Phytolacca dioica has a primary vascular system which includes medullary bundles. The primary structure of these bundles is composite, consisting of two to four collateral vascular strands with their phloem poles oriented toward a common center. A cambium is formed between the xylem and phloem of the strands and extends to enclose the phloem of the whole bundle. After a period of cambial activity the medullary bundles become amphivasal. As is typical of species with helical phyllotaxy, the primary vascular system is organized into sympodia. The medullary bundles form the distal portions of the median leaf traces and continue in a medullary position for the number of nodes equal to the denominator of the phyllotactic fraction characterizing a given stem. As a medullary bundle passes out into a leaf, two or three vascular strands pass inward from the vascular cylinder to form a new medullary bundle. The number of medullary bundles in a stem is, thus, maintained. Variations of this pattern occur in the basal regions of juvenile shoots and in the basal and apical regions of adult flowering shoots. The relationship between leaf arrangement and the passing of vascular strand into the pith is discussed and a new classification of vascular systems with medullary bundles is proposed.