Directional solidification experiments were carried out on Al-12.3 at% Li alloys near the cell/dendrite transition. Experiments were performed both in microgravity and on the ground in vertical upward configuration. Reliable data for 3D solidification with a purely diffusive transport regime in the liquid have thus been obtained that are used to test current theories relative to primary spacing, tip undercooling and microsegregation. In particular, we show that the cell tips region can be satisfactorily described by the Saffman-Taylor equation for viscous fingering. Further, comparison of ground samples with those elaborated in microgravity has enabled the effects of solutal convection to be evidenced, both at the macroscopic scale (solid/liquid front shape, macrosegregation) and at the microscopic scale (cell tip composition, intercellular liquid fraction, primary spacing).