Taxonomy and Conventional Systematics
Science needs a precise identification of its study objects, therefore taxonomy is a prerequisite for all other biological disciplines. Names of plants must comply with the rules of the International Code of Nomenclature, and their correct application is governed by a certain specimen (called 'type specimen'), which has been deposited in a certain herbarium. Taxonomists record the diversity of life and write identification keys, floras and monographs, which are used by other scientists to identify their study objects.
Hasn't that all been done long ago? No way! It seemed like that for a while in Europe and North America, but not in other parts of the world. And now the flora changes at an unprecedented speed. On the one hand we see the worst mass extinction since the meteorite impact at the end of the Cretaceous, on the other more species than ever before are transported to other parts of the world, deliberately or inadvertantly, by tourism and trade. And because of climate change many of them for the first time have a chance to become established outside their native range.
Classical systematics used to evaluate the relationships among plants based on morphological and anatomical characters. Today molecular systematics allows more precise phylogenetic analyses and has led to numerous changes, but it has also confirmed earlier hypotheses to a large degree. Still morphology yields the most important characters for plant determination and their (at least preliminary) placement in the tree of life. Above all, however, morphological characters tell us much more about the adaptations of plants to their environment than DNA sequences can.