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The Independence of Cell Shape and Overall Form in Multicellular Algae and Land Plants: Cells Do Not Act as Building Blocks for Constructing Plant Organs

We evaluate here whether the mechanisms regulating cellular morphogenesis in planar structures are also responsible for dictating their overall shapes. A comparative survey of 15 multicellular algae and land plants was performed in order to identify the common features associated with cellular morphogenesis in their immature planar structures. The superficial layers of all such structures are partitioned into polygonal cellular arrays that are characterized by specific mean numbers of walls per cell, narrow ranges of cell sizes, and regular cell shapes. The ability to generate these polygonal arrays is independent of overall shape, evolutionary lineage, cell layer number, cell division pattern, and cell wall composition. Instead, geometric principles dictate that the superficial layers of plant structures exhibit mean numbers of ca. 6.0 walls per cell, which result from the tendency of cell plates in most dividing plant cells to avoid existing three-way vertices. In contrast, the superficial layers of algal structures exhibit mean numbers between 5.5 and 5.6 walls per cell, which depend on the ability of cleavage furrows in dividing algal cells to arise at existing three-way vertices. Sequential observations of developing prothalli of the fern Onoclea sensibilis were used to characterize the regulation of cell size and cell shape in this representative planar structure. Cell biological mechanisms appear to dictate cell size via the control over the symmetry, timing, and orientation of cell division. Biomechanical mechanisms regulate cell shape in a remarkable manner. Cell walls in the region of isodiametrical expansion pivot around their vertices so that the observed angles approach the ideal angles calculated for equiangular polygons. Additional divisions alter the wall numbers of the three cells around each vertex, but the observed angles continue to adjust so that they parallel the predicted ideal angles. Angle rotation is accompanied by differential amounts of relative wall expansion, which are inversely dependent on wall length. It appears as if the vertices of dividing cells undergoing isodiametrical expansion are attempting to reach the condition of local mechanical equilibrium. But the vertices in older cells undergoing preferential elongation along one axis tend to revert to the original rightangle configuration in the attempt to accommodate to global growth stresses. Thus, biomechanical mechanisms regulating cell shape allow the prothallus to approach mechanical stability. Last, the polygonal cellular array in planar structures is viewed as the morphogenetic equivalent of a ground state; i.e., it is generated without any evident input of specific genetic instructions. We conclude that the local regulation of cellular morphogenesis is completely independent of the global regulation of the overall form of planar structures.