Stomatal structure is highly conserved across land plants – a symmetric pair of specialized guard cells delimits a central pore. However, when viewed from a developmental perspective, the patterning of the stomatal complex (the stoma and neighbouring cells) differs among taxa. Most hypotheses of stomatal evolution in angiosperms are based on comparative studies of mature stomata of both extant and fossil taxa, with a primary focus on three widely recognized stomatal types – anomocytic, paracytic and stephanocytic – which differ in the patterning of their neighbour cells.
Understanding evolutionary pathways requires a more explicit phylogenetic context than over-simplistic comparisons between dicotyledons (a non-monophyletic group) and monocotyledons. Such comparisons are most commonly exemplified by the model organisms Arabidopsis and rice, respectively. A recent paper in Annals of Botany presents a novel ultrastructural study of developing stomata in leaves of Amborella (Amborellales), Nymphaea and Cabomba (Nymphaeales), and Austrobaileya and Schisandra (Austrobaileyales), which represent the three earliest-divergent lineages of extant angiosperms. The authors show that similar mature stomatal phenotypes can result from contrasting morphogenetic factors. Loss of asymmetric divisions in stomatal development could be a significant factor in land plant evolution, with implications for the diversity of key structural and physiological pathways.