PALEOBIOLOGICAL IMPLICATIONS OF A FUNCTIONAL DESIGN SPACE OF SINGLE VEINED LEAVES

A novel approach is here taken to investigate the morphological evolution of leaves: rather than focusing upon morphology as empirically documented or theoretically possible, an iterative model has been used to determine the relative leaf sizes and shapes that are functionally possible along several hydraulic variables. The model is of a single-veined leaf and variables include the hydraulic resistances associated with vein axial and radial transport as well as with water movement through the mesophyll and the leaf surface. Evaluation for these four parameters of the possible leaf morphologies for which the entire lamina can be supplied with sufficient water defines a functional design space that includes all single-veined leaf shapes found in nature, including scale-, awl-, and needle-like morphologies. This exercise demonstrates that hydraulic parameters have dissimilar influences: surface resistance only affects leaf size, while radial and mesophyll resistances primarily affect leaf shape--distinctions which may provide insight into patterns of morphological and anatomical evolution documented in conifers and other groups with linear and simple multi-veined leaves. Other conclusions can be generalized to more complex laminar morphologies. Small modifications to physiological parameters can result in large, abrupt shifts in morphology, while continuous morphological change may require complicated changes to physiological parameters, affecting interpretation of a number of patterns both documented and inferred in the paleobotanical record. Also, sequences of morphological change repeated during the convergent evolution of leaves in independent lineages may represent a rise in developmental complexity that allowed a more physiologically permissive set of morphological possibilities.