This paper describes the distribution, structure and histochemistry of the colleters in vegetative and reproductive apices of Alibertia sessilis (Vell.) K. Schum., a native Rubiaceae species of the Brazilian "cerrado". In this family, dendroid colleters have been associated with the presence of bacterial nodules in the leaves. Samples were prepared according to common light and scanning electron microscopy techniques, and histochemical assays involved freshly collected material. Colleters are dendroid and occur on the adaxial surface of stipules, bracts and sepals; they consist of a multicellular and multiseriate central axis of parenchymatous nature, covered by digitiform or sharply pointed epidermal cells of irregular sizes, joined together only in the proximal portion and separated from each other in the distal portion. The columnar cells are axially elongated, with thick cell walls, prominent nucleus and reduced cytoplasm, developed vacuole with lipid inclusions and phenolic compounds. The epidermal cells have thin walls, a conspicuous nucleus, abundant cytoplasm and a poorly developed vacuome. The colleters are devoid of cuticle. The secretion is more abundant in the vegetative apex, impregnating the leaf buds and the stipules, forming a spherical cap that is hard, bright, and hydrophobic. In all analyzed samples, the histochemical assays detected polysaccharides, proteins and lipids in the protoplast of both epidermal and columnar cells; phenolic compounds were detected solely in the columnar cells. The resinous nature of the exudate was confirmed with the use of the diethylic ether solvent. Bacterial leaf nodules were not found.
Alibertia sessilis; anatomy; dendroid colleters; histochemistry; Rubiaceae
