The influence of grain boundaries and natural convection on the birth and development of morphological instability is investigated during the directional solidification of succinonitrile-based alloys (succinonitrile–0.5 wt% acetone and succinonitrile–1.5 wt% water) in a cylindrical crucible. Continuous and in situ observation of the solid–liquid interface by optical methods is carried out through the melt. It is in particular shown that grain boundaries do not always cause morphological instability early and that a dynamical grooving of grains may occur by the propagation of channels, emanating among others from depressions in the solid ridges aside grain boundaries. Then, the accommodation of a grain boundary in the cellular array is presented. As convection in the melt is strong enough in the experiments for solute sweeping over the growth front to be significant, a delay of morphological instability is furthermore observed downstream in regions adjacent to well-oriented solid ridges.