A review of the previous literature on the morphology and evolution of monocotyledonous leaves reveals a number of interpretive controversies and contradictions. The phyllode theory, which has been the prevailing interpretation of the origin and evolution of monocotyledonous leaves, is evaluated and is shown to have been biased by a preoccupation with temperate monocotyledons and the exclusive study of mature leaf morphology and vascular anatomy. Recent investigations of comparative development have provided a broader basis for the evaluation of structural relationships of leaves in the monocotyledons and dicotyledons and have served to eliminate some of the interpretive inconsistencies that were based on the study of vascular anatomy only. The most controversial leaf type in the monocotyledons has been the unifacial (radially symmetrical) leaf found in a number of divergent families. Contrary to the original Phyllode theory of the Candolle (1827) and Arber (1918), developmental comparison of unifacial ap...