The angiosperms are the largest, most highly diversified, and most successful major group of land plants. They contain an estimated 225000 to 350000 species, some 12000 to 13000 genera, and 250 to more than 500 families, depending on the taxonomic concepts applied. In retrospect, it is most surprising that the general outline of their classification had already been recognized by the time of Bentham, more than a century ago. Many elements of this classification have not only been incorporated into the influential system of Engler and Prantl (1887-1915), but have persisted up to the present day. This does not mean, however, that systematic botany has come to a standstill: on the contrary, apart from completing the inventory, botanists have opened many novel fields of data inquiry. They have developed highly elaborate approaches to data processing, have clarified the objectives of biological classification, and have given consideration to evolutionary aspects.
At the time of Engler, the weighting of characters was considered as the most important step in classification. Later on, the rising tide of data from various novel sources, such as micromorphology, cytology, and phytochemistry, coincided with the claim for basing classifications upon as many different data as possible. This led to the development of numerical methods, producing so-called phenetic classifications, in which equal rank was given to all characters. These classifications were sometimes explicitly aphyletic, aiming at homogeneity and stability of taxa instead of their proven or alleged monophylesis. In the last two decades the pendulum has swung to the opposite direction, and it is felt that virtually all facets of plant life can only be understood against an evolutionary background. While in the 19th century hierarchical classifications were considered to provide indirect evidence for the fact that evolution had really occurred, more recently the relationship between classification and evolution has drastically changed. Today, the idea of evolution is so firmly established that it does not need independent support; one rather tries to base classifications on the presumed phylogeny of the respective group of organisms. This, however, demands the recognition of monophyletic taxa and knowledge about the polarity of character states, and the argument employed is not always without circularity.
At present, various forms of strictly phylogenetic approaches such as "cladistics" or "numerical parsimony analysis", are nearly universally considered as providing the most appropriate methodology for generating phylogenetic hypotheses as the basis of biological classifications. Most recently, attention shifts away from the phenotype towards the study of the genetic material itself by analyzing restriction sites or by sequencing - for the time being - small portions of the genome. These powerful tools introduce the greatest precision into the study of phylogeny. It is too early to decide whether they create a truly novel dimension of the study of plant syste-matics. Yet despite all the excitement about recent developments, one should not overlook that the very basis of our science was laid without commanding today's conceptual and technical tools, and that many seemingly novel ideas have a long history in biological classification: many of the modern principles, such as the use of apomorphic characters, monophyletic taxa, and symmetric groupings have been inherent in serious taxonomic work for more than 100 years. Also in this sense systematic botany appears as a never-ending synthesis.
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