Bark anatomy and intercellular canals in the stem of Delarbrea paradoxa (Araliaceae)

The anatomy of the primary tissues and secondary phloem in the stem of Delarbrea paradoxa Vieill. (Araliaceae) was examined with emphasis on structure and topography of secretory canals. Secretory canal systems of primary (axial canals in cortex and pith, radial canals in medullary rays) and secondary origin (axial canals in secondary phloem, radial canals in rays of secondary phloem and secondary xylem) were distinguished. Two distinct types of axial parenchyma (sheath parenchyma near axial secretory canals, and phloem parenchyma associated with conducting elements) occur in the secondary phloem. Distribution, size and number of cells per strand, occurrence of starch, and mode of transformation during phloem collapse serve to distinguish these two types. Three stages of secretory canal development (canal formation, active secretion, and senescence) were distinguished on the basis of TEM observations. The secretory canal lumina are formed simultaneously with the differentiation of meristematic cells into epithelial secretory cells. During the active secretion phase the epithelial cells contain leucoplasts aggregated into small groups, each accompanied by 2 to 3 mitochondria. These aggregations indicate terpene production in the cell. The secretion of terpenes is accompanied by swelling and loosening of the cell walls facing the canal lumina. Secretory processes were not indicated in the highly vacuolated senescent epithelial cells.