Longitudinal Symmetry and Zonation of Leaves Part I: Leaf Zones

This chapter characterizes the morphology of a leaf in terms of its longitudinal symmetry. Also, as in shoots, most leaves are longitudinally asymmetric, with each subdivision having a different shape and length along the extension of the leaf. While the concepts of upper and lower leaf zones originated with reference to the leaves of angiosperms, similar morphological expressions can be found in the leaves of pteridophytes. The significance of the early delimitation of the primordial leaf into upper and lower leaf zones is that serial changes in leaf form along the length of a shoot of many species can be a result of differential expansion or suppression of the developmental derivatives of these two zones of the leaf primordium. The hypothesis that spatial relationships may play a role in the degree of union between adjacent lateral stipules to form an interpetiolar stipule is suggested by the shoot of Pelargonium peltatum.