rystal growth experiments were conducted using potassium alum and calcite crystals in aqueoussolution under both non-stirred and stirred conditions to elucidate the mechanism for size-dependent(proportionate) and size-independent (constant) crystal growth. Growth by these two laws can be dis-tinguished from each other because the relativesize difference among crystals is maintained duringproportionate growth, leading to a constant crystal size variance (b2) for a crystal size distribution(CSD) as the mean size increases. The absolutesize difference among crystals is maintained duringconstant growth, resulting in a decrease in size variance. Results of these experiments show that forcentimeter-sized alum crystals, proportionate growth occurs in stirred systems, whereas constant growthoccurs in non-stirred systems. Accordingly, the mechanism for proportionate growth is hypothesizedto be related to the supply of reactants to the crystal surface by advection, whereas constant growth isrelated to supply by diffusion. Paradoxically, micrometer-sized calcite crystals showed proportionategrowth both in stirred and in non-stirred systems. Such growth presumably results from the effects ofconvection and Brownian motion, which promote an advective environment and hence proportionategrowth for minute crystals in non-stirred systems, thereby indicating the importance of solution veloc-ity relative to crystal size. Calcite crystals grown in gels, where fluid motion was minimized, showedevidence for constant, diffusion-controlled growth. Additional investigations of CSDs of naturallyoccurring crystals indicate that proportionate growth is by far the most common growth law, therebysuggesting that adection, rather than diffusion, is the dominant process for supplying reactants to crystal surfaces.