What I study is the evolution of the genus Tilia in North America. Tilia contains all the trees known as basswood, linden, lime (not the citrus lime, the English lime), tilo, sirimo, and many other names. They all look pretty much the same, like this:
There are roughly 25 species of Tilia worldwide. Although it's in the largely tropical Malvaceae family (along with hibiscus, cotton, and mallow), the genus Tilia is found entirely in the temperate northern hemisphere, and all its species are trees. The North American basswoods are found from the Atlantic to the 100th meridian (eastern Dakotas and straight south), southern Canada to the Gulf coast, and in mid-altitudes throughout Mexico. The trees can get really tall, typically 23-40 m at maturity. If not disturbed, they can live up to 200 years. However, since the roots resprout readily, a basswood that has been cut and resprouted may be much older than that.
Old-growth basswood in Chippewa National Forest, Minnesota // A resprouted basswood; this form is known as a coppice
All members of the genus have more or less toothed, heart-shaped leaves but the real key to identification is the leafy bract that subtends the cymose inflorescence. (Cymose means the inflorescence is a cyme, which is something like an umbel but is determinant; it doesn't have more embryonic flowers emerging in the center.) The inflorescences are heavy and hang down below the bract. Basswood is one of the few insect-pollinated forest trees in the eastern U.S. It is just finishing its flowering in the Chicago area right now. The trees emit a heavy, sweet nectar smell and are a favorite with bees. The two shades of green, leaves and bracts, make basswoods easy to spot.
a typical basswood specimen, this one from the western tip of Virginia
I have myself visited and sampled over 50 natural populations of basswood in North America. Other very helpful people have sampled sites for me when I could not visit myself. I collected the final specimen from a forest preserve near Chicago a few days ago and now have over 700 samples. Next time I will tell you what I do with them!