The entire plant kingdom demonstrates an immense variation in forms, structures, colours and flavors. This is the result of evolution over the course of time. The manifestation of a plant (phenotype) is determined by a combined action of environment and genetic ability (genotype). Different phenotypes belonging to a definite genotype are an expression of internal plasticity. In systematics these modifications must be recognized, but for natural classification genetically determined variation is used.
The main task of biological systematics is that of defining, delimiting, describing and naming taxa, and their incorporation in a hierarchiacal classification expressing different levels of similarity. A taxon is a group of organisms--of any taxonomoic rank--which can be recognized in nature as a unity of the total pattern of variation (mint, labiates).
As far as possible, biological classification should be natural, that is, based on the totality of the knowledge of taxa. At the same trime botanical classification should reflect evolutionary lines within the taxa concerned (phylogenetic classification). Finally, botanical classification must follow internationaly accepted rules concerning the systematical categories division, class, order, family, genus, species and subcategories) and nomeclature.