Variable timing of developmental progression in the stomatal pathway in Arabidopsis cotyledons.

•  Stomatal production depends on the rates of precursor cell formation (e.g. the formation of meristemoids by asymmetric division), and of developmental progression (e.g. the division of guard mother cells). It is not known whether these rates follow steady-state kinetics or are variable. •  The timing of development was scored in Arabidopsis cotyledons in fixed and living tissue using the dental resin impression method. •  Cotyledons exhibited much less of a longitudinal gradient in stomatal formation than leaves. The timing of the appearance of stomatal and precursor cells during cotyledon development varied between individual plants. Precursor cell formation ceased much earlier in the adaxial than in the abaxial epidermis. Meristemoids are precursors that form guard mother cells. The ratio of these cell types varied greatly in different plants as well as in the same epidermis through time. There was also considerable variability in the duration of the meristemoid stage between individual cell lineages. •  Precursor cell production follows non-steady-state kinetics. Early steps in the pathway are not necessarily synchronized, but later steps, such as the conversion of meristemoids to guard mother cells, sometimes appear to be coordinated.