The development of the first pair of leaves of the Runner-bean (Phaseolus multiflorus Willd.) has been investigated from the unfolding of the leaves to the end of lamina expansion. The growth of the petiole is relatively fast and ceases first. Extension growth of the lamina continues longer. In the growth of the xylem, the average area of the vessels formed increases most markedly at an early developmental stage. Vessel number may continue to increase after lamina growth has ceased. Both the number of vessels and the xylem area per unit area of lamina diminish as the leaf matures.