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 are often terete or approach the terete form. The full description of foliar anatomy concentrates on the unusual epidermal features and internal canal. The exposed epidermis is abaxial, but in a few species, in addition to the abaxial epidermis, a small amount of adaxial epidermis is also exposed in a foliar groove. In those leaves covered entirely by abaxial epidermis, the adaxial epidermis is still present but immersed within and surrounded by the mesophyll. It flanks the walls of an internal canal represented as a lacuna on cross sections. The bilaterally symmetrical arrangement of the vascular bundles, a characteristic of all examined leaves in section Rhizobium, divides the leaf into equilateral 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 lacking a canal, passing through a phase featuring an eccentrically located canal, and culminating in a centrally sited canal. Anatomical data support the monophyly of section Rhizobium, and the unifacial-leaved species constitute a distinctive clade within the section.