Fig. 10.3-18. Transverse section of leaf of kafir-lily (Clivia, an ornamental monocot). Examine the guard cells (horizontal arrows) and their outer ledges (vertical arrows): the cuticle on the guard cells differs from that over all the rest of the leaf, having accumulated to a tremendous thickness. When guard cells lose turgor and shrink, they move toward each other, closing the stomatal pore as the facing walls of the guard cells push against each other. But in species that have ledges like this, the ledges are the first region to make contact and seal the pore, then the facing walls push against each other: there are actually two seals that keep water from squeezing out through a closed stoma. Some species even have inner ledges also, providing a third seal.
Even when the stomatal pore is open, the ledges help retain water by acting somewhat like the grooves of yucca leaves or the overarching subsidiary cells of Hakea: because the ledges are so tall, they create a miniature tube with the stomatal pore at one end.The guard cells are much smaller -- both narrower and thinner -- than the surrounding epidermis cells (from this view, we cannot tell if they are ordinary epidermis cells or subsidiary cells). (Dark red bodies in the guard cells are nuclei; the dots in the mesophyll cells are chloroplasts.)