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Variation in leaf micromorphology can aid delimitation of taxonomically difficult groups. The woody genus Metrosideros (Myrtaceae) dominates Hawai'i's native forests and comprises striking, predominantly infraspecific, macromorphological variation, including many forms unrecognized in current taxonomic treatments. On taxonomically rich O'ahu, 10 taxa (varieties or unnamed morphotypes of M. polymorpha and other species) occur in a predictable sequence with overlapping ranges from low to high elevation along the numerous leeward ridges of the Ko'olau Range. We used scanning electron and light microscopy to examine mature, nonsenescing sun leaves from each of these 10 taxa. Parametric and nonparametric tests were used to compare stomatal complex (SC) traits and those of a newly described secretory structure (SS) across taxa. Correlations among traits and discriminant analyses were also done. Density and length of SCs were inversely related overall, and both measures varied significantly across taxa. Although abaxial SSs were consistently present only in the two highest-elevation glabrous taxa, adaxial SSs were present in all 10 taxa. Length and density of adaxial SSs and density of abaxial SSs varied across taxa, and densities of both SCs and SSs were greatest at high elevation. Combined, the SC and SS traits differentiated 100% of four glabrous varieties and two of three pubescent varieties of M. polymorpha. Variation in leaf micromorphology, including presence and density of a newly described SS, aids delimitation of closely related Hawaiian Metrosideros taxa and may refilect differential local adaptation across a heterogeneous landscape.