The spacing between cells and dendrites in directional solidification of binary alloys is analyzed and controlled experimentally. An organic alloy analogue with laser dye allows in situ imaging and perturbations of the cell and dendrite structure. Perturbations permit accurate control of the microstructure, either by guiding the initial formation of the pattern or by triggering subcritical transitions between stable micro structures. This control of microstructure through perturbation allows us to investigate the modes of instability of crystal growth patterns. (C) 2004 Elsevier B.V. All rights reserved.