origin. It then follows as a logical consequence that the gynoecium is acarpellate. Whether this acarpellate condition is primitive or derived from a carpellate organization remains unresolved. In the case of a derivation from a carpellate gynoecium, one would have to postulate a phylogenetic shifting of ovule inception and (or) the reduction of the adaxial portion of a carpel whose margins were confluent and thus formed a more or less cylindrical primordium. Congenital or phylogenetic fusion of the cross zone of a peltate carpel with the floral apex is not even a theoretically possible hypothesis since it is not in agreement with any conceivable ontogenetic and phylogenetic processes.