1. The process of establishing and delimiting taxa within the hierarchy of classes. The resulting system of classification should either help in the rapid identification of organisms or should express their natural interrelationships (preferably both). Four kinds of classification can be recognized: artificial, natural, phenetic, and phylogenetic. Artificial systems are based on a few predetermined characters usually selected to aid identification and consequently may not reflect the true relationships of the plants. The sexual system of Linnaeus based on the numbers of stamens and carpels is an example (see also key
). Natural systems of classification are based on many characters and have a predictive value. Thus if one species of a particular genus is found to possess a certain character that was not used when the genus was originally defined, it is nevertheless quite likely that other species within the genus will also possess the character. *Phenetic
systems tend to be general purpose, incorporating data from many sources. *Phylogenetic
systems are also broadly based, but have the additional intention of reflecting the evolutionary history and relationships of a group of taxa. Most modern systems aim to be phylogenetic.
2. The branch of ecology concerned with allocating plant communities to distinct units or *associations
. For example, a stretch of moorland may be described as a Calluna/Pteridium association. Stands of floristically similar vegetation with certain characteristic species in common are placed together in associations, the names of which end with the suffix -etum. Thus associations dominated by pine or oak are grouped as Pinetum or Quercetum respectively. The method provides a useful summary of the vegetation types found in a region. However it has the disadvantage that vegetation intermediate between two recognizable associations may be ignored for convenience' sake. It is a very subjective method of assessing vegetation. Compare ordination