Résumé
Two species of Lathyrus (L. latifolius and L. sylvestris) were compared using allozymes and quantitative vegetative and reproductive traits. The variation in these characters was analysed in several populations of the two species located in south-western, central and northern France. In a site where the two species are syrapatric, intermediate plants are described. The two species can be identified by discriminant alleles. The genetic structure observed for intermediate plants in the zone of sympatry indicates, paradoxically, the occurrence of a strong reproductive barrier between the two species. The discriminant analysis performed on the quantitative traits readily distinguishes the two species and confirms the reproductive isolation between them. The quantitative character that best discriminates between the species is the number of ovules per ovary, a variable which has non-overlapping distributions in the two species. We discuss how this character, usually ignored, may be better for distinguishing species than some traditionally used reproductive traits such as the number of seeds per fruit. The vegetative traits studied (e.g. degree and intensity of branching) also differ significantly between the two species. Divergent evolution of these traits in the two species is discussed in relation to differences in the original habitats of the two species.