An investigation has been made of the relation between the sizes of the lamina and of the petiolar xylem of both mature and immature leaves of the runner bean (Phaseolus multiflorus Willd.).The ratios xylem area/lamina area and the number of vessels/lamina area are lowest for mature leaves. Immature leaves gave higher but somewhat more variable values for these two ratios.There is a constant growth ratio between the lpminn area and the xylem area, such that k is approximately o·61 in the allometry formula.The significance of the results is briefly considered from the point of view that the xylem area is related to the water requirements of the leaf.