Stipules and colleters of the mangrove Rhizophoraceae: morphology, structure and comparative significance.

One characteristic possessed by the entire mangrove Rhizophoraceae is the strong enclosure of its young shoots by conspicuous rounded or flattened stipules. Young leaves are conspicuously immersed in resinous to milky exudates from colleters (multicellular external secretory emergences) located at the adaxial stipule bases. We systematically studied and compared morphological and structural features of stipules and their colleters from 18 taxa of the mangrove Rhizophoraceae. Three types of sclereid idioblasts (Ceriops and Rhizophora), collenchymas, and thick cuticles, were found to provide a structural basis for the mechanical support of stipules. Several to hundreds (35-580) of finger-like colleters aggregate into genus-specific shapes: rectangular to trapezoidal (Bruguiera), triangular (Ceriops and Kandelia), or as a band (Rhizophora). Number of rows, total number of colleters per stipule, and their individual size, also vary by taxon, and have taxonomic value. All colleters of this subfamily are considered anatomically 'standard', regardless of whether they appear as short-stalked rods (Bruguiera), long-stalked rods (Ceriops, Kandelia) or acuminate rods (Rhizophora, no stalk). Colleters in these taxa are comprised of a central axis of slender, elongated cells and an outer palisade-like epidermis, with a secretory function, perpendicular to the axis. Based on stipule and colleter characteristics, Ceriops and Kandelia are closely related, but Rhizophora shows more derived features. The structural and mechanical protection provided by stipules and colleter exudates may help shield the young shoots of these mangrove plants from their harsh environments.