Taxonomic groups of any rank will, in this Code, be referred to as taxa (singular: taxon).

2.1. Every individual plant is treated as belonging to an indefinite number of taxa of consecutively subordinate rank, among which the rank of species (species) is basic.

3.1. The principal ranks of taxa in descending sequence are: kingdom (regnum), division or phylum (divisio, phylum), class (classis), order (ordo), family (familia), genus (genus), and species (species). Thus, except for some fossil plants (see Art. 3.3), each species is assignable to a genus, each genus to a family, etc.

3.2. The principal ranks of nothotaxa (hybrid taxa) are nothogenus and notho-species. These are the same ranks as genus and species, only the terms denoting the ranks differing in order to indicate the hybrid character (see App. I).

3.3. Because of the fragmentary nature of the specimens on which the species of some fossil plants are based, the genera to which they are assigned are not assignable to a family, although they may be referable to a taxon of higher rank. Such genera are known as form-genera (forma-genera).

Ex. 1. Form-genera: Dadoxylon Endl. (Coniferopsida), Pecopteris (Brongn.) Sternb. (Pteridopsida), Stigmaria Brongn. (Lepidodendrales), Spermatites Miner (seed-bearing plants).

Ex. 2. The following are, however, not form-genera: Lepidocarpon D. H. Scott (Lepidocarpaceae), Mazocarpon M. J. Benson (Sigillariaceae), Siltaria Traverse (Fagaceae).

Note 1. Art. 59 provides for form-taxa for asexual forms (anamorphs) of certain pleomorphic fungi, at any rank.

3.4. As in the case of certain pleomorphic fungi, the provisions of this Code do not prevent the publication and use of names of form-genera of fossils.

4.1. The secondary ranks of taxa in descending sequence are tribe (tribus) between family and genus, section (sectio) and series (series) between genus and species, and variety (varietas) and form (forma) below species.

4.2. If a greater number of ranks of taxa is desired, the terms for these are made by adding the prefix sub- to the terms denoting the principal or secondary ranks. A plant may thus be assigned to taxa of the following ranks (in descending sequence): regnum, subregnum, divisio or phylum, subdivisio or subphylum, classis, subclassis, ordo, subordo, familia, subfamilia, tribus, subtribus, genus, subgenus, sectio, subsectio, series, subseries, species, subspecies, vari-etas, subvarietas, forma, subforma.

4.3. Further ranks may also be intercalated or added, provided that confusion or error is not thereby introduced.

4.4. The subordinate ranks of nothotaxa are the same as the subordinate ranks of non-hybrid taxa, except that nothogenus is the highest rank permitted (see App. I).

Note 1. Throughout this Code the phrase "subdivision of a family" refers only to taxa of a rank between family and genus and "subdivision of a genus" refers only to taxa of a rank between genus and species.

Note 2. For the designation of certain variants of species in cultivation, see Art. 28 Notes 1 and 2.

Note 3. In classifying parasites, especially fungi, authors who do not give specific, subspecific, or varietal value to taxa characterized from a physiological standpoint but scarcely or not at all from a morphological standpoint may distinguish within the species special forms (formae speciales) characterized by their adaptation to different hosts, but the nomenclature of special forms is not governed by the provisions of this Code.

5.1. The relative order of the ranks specified in Art. 3 and 4 must not be altered (see Art. 33.5 and 33.6).