Why do we need a taxonomic classification

"A plant's name is the key to its literature - in other words, the key to what we know about it."
(C. G. G. J. van STEENIS, 1957)

It is probably a human endeavor to bring order into every diversity, because one can only try to understand what one can overlook. Already in the earliest human cultures the diversity of forms of animate and inanimate nature was dealt with, and one began with the naming of objects and organisms. The formation of the term was from the beginning a main concern and at the same time a point of contention of any classification. One of the immortal achievements of PLATON and ARISTOTLE is the realization that objects (and organisms) are to be assigned to certain ranks of different hierarchy levels. They differentiate between upper and lower terms. Individuals that look the same belong to one species, species that are similar to one another belong to a higher category. For example, the olive trees belong to the trees. These in turn belong - together with the herbs - to the plants. After all, plants and animals are living beings. In the hierarchy of categories, living beings are the highest category, plants, trees and olive trees are subordinate categories in descending order.

A classification can only seldom - or only with restrictions - be regarded as complete once and for all, because with the advancement of science and the use of increasingly difficult methods, our knowledge of the diversity of organisms increases, but we are constantly faced with new problems of classification of these findings.

ARISTOTLE, C. v. LINNÉ, C. DARWIN, for example, had no idea that cytological knowledge or biochemical data would be used today to gain deeper insight into the nature of a species.

According to E. MAYR (1975) the theory and practice of the classification of organisms is called taxonomy, and the science of diversity and the comparison of organisms is called systematics. The essence of the system is (according to the memorandum of the German Research Foundation: Biological Systematics, 1982):

"... primarily in the recording and ordering of the diversity of forms in living nature. It creates the prerequisites for the identification of organisms and enables access to the information available about them. It thus provides the basis for ecology and other experimental biological disciplines for the comparison and the reproducibility of results ... (She) deals further with the clarification of the relationships and the justification of the higher units, as well as with the question of the causes of the diversity of the organisms in the Aristotelian sense, the classification as the first goal. Rather, based on the diversity of the organisms, it also has the task of reconstructing the course of phylogenesis. "

A critical aspect that repeatedly gives rise to controversy is the different weighting or evaluation of morphological, cytological and biochemical data. Another problem is the fact that names and terms have changed meaning over the centuries; Translations into national languages ​​led to further confusion. It is thanks to C. v. LINNÉ, to have developed a uniform binary nomenclature and a diagnosis of the species (in Latin) and to have tested it on approx. 7700 plants collected worldwide by himself or by his employees.

Because he deposited the material he worked on in a herbarium (now owned by the Linnean Society in London) and used other herbarium evidence that is also still available, we know exactly which specimens his names are linked to. Building on this approach, the so-called type method was developed. It describes a very specific way of working of the systematists, which only serves to name plants. Plant names are therefore linked to very specific specimens (types) deposited in herbaria. This procedure is a nomenclature trick because it is usually not possible to capture the full range of variability within a species; in addition, there is only a very small number of species whose genetic data are known.

The type method must not be confused with the typological concept of species, which assumes that all individuals of a species conform to a certain norm. This approach no longer plays a role in modern systematics. Rather, there is consensus that a species can be viewed as a reproductive community that is discontinuous from a related species. The grouping of species into superordinate hierarchically arranged groups (taxa) is usually much more difficult to justify.

Every naming of organisms must follow certain rules. There is now an international botanical nomenclature code that takes this into account. It expresses conventions that scientists have agreed on. The taxonomy of plants is based on different criteria than those of animals or microorganisms. For example, metabolic pathways and products play a decisive role in the systematics of bacteria, while biochemical analysis data have contributed little to the description of the species of plants. Biochemical data there proved to be suitable for characterizing higher-level taxa or for clarifying the population structures (proportion of heterozygosity and polymorphism) of individual species.

The use of modern electron microscopic techniques, e.g. scanning electron microscopy, made it possible to gain a little more clarity in the area of ​​unicellular, small organisms, e.g. diatoms (Bacillariophycea), whose species differ primarily through the sculpture of their surface, cell shape and size. Since it is precisely the subtleties of the surface structure that are at the limit of resolution of the light microscope, and can often only be recognized with difficulty and only after a lot of practice, the scanning electron microscope has proven to be of great help. Particularly in the case of unicellular, asexually (or only exceptionally sexually) reproducing organisms, there are still great uncertainties about the species affiliation (individual individuals or populations) and the intraspecific variation. In the following we will first have to ask ourselves,

But we must also be clear about the fact that the individual worker of a group of plants with the great diversity of forms and species has neither the time nor money, and usually also not the prior knowledge and the technical prerequisites, to each species according to every conceivable criterion examine. The diagnosis must be carried out as quickly as possible and as carefully as necessary. It also depends on the actual goal of the investigation, because it makes a difference whether you want to inventory (classify) a species or whether you want to determine its phylogenetic position. If you are interested in the latter, you need all available data.

James L. REVEAL offers a detailed and up-to-date compilation of central questions about the taxonomy of angiosperms in a series of "Lecture Notes" ("Notes" is a massive understatement here)

James L. REVEAL (Norton-Brown Herbarium, University of Maryland)