Bennettitalean stomata

Not only the topography, but also the size of stomata is fairly constant in the Bennettitales. Occasional departures from the typical dimensions were probably related to polyploidy. Specialized subsidiary cells, examined with SEM, formed a cupola over the guard cells. The ontogeny of syndetocheilic stomata as deduced from aborted apparatuses was mesogenous-tetralabrate. Distribution of compressed stomata on the margins of stomatal zones suggests higher concentration of a morphogen responsible for positional information near the veins and the direction of positional signals spreading from them. More orderly arrangement of stomata in the median portion of an intercostal zone is attributed to inhibitory action of developing meristemoids, while the differences in stomatal frequencies on the opposite margins of a stomatal zone were probably caused by microshades from the ribbed costae.