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Abstract Planar structures dramatically increase the surface‐area‐to‐volume ratio, which is critically important for multicellular organisms. In this study, we utilize naturally occurring phenotypic variation among threeSansivieriaspecies (Asperagaceae) to investigate leaf margin expression patterns that are associated with mediolateral and adaxial/abaxial development. We identified differentially expressed genes (DEGs) between center and margin leaf tissues in two planar‐leaf speciesSansevieria subspicataandSansevieria trifasciataand compared these with expression patterns within the cylindrically leavedSansevieria cylindrica. TwoYABBYfamily genes, homologs ofFILAMENTOUS FLOWERandDROOPING LEAF, are overexpressed in the center leaf tissue in the planar‐leaf species and in the tissue of the cylindrical leaves. As mesophyll structure does not indicate adaxial versus abaxial differentiation, increased leaf thickness results in more water‐storage tissue and enhances resistance to aridity. This suggests that the cylindrical‐leaf inS. cylindricais analogous to the central leaf tissue in the planar‐leaf species. Furthermore, the congruence of the expression patterns of theseYABBYgenes inSansevieriawith expression patterns found in other unifacial monocot species suggests that patterns of parallel evolution may be the result of similar solutions derived from a limited developmental toolbox.