From a review in the Tribune-Review of The Naming of Names: The Search for Order in the World of Plants, by Anna Pavord:

Until the Middle Ages in Europe, plants were defined by their usefulness, for food, medicine, magic. The earliest attempt at classifying was by the Greek philosopher Theophrastus, schoolmate and colleague of Aristotle, who described 500 plants, of the 422,000 species identified today.

Theophrastus began with the concept of a plant as an animal with its feet in the air and its mouth in the ground, and he followed the prevailing notion that plants, like tadpoles or caterpillars, could transmute -- wheat and barley, for example, into worthless darnel. That idea remained common through the end of the 17th century.

Theophrastus wrestled with a problem that would daunt herbalists for the next 1,500 years: The same plant would wear many different names, "a great tangled knot of competing synonyms," complicated when writers attempted to compile knowledge from other countries and languages. (The widespread marsh marigold, for example, has 60 names in France, another 80 in Britain, and at least 140 in Germany, Austria and Switzerland.)

Ironically, Theophrastus' name was lost to the west for 1,500 years, his work "shamelessly plagiarized and regurgitated" by the Roman Pliny, who for centuries remained for Europe the towering ancient authority, while Theophrastus was preserved in Muslim libraries, circulating back into Europe during the Renaissance.

This gives me the opportunity to mention the Theophrastus Project, based at UCL ... the site is mostly 'announcement' type stuff, but the project itself looks interesting.