Apparition de l'asymétrie foliaire chez les plantules du Begonia subvillosa et du B. fagifolia (Begoniaceae)

Abstract

The leaves of the majority of Begonia species are asymmetrical. To shed some light on the nature of this asymmetry, the authors followed the development of the first forming leaves in two species with very different adult shapes: Begonia subvillosa, which has strongly asymmetrical leaves and palmate venation, and B. fagifolia, which has pinnate venation and symmetrical leaves. Key stages of the development were identified. First, there is no asymmetry within the embryo. In both species, the leaves of the first two nodes are asymmetrical in the early stages. However, they lose this asymmetry during development and show a palmate venation. From the third node on, the two species show some differences. The leaf of B. subvillosa keeps its initial asymmetry and part of its blade develops into an auricle, unlike the leaf of B. fagifolia. From the fourth node on, the leaf of B. fagifolia is pinnately veined. The central area of the leaf blade of both species settles to its adult shape much earlier in development than other areas of the leaf. The different parts of the blade acquire their adult characteristics successively according to different timing sequences. Key words: Begoniaceae, development, leaf, morphogenesis, symmetry.