Descriptions of Silurian and Lower Devonian stomata based on cuticles, coalified compressions and permineralizations reveal similarities with those in mosses and certain ferns, and facilitate understanding about the mechanism of guard cell movements.
A detailed survey of stomatal complexes, substomatal chambers and cortical tissues in Rhynie Chert plants suggest adaptations to reduce water loss in peristomatal regions and these, together with a specialized parenchymatous tissue with an extensive intercellular space system at the base of the substomatal chamber, point to high water use efficiency.
Lower stomatal frequencies are discussed in relation to water stress and postulated high atmospheric carbon dioxide concentrations. Stomatal numbers and distribution in axial Silurian and basal Devonian fossils form the basis for speculation on the selective pressures (e.g. the generation of a transpiration stream, H2O and nutrient acquisition, temperature control) that led to the evolution of stomata, although the fossil record provides no direct evidence for evolutionary pathways.