Vascular continuity in the primary and secondary stem tissues of Bougainvillea (Nyctaginaceae)

An investigation of stem structure of Bougainvillea by serial sections and cine-photography shows that the medullary system of the inner area of young stems is the sole vascular system directly continuous into the lateral appendages (leaves, axillary buds and axillary thorns) via complex nodal anastomoses. The vascular system at the periphery of the primary bundles is not directly continuous into these appendages. In secondary growth, there is direct continuity between vascular bundles within a single ring, in a tangential direction via either xylem alone or both xylem and phloem, and between rings in a radial direction always via xylem and phloem, even though the rings are derivatives of successive cambia.