Developmental patterns of leaves in Podostemaceae compared with more typical flowering plants: saltational evolution and fuzzy morphology

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Abstract
Leaves and stems of flowering plants have been widely accepted as clearly distinguishable structural categories. Evolution, however, can blur the stem–leaf (axis–appendage) distinction. Compound leaves with apical growth and three-dimensional branching may be seen as developmental mosaics sharing some growth processes with leafy stems (shoots). To cope with fuzzy boundaries between structural categories, fuzzy morphology is proposed as a complementary way of looking at higher plant architecture. Fuzzy morphology treats structural categories (e.g., leaves and stems) as concepts with fuzzy (not sharp) boundaries. The developmental morphology of compound leaves is described comparing Apium repens (Apiaceae) with less typical angiosperms ("misfits") of the rheophyte family Podostemaceae, especially Marathrum rubrum, Mourera fluviatilis, and Tristicha trifaria. In some taxa within the Podostemaceae, typical leaf characteristics may be replaced by new properties, e.g., 90° switch of symmetry plane of the leaf primordium with production of pinnae or lobes along the adaxial and abaxial margin in Marathrum rubrum and Mourera fluviatilis. In Tristicha trifaria, compound photosynthetic appendages (called ramuli) combine developmental characters of typical leaves and typical stems. Thus, they may be called leaf-stem intermediates. Fuzziness of structural categories such as leaf and stem is especially obvious in Podostemaceae, which can be seen as the result of saltational evolution. The structural categories of typical flowering plants are transcended in the Podostemaceae because of developmental changes, resulting in organs with unique combinations of features. Key words: leaf development, developmental mosaic, Apium, Marathrum, Mourera, Tristicha.