The evolution of plant body plans - A biomechanical perspective

Defining 'plants' inclusively as 'photosynthetic eukaryotes', four basic body plans are identifiable among plant lineages (unicellular, siphonous, colonial and multicellular). All of these body plans occur in most plant lineages, but only the multicellular body plan was carried onto land by the embryophytes. 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 plan, each of the other body plans involves an 'open and indeterminate' ontogeny capable of modifying body size, shape and geometry regardless of how organized growth is achieved. In terms of unicellular species, the available data indicate that size-dependent variations in surface area, metabolic constituents (e.g. photosynthetic pigments), and reproductive rates limit maximum body size in nutrient poor habitats or those that change rapidly or unpredictably. This maximum size can be exceeded in more stable niches by either the cooperation of conspecific cells sharing a common extracellular matrix (i.e. the 'colonial' body plan) or by repeated mitotic cellular division associated with sustained cytoplasmic (symplastic) continuity (i.e. multicellularity). The siphonous plant body plan may have been evolutionarily derived from a unicellular or multicellular ancestral life form. Each of the plant body plans is reviewed in terms of its biomechanical advantages and disadvantages. Variants of the multicellular body plan, especially those of the Chlorophyta, Charophyta, and Embryophyta, are given special emphasis. (C) 2000 Annals of Botany Company.