Summary• Atriplex halimus , a monoecious Chenopodiaceae , produces flowers displaying two basic architectures.• The first architectural pattern is made of staminate pentamerous flowers with an external whorl of yellowish tepals and an internal whorl of stamens. The second architectural pattern consists of female flowers with a single carpel enclosed within two opposite bracts. In both architectures, bisexual flowers and flowers of the unexpected sex were detected leading to the occurrence of up to six floral phenotypes on the same individual.• Daylength and light intensity affected sex ratio and flower distribution between both architectural patterns. Short days and low light irradiance promoted femaleness and bracteate floral architecture. Flower position on a reproductive axis and geographical origin of the plant (genotype) also affected sex and architecture ratios.• Thus, all the genetic information required for the production of both floral architecture and sexual organ types is present in each A. halimus plant but endogenous and environmental cues determine the fate of the floral meristems. These results are discussed in relation to classical models of genetic control of floral morphogenesis.

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Classiquement, la fleur femelle d'Atriplex halimus L. est sans périanthe. Elle possède deux bractéoles, un ovaire, et un style. D'après les bractéoles, on distingue, chez chaque individu de deux populations tunisiennes, trois phénotypes de fleurs femelles, chacun se distribuant en trois types selon la longueur du style. La fleur mâle produit trois catégories de pollen. C'est la seule hétérostylie intra-individuelle connue qui, de plus, s'exprime chez des fleurs unisexuées. Ces résultats remettent en cause la notion classique d'individu. Les auteurs discutent la possibilité d'un processus impliquant, indirectement, hybridation et introgression entre A. halimus et A. nummularia.

Abstract.-Atriplex halimus L. is a highly polymorphic species that is widespread, perennial, monoecious and polygamous. We have shown an intra-and interindividual polymorphism for several floral morphological characters including styles, ovule types and radicle orientation. A careful analysis of the female flowers with two bracteoles is performed in one plant population from Tadjerouine (Tunisia). It shows that the ovules of different flowers of individual plants are either amphitropous or campylotropous. An anatomical and histological study of pollinated ovules is done in order to search for putative variation in the embryogenesis process correlated with the ovule type. We also found unique features, both a suspensor consisting in two cellular rows and the lack of a typical torpedo stage. Moreover, this study provides evidence for some abnormal development of the embryos and for polyembryony in the amphitropous ovules. Mots clés : Atriplex halimus -polymorphisme ovulaire -embryogenèse zygotique -polyembryonie.