The structure of (Au) bipyramids, very promising for their specific plasmonic properties, is fully analyzed for gradual stages of growth. From combined use of HRTEM (High resolution transmission electronic microscopy) in specific conditions and SAED (selected area electronic diffraction) analysis, the atomic structure of these elongated objects of different sizes is extracted (from 2 to 60 nm). In the case of silver(I)-assisted growth from citrate capped seeds, a clear penta-twinned structure, made by the superimposition of specific pairs of crystallographic zones is found for the different size nanoparticles representing the different steps of the growing bipyramids. The experimental results were analyzed in the framework of a recent atomistic approach developed for metal-environment interactions to account for the stability of multi-twinned nanorods or bipyramids in a complex environment. Efficient in describing the multi-twinned nanoparticles and the drastic effects of environment on the stabilization of elongated multi-twinned shapes, this model supports the experimental features of bipyramids and nanorods formation from citrate seeds in function of the nature of the surface chemical ligands: citrate seeds in a growth solution prepared without silver(I) yields the formation of a few penta-twinned nanorods, while the silver(I)-assisted growth process mainly leads to penta-twinned bipyramids. The conservation of a bulk penta-twinned structure inherited from isotropic decahedral (i-Dh) seeds all along the growth is retrieved by this approach for two different environments.