Stomata are microscopic structures which exist on the surfaces of many land plants that enable the uptake of CO2 and regulation over water release. The fossil evidence suggests that stomata are ancient as they have been observed on early land plants which existed well 400 million years. Whilst much is known about stomata and their development in vascular land plants, relatively little is known in non-vascular land plants. This thesis identifies a core toolbox of stomatal developmental genes in the non-vascular land plant Physcomitrella patens which share orthology with vascular land plants gene equivalents. To ascertain whether P. patens shares the same genetic mechanisms to alter its stomatal development under different CO2 conditions experiments were conducted on mutants of key patterning genes required for stomatal development. It was found that P. patens does not respond using the same genetic equivalents to that of Arabidopsis thaliana. In conclusion, this thesis outlines the molecular mechanisms required to build stomata are very ancient and shared between distant land plant lineages and as such adds more evidence for a monophyletic origin of stomata.