The evolution of plant body plans - A biomechanical perspective

Defining 'plants' inclusively as 'photosynthetic eukaryotes', four basic bo dy plans are identifiable among plant lineages (unicellular, siphonous, col onial and multicellular). All of these body plans occur in most plant linea ges, but only the multicellular body plan was carried onto land by the embr yophytes. Extensive morphological and anatomical homoplasy is evident among species with different body plans. This is ascribed to the facts that the acquisition of nutrients and radiant energy is affected by plant body size, shape and geometry, and that, with the exception of the unicellular body p lan, each of the other body plans involves an 'open and indeterminate' onto geny capable of modifying body size, shape and geometry regardless of how o rganized growth is achieved. In terms of unicellular species, the available data indicate that size-dependent variations in surface area, metabolic co nstituents (e.g. photosynthetic pigments), and reproductive rates limit max imum body size in nutrient poor habitats or those that change rapidly or un predictably. This maximum size can be exceeded in more stable niches by eit her the cooperation of conspecific cells sharing a common extracellular mat rix (i.e. the 'colonial' body plan) or by repeated mitotic cellular divisio n associated with sustained cytoplasmic (symplastic) continuity (i.e. multi cellularity). The siphonous plant body plan may have been evolutionarily de rived from a unicellular or multicellular ancestral life form. Each of the plant body plans is reviewed in terms of its biomechanical advantages and d isadvantages. Variants of the multicellular body plan, especially those of the Chlorophyta, Charophyta, and Embryophyta, are given special emphasis. ( C) 2000 Annals of Botany Company.