The flower ofRanalisma humileis comparable in morphology to those of other alismads. The vascular anatomy is noteworthy. Flowers ofR. humileare provided with a major (central) vascular system supplying androecial and gynoecial vasculature and a variable amount of the sepal and petal vasculature, and a minor (peripheral) system which supplies the rest of the perianth vasculature. There is very little anastomosis between the two systems. It is suggested that the variability of perianth vascular supply reflects variability in timing of differentiation events. The occurrence of an almost independent peripheral vascular system is probably of significance in a consideration of the relationship ofRanalisma humileto other alismads.