Comparative vegetative anatomy and systematics of Vanilla (Orchidaceae)

Vanilla is a pantropical genus of green-stemmed vines bearing clasping (aerial) and absorbing (terrestrial) roots. Most vanillas bear normal, thick foliage leaves; others produce fugacious bracts. Seventeen species, including both types were studied. Foliage leaves of Vanilla are glabrous, have abaxial, tetracytic stomatal apparatuses, and a homogeneous mesophyll. Species may or may not have a uniseriate hypodermis. Crystals occur in the foliar epidermises of some species, but all species have crystalliferous idioblasts with raphides in the mesophyll. Vascular bundles in leaves are collateral and occur in a single series alternating large and small. Sclerenchyma may or may not be associated with the vascular bundles. Scale leaves may be crescent or C-shaped and usually have abaxial stomatal apparatuses. A hypodermis may or may not be present; the mesophyll contains raphide bundles in idioblasts. Vascular bundles are collateral and occur in a single row sometimes aligned close to the adaxial surface. They may or may not be associated with sclerenchyma. Stems of leafy vanillas show a sclerenchyma band separating cortex from ground tissue; stems of leafless vanillas do not show a sclerenchyma band. Ground tissue of the stem may consist solely of assimilatory cells or mixed assimilatory and water-storage cells. In some species centrally located assimilatory cells are surrounded by layers of water-storage cells. A uniseriate hypodermis is present in all stems. Sclerenchyma may completely surround the scattered collateral vascular bundles, occur only on the phloem side, or be absent. Both aerial and terrestrial roots are notable for their uniseriate velamen the cell walls of which may be unmarked or ornamented with anticlinal strips. Exodermis is uniseriate; the cells vary from barely thickened to strongly thickened. Only the outer and radial walls are thickened. Cortical cells of aerial roots generally have chloroplasts that are lacking from the same tissue of terrestrial roots. Raphide bundles occur in thin-walled cortical idioblasts. Endodermis and pericycle are uniseriate; pericycle cells are all ?-thickened opposite the phloem. Cells of the endodermis are either ?- or ∪-thickened opposite the phloem. Vascular tissue may be embedded in thin- or thick-walled sclerenchyma or in parenchyma. Metaxylem cells are always wider in terrestrial than in aerial roots of the same species. Pith cells are generally parenchymatous but sclerotic in a few species.