In addition to the binomial, which names a flowers species, each plant has a name for each higher-level group to which it belongs. Each plant belongs to a genus, each genus to a family, each family to an order, each order to a class, each class to a phylum, each phylum to a kingdom, and each kingdom to one of the three domains of life: Archaea, Bacteria (both made up of microorganisms formed by prokaryotic, or nucleus-free, cells), and Eukarya. The domain Eukarya, made up of organ¬isms with cells that have nuclei, contains four king¬doms of life: Protista (protists, mainly molds and al¬gae), Fungi (mainly nonphotosynthetic organisms), Plantae (plants, both nonvascular and vascular), and Animalia (animals).
Names for the higher-level flower groups, or taxa, are all created according to rules of the ICBN. The rules for naming higher-level groups do not indi¬cate which names are best or most correct. Unlike the binomial genus-species names of flowers, on which scien¬tists generally agree, the best name for the higher-level groups to which these genera belong can sometimes be controversial. Therefore, some of the higher-level groups have more than one proposed name of flowers. Neither of the names is necessarily more cor¬rect than the other. Usually the different names of flowers re¬flect different ideas about how the higher-level groups are related to each other. In some cases, the higher-level names that are listed were selected from several proposed flowers names. Other sources may classify some of these genera under slightly differ¬ent higher-level group names, as a result of the on¬going studies, discussions, and controversies over classification. The binomial genus-species name, however, will nearly always be the same. The exis¬tence of more than one name for some of the higher levels of flowers classification is simply a reminder that botanists are constantly learning new things about plants and occasionally change their ideas about how plants should be named.
Each of the organisms (bacteria, fungi, and plants) listed in this appendix is alphabetized by its binomial scientific name (far left-hand column); the most often used common name appears in the mid¬dle column. Finally, the far-right column identifies the kingdom (k.), phylum (p.), class (c.), order (o.), and family (f.) in which the species is commonly classified, along with some notable characteristics. All organisms can be assumed to belong to the do¬main Eukarya unless one of the other domains (ei¬ther Archaea or Bacteria) is identified. The abbrevia¬tion g., for "group," indicates a group name that is "artificial"—that is, not based on evolutionary rela¬tionships but rather on some common characteris-tics that have made it convenient for researchers to regard these organisms as a group. The abbrevia-tion spp. stands for "species" (plural).