Tilia Linnaeus. Subfamily: Tilioideae.
A genus of about 25-45 species, trees, of temperate regions of North America, Europe and Asia. Hardin's (1990) treatment of AmericanTilia seems a practical and reasonable approach; it gives taxonomic status to the more distinctive (and geographically based) elements of variation, while recognizing the intergradational nature of the variation; McCarthy (2012) agreed with Hardin’s entities and their ranks. Pigott (2012), however, differed in his interpretation (see synonymy). Further investigation of this complex group is, however, warranted. Xie et al. (2023) found phylogenetic incongruence and complicated reticulation in the genus as a whole; in their North American sampling, species and variety taxonomy was not well resolved. The section and subsection taxonomy of the genus shown here follows Xie et al. (2023).
ID notes:While the varieties treated below are broadly distinctive and have definite geographic distributions across e. North America, they are imperfectly distinct in geographic areas of overlap. In our area, their identification is particularly problematic in Virginia, where individuals in many parts of the state show intergradation between the northern var. americana and the Southern and Central Appalachian var. heterophylla. When not in flower or fruit, Tilia and Morus are often confused. They can be easily told apart by leaf venation. Morus has the main leaf veins splitting towards the margin but then rejoining to form a rather prominent, looping (scalloped) marginal vein; the basal veins 3, palmate, sometimes an additional prominent vein on each side joining the lateral vein above its divergence from the petiole end; and the main lateral leaf veins (above the basal veins) mainly alternate. Tilia has the main leaf veins splitting several times towards the leaf margin and leading into the teeth without rejoining and forming a marginal vein; the basal veins 5, palmate, all joining together at the summit of the petiole; and the main lateral leaf veins (above the basal veins) usually opposite
Ref: Bayer & Kubitzki In Kubitzki & Bayer (2003); Haines (2011); Hardin (1990); McCarthy (2012); Pigott (2012); Stace (2010); Strother (2015b) In Flora of North America Editorial Committee (2015); Xie et al. (2023). Show full citations.