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THE EVOLUTION AND ALLOMETRY OF PLANT BODY PLANS.  Karl J. Niklas.  Department of Plant Biology, Cornell University, Ithaca NY 14853.

     Plants (defined as eukaryotic photoautotrophs) have evolved multiple times.  Most lineages originated in deep geological history (i.e., the algae); the most recent lineage is that of the land plants (i.e., the embryophytes).  Across all lineages, only three basic body plans exist (i.e., unicellular, colonial, and multicellular), although important variants exist (e.g., siphonous algae and vascular plants). Convergence and divergence is common within and across the major algal lineages.  However, the multicellular body plan is the only one successful on land.

     This convergence and divergence within the algal lineages is attributed to the capacity of each body plan to maximize its surface area with respect to an increase in body volume, thereby minimizing nutrient transport time and maximizing growth rates in an aquatic and essentially gravity-free environment.  This ‘allometric tactic’ is adjusted significantly in an aerial habitat, which biophysically requires water conservation and thus a reduction in body surface area with respect to body volume.  Additionally, an aerial habitat necessitates mechanically supportive/nutrient conducting tissue types as body size either ontogenetically or evolutionarily increases. 

     Despite their tremendous diversity in size, shape, geometry, and internal structure, all plants, regardless of phyletic affiliation or habitat preference, conform to the same general scaling (allometric) ‘rules.’  Thus, across and within the plant lineages, growth scales, on average, as the 3/4–power of body mass and isometrically with respect to either cell photosynthetic pigment concentration (algae) or foliage biomass (tracheophytes).  These ‘rules’ are attributed to unavoidable biophysical constraints  that have shaped all of plant evolution (e.g., Fick’s law of diffusion). 

     (Supported by an Alexander von Humboldt Forschungspreis.)