Organogénie de la feuille du Begonia radicans Vellozo et du Begonia scabrida (Begoniaceae)

Abstract

The asymmetrical leaf of Begonia raises both biological and mathematical questions. The leaf organogenesis of two species of Begonia is analyzed in this study, one with asymmetrical, palmately nerved leaves, Begonia scabrida A.DC, the other with quasi-symmetrical, pinnately nerved leaves, Begonia radicans Vellozo. Additional observations on Begonia fagifolia Fischer, a species of the second type, are included for comparison. In the three species studied, the leaf primordia are asymmetrical at initiation, notwithstanding mature leaf symmetry or nervation. In B. radicans, the leaf is asymmetrical at initiation and becomes quasi-symmetrical, while in B. scabrida, it remains asymmetrical throughout. The Begonia leaf thus represents a true case of asymmetry, related to the morphogenetic properties of the vegetative apex. None of the present theories of leaf organogenesis are able to fully account for this phenomenon. Leaf asymmetry represents an ontogenetic constraint in the evolutionary history of Begoniaceae. The hypothesis is presented that the palmately nerved leaf is a plesiomorphic state and the pinnately nerved leaf an apomorphic one that appeared repeatedly within the family. Key words: Begoniaceae, leaf, morphogenesis, angiosperm, phylogeny, apex.