The epidermis in Passerina (Thymelaeaceae): structure, function and taxonomic significance

Epidermal features were studied in all 17 species of Passerina, a genus end emic to southern Africa. Leaves in Passerina are inversely ericoid, the ada xial surface concave and the abaxial surface convex. Leaves are inversely d orsiventral and epistomatic. The adaxial epidermis is villous, with unicell ular, uniseriate trichomes and relatively small thin-walled cells, promotin g flexibility of leaf margins owing to turgor changes. in common with many other Thymelaeaceae, abaxial epidermal cells are large and tanniniferous wi th mucilaginous cell walls. The cuticle is adaxially thin, but abaxially we ll developed, probably enabling the leaf to restrict water loss and to tole rate high light intensity and UV-B radiation. Epicuticular waxes, present i n all species, comprise both soft and plate waxes. Epidermal structure prov es to be taxonomically important at family, genus and species levels. Inter specific differences include arrangement of stomata and presence or absence of abaxial epidermal hair. Other diagnostic characters of the abaxial epid ermal cells are arrangement size and shape, cuticular ornamentation and pre sence or absence of wax platelets. Two groups of species on the basis of ab axial epidermal cell orientation are recognised. Many leaf epidermal featur es in Passerina are interpreted as structural adaptations to the Mediterran ean climate of the Cape.