Horsetails are an ancient group of land plants that possess many unusual features, including the structure and development of the apertures (stomatal pores) in the epidermis. In addition to a symmetric pair of guard cells, stomata in Equisetum are delimited by an overlying pair of neighbour cells with characteristic vault-like radiating thickenings. Stomatal development involves a well-defined series of asymmetric and symmetric mitoses.
The results of Cullen and Rudall contribute to our understanding of the diverse patterns of stomatal development in land plants. They add to a considerable catalogue of highly unusual traits of horsetails – one of the most evolutionarily isolated land-plant taxa.