Several developmental and morphogenetic factors govern the evolution of stomatal patterning in land plants

Summary We evaluate stomatal development in terms of its primary morphogenetic factors and place it in a phylogenetic context, including clarification of the contrasting specialist terms that are used by different sets of researchers. The genetic and structural bases for stomatal development are well conserved and increasingly well understood in extant taxa, but many phylogenetically crucial plant lineages are known only from fossils, in which it is problematic to infer development. For example, specialized lateral subsidiary cells that occur adjacent to the guard cells in some taxa can be derived either from the same cell lineage as the guard cells or from an adjacent cell file. A potentially key factor in land‐plant evolution is the presence (mesogenous type) or absence (perigenous type) of at least one asymmetric division in the cell lineage leading to the guard‐mother cell. However, the question whether perigenous or mesogenous development is ancestral in land plants cannot yet be answered definitively based on existing data. Establishment of ‘fossil fingerprints’ as developmental markers is critical for understanding the evolution of stomatal patterning. Long cell–short cell alternation in the developing leaf epidermis indicates that the stomata are derived from an asymmetric mitosis. Other potential developmental markers include nonrandom stomatal orientation and a range of variation in relative sizes of epidermal cells. Records of occasional giant stomata in fossil bennettites could indicate development of a similar type to early‐divergent angiosperms.ContentsSummary 598 I. Introduction 599 II. One‐cell spacing 601 III. Paired guard cells 601 IV. Asymmetric cell divisions 604 V. Leaf development and stomatal orientation 606 VI. Lateral subsidiary cells 607 VII. Amplifying divisions 608 VIII. Conclusions 610Acknowledgements 611References 611