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The Problem of Leaf Morphology and Evolution in the Monocotyledons

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 appendages of monocotyledons with rachis leaves of Umbelliferae suggests that the terete leaf axis of the former is not simply an elaborated petiole but is positionally equivalent to the laminar region of a dicotyledonous leaf. Instead of being expanded in surface by marginal growth, the unifacial moncotyledonous leaf is thickened developmentally and hence has become nearly radial in summetry. Thus the original phyllode concept of the loss of the blade retion and the expansion and assumption of its function by the petiole no longer seems valid for monocotyledonous leaves. In addition, comparative developmental studies, as well as the analysis of serial changes in leaf morphology, have confirmed that where dorsiventral, laminar leaves are developed in the monocotyledons, they are developmental elaborations of the basal, meristem-encircling part of the leaf primordium, and the distal upper leaf zone remains rudimentary. Therefore, blade regions of such dorsiventral appendages are not homologous with those of dicotyledons because they have a different developmental origin; laminae of dicotyledons are derived from the upper leaf zone. It is suggested that the basic construction of monocotyledonous leaves consists of two regions: (1) a distal unifacial upper sector; and (2) a proximal bifacial sheath; variations in leaf morphology in a range of monocotyledonous taxa are therefore due to reciprocal elaboration, or suppression of these two sectors, or both. It is emphasized, however, that before any model of this sort can be extended to the monocotyledons as a whole, it must be evaluated rigorously in the broadest range of representatives from each major family.