ANATOMY OF THE THICK LEAVES IN DENDROBIUM SECTION RHIZOBIUM (ORCHIDACEAE)

Plants of section Rhizobium in the genus Dendrobium (Orchidaceae) are characterized by thick and fleshy leaves or by leathery leaves that ar e often terete or approach the terete form. The full description of fo liar anatomy concentrates on the unusual epidermal features and intern al canal. The exposed epidermis is abaxial, but in a few species, in a ddition to the abaxial epidermis, a small amount of adaxial epidermis is also exposed in a foliar groove. In those leaves covered entirely b y abaxial epidermis, the adaxial epidermis is still present but immers ed within and surrounded by the mesophyll. It flanks the walls of an i nternal canal represented as a lacuna on cross sections. The bilateral ly symmetrical arrangement of the vascular bundles, a characteristic o f all examined leaves in section Rhizobium, divides the leaf into equi lateral halves, the canal being centered along the midplane. Xylem of all vascular bundles faces the midplane of the leaf. We postulate that the canal, lined with cells of the adaxial surface, is the product of an evolutionary sequence of events commencing with the grooved leaf l acking a canal, passing through a phase featuring an eccentrically loc ated canal, and culminating in a centrally sited canal. Anatomical dat a support the monophyly of section Rhizobium, and the unifacial-leaved species constitute a distinctive clade within the section.