Species' is the fundamental unit in biological systematics, but there are different definitions. Frequently, the biological species concept is used, according to which a species is defined as a cluster of natural populations which cross fertile among each other and which are reproductively isolated from other populations. However, especially in the plant kingdom the use of the biological species concept is not unproblematic, among other, because numerous taxa reproduce purely asexual (apomictic) (e. g. many species in the genus Hieracium – Hawkweed). Furthermore, crossbreeds between different 'species' frequently occur (e. g. in Salix – Willows). Species are named with a binary nomenclature (e. g. Quercus petraea – Sessile Oak).