The induction of the well known and widespread phenomenon ‘stomatal patchiness’ has been attributed to a variety of potential causes: from low PPFD levels, all kinds of stress conditions to CO 2 -changes and even fungal infections. A mechanism which explains the occurrence of reproducible patterns of static (i.e. stable) stomatal patchiness at low PPFD levels is proposed. Further, experimental evidence is presented for the hypothesis that dynamic (i.e. not stable) stomatal patchiness is a consequence of heterogeneous water status in different parts of the leaf and can be induced by all ambient factors which cause such heterogeneities.