The micromorphologies of seed coats and petal trichomes of the taxa of Viola subsect. Boreali-Americanae (Violaceae) and their utility in discerning orthospecies from hybrids

Viola subsect.Boreali-Americanae, confined to North America including northern México, has long been considered one of the most taxonomically difficult temperate groups of the angiosperms. Hybridization, followed in some localities by introgression, has made it difficult for systematists to discern the “core” species by using only standard macromorphological characters. Analyses that employed scanning electron microscopy (SEM) of seed coats and petal trichomes generated new characters. SEM of the surface of seeds of each of the orthospecies revealed a suite of primary and secondary sculpture characters unique to each species that are correlated with a suite of macromorphological characters. In a number of species, SEM of petal trichomes provided additional unique characters. SEM of the surface of seed coats enabled, also, the identification of instances of hybridization by revealing blends of structures of the primary and secondary sculpture typical of different orthospecies and instances of putative introgression by revealing deviations in surface structures from those typical of orthospecies. A procedure based on iterative examinations of macromorphology and micromorphology, which was developed in this research, enable the discernment of hybrids and putative introgressants. Combining the micromorphological and macromorphological data with ecological and phytogeographical data enabled the delimitation of sixteen orthospecies.