All known plant species have names. Unfortunately, outside of flower shops and botanical gardens, they do not come with nametags. Therefore, it is often necessary to identify an unknown plant, that is, to determine the species to which it belongs and thus its name. Identification assumes that the plants have already been classified and named. When you identify a plant, you are basically asking: "Of all known species, which one most closely resembles this individual in my hand?"

Professionals and serious amateurs identify plants by keying. This is a stepwise process of elimination that uses a series of paired contrasting statements, known as a dichotomous key. Keying is like a trip down a repeatedly forking road: If at the first fork you turn right, you cannot possibly reach any of the towns that lie along the left fork. Each successive fork in the road...