The development of dendrites from an initially flat, stationary solid/liquid interface in a transparent material was observed and photographed. It was found that the dendrites originated from preferred areas on the interface immediately adjacent to grain boundaries, subboundaries, and trapped foreign particles. The built-in large-amplitude distortions of the interface adjacent to these imperfections initiated dendritic growth under conditions which did not produce dendritic growth on imperfection-free regions of the interface. It is concluded that considerations of the stability of planar interfaces must take into account the influence of the imperfections which are present in almost all solids.