For your reference, some terms related to plant classification are defined below. The dynamic process of classification is forever in flux, as entire classification systems and the categories within them evolve and new relationships among organisms come to light. While the terms and categories below are very much not written in stone, they give us a working vocabulary that allows us to identify and compare organisms, including plants. Having a basic understanding of plant classification can give us many insights into seed saving, can help us appreciate plants and natural processes, and can improve our environmental awareness, as well.
This page is a work in progress, and definitions may be added and updated periodically. Please don’t hesitate to contact us with corrections or if you have anything to add.
➤Classification Organizing things into classes
➤Taxonomy The science of classifying things, especially organisms, by name
➤Taxon (plural taxa) A group of organisms
➤Systematics The ongoing process of classifying organisms in a way that best represents their phylogeny
➤Phylogeny (the tree of life) – From The Dinosaur FAQ (1): “The hierarchical structure by which every life-form is related to every other life-form. The word phylogeny refers to the reality of that tree – the one, true, tree – as opposed to the theories that people make about it. So phylogeny is not an activity (something that we do), but a fact (something that we try to discover.).”
➤Cladistics -See The Dinosaur FAQ (1), Google Dictionary (2), and UCMP’s An Introduction to Cladistics(3): “A relatively new way of doing systematics… It works by analysing different taxa to find objective similarities and differences between them, and using those similarities and differences to derive a hierarchical structure showing which taxa are most similar to others…” (1) “It is assumed that the higher the proportion of characteristics that two organisms share, the more recently they diverged from a common ancestor.” (2) “The basic idea… is that members of a group share a common evolutionary history, and are ‘closely related,’ more so to members of the same group than to other organisms. These groups are recognized by sharing unique features which were not present in distant ancestors,” i.e. derived characteristics. (3)
The Cladistics system developed alongside the computer age, and is helped along by “the availability of cheap, powerful computers to run the analyses.” (1) The resulting relationship maps are complex and ever-changing, which makes formal ranking impractical. So, cladistics is less rank-intensive than earlier biological classification systems. (1) “It remains to be seen how nomenclatural practices will change, but what seems to be emerging by default is an approach in which some of the more ‘important’ nodes… get quietly labelled as families, orders, etc. according to a blend of historical precedent and what seems to be useful at the time.” (1) (See “Linnaean classification.”)
➤Clade See The Dinosaur FAQ and Wikipedia’s “Clade” article: A branch of the tree of life (from Greek klados, ‘branch’.) (4) “A group consisting of an ancestor together with all its descendents.” (1) “Clades are nested, one in another, as each branch in turn splits into smaller branches. These splits reflect evolutionary history as populations diverged and evolved independently.” (4)
➤Monophyletic “(Greek: ‘one clan’)…consist[ing] of an ancestral species and all its descendants.” (4) Clades are monophyletic.
➤Paraphyletic “A paraphyletic group consists of all of the descendants of a common ancestor minus one or more monophyletic groups. Thus, a paraphyletic group is ‘nearly’ monophyletic (hence the prefix ‘para’, meaning ‘near’ or ‘alongside’.)” (4)
➤Polyphyletic “A polyphyletic group is characterized by convergent features or habits (for example, …fruit trees…); the features by which the group is differentiated from others are not inherited from a common ancestor.” (4)
➤Linnaean classification – See The Dinosaur FAQ: Our foundational system of biological classification, developed by the 18th-century Swedish doctor, zoologist, and botanist Carl Linnaeus. The Linnaean system introduced a conceptual superstructure, or tree-like pattern, from which all life-forms branched out. In this system, nested groupings of organisms are arranged hierarchically, in ranks– like trunks, branches, and twigs of the tree of life. These ranks include, in descending order: kingdoms, phyla (or, in botany, divisions,) classes, orders, families, genera, and species.
The Linnaean system also gave us a universal and precise way to name particular organisms, by introducing binomials, or standardized two-worded names, such as Phaseolus vulgaris for the common bean. The first word in a binomial indicates an organism’s genus, while the second word indicates its species. This naming method is still in use today.
The higher rankings in the Linnaean system were never made completely uniform, and can be described as a “maze of similar-sounding ranks, semi-standardised endings and contradictory ‘rules’.” In recent decades, it has become standard to instead describe the broader taxonomic groupings (especially above the level of order) in terms of unranked clades (see “clade” and “cladistics.”)
➤Domain (5) Currently, this is widely accepted as the highest rank (or broadest grouping) of biological organisms. There are three domains:
Archaea – Prokaryotes, or microorganisms consisting of one cell with no nucleus
Bacteria – Another class of prokaryotes
Eukarya – Life forms consisting of one or more cells with nuclei and organelles (specialized compartments within a cell) bounded by membranes. All multicellular organisms are in is domain. These include animals, plants, fungi, algae, and some protists.
For example, all plants are in the Eukarya domain.
***What about viruses? Viruses do not have cells and are not currently included in mainstream biological classification systems. Cells have traditionally been defined as the building blocks of life. However, a broadening of the domain system has been proposed in recent years. (5)
➤Kingdom (6) In the Linnaean classification system, a rank below domain and above phylum/division. All green plants–including green algae (which are mostly aquatic) and land plants– are in the plant kingdom, Plantae. Traditionally, this kingdom also included fungi and all algae (including brown and red algae.) The plant kingdom (and Viridiplantae clade) includes green algae, mosses, liverworts, hornworts, clubmosses, ferns, gymnosperms, and angiosperms. Of these, only the last two– gymnosperms and angiosperms– are seed-bearing.
For example, Brassica oleracea (cabbage) is one of more than 350,000 species in the plant kingdom or Viridiplantae clade.
Other kingdoms include archaea/ archaeabacteria, bacteria / eubacteria, fungi, and animals. Protists, once categorized as a kingdom, are no longer considered an official kingdom, but a loose assortment mostly defined by what they are not. Outside of the U.S., archaea and bacteria are typically combined into the monera kingdom. Kingdoms may now be classified differently, as supergroups.
➤Phylum/Division (7,8) A rank below kingdom and above order. Phyla/Divisions of seed-bearing plants include:
THESE GYMNOSPERM DIVISIONS–
Ginkgophyta (ginkgo, maidenhair tree) – Have seeds not protected by fruit. There is only one living species left.
Cycadophyta (cycads) – Have seeds and a crown of compound leaves
Pinophyta (aka Coniferophyta, conifers) – Have cones containing seeds and wood composed of tracheids, or water-conducting cells with no perforations in their walls (as opposed to water-conducting vessels.)
Gnetophyta (gnetophytes) – Have seeds and a woody vascular system with vessels
AND THIS ANGIOSPERM DIVISION–
Magnoliophyta Having flowers and fruit, vascular systems with vessels
➤Class (9) A rank below phylum/division and above order
➤Order (10) A rank below class and above family. There are no hard rules for classification at the order level. Rather, consensus is reached over time by the taxonomist community. The suffix -ales is used in the names of plant orders.
For example, cabbage is in the order Brassicales, while the common bean plant is in the order Fabales.
➤Family (11) A rank below order and above genus. There are no hard rules for classification at the family level. Rather, consensus is reached over time by the taxonomist community. The suffix -aceae is used in the names of plant families. Each plant family also has a common name that highlights one of its key members or describes all members.
For example, cabbage is in the family Brassicaceae. Brassicaceae is also called the mustard family. The common bean plant is in the family Fabaceae, or the legume family.
➤Genus (12) A rank below family and above species. There are no hard rules for classification at this level, but genus classification usually meets these criteria:
-The genus is distinct.
-The genus is not too expansive.
-Hypothesized descendants of the same ancestor are grouped together. See “monophyletic.”
-Species within the genus have similar physical attributes and/or DNA sequences.
For example, cabbage is in the Brassica genus. Other plants in this genus include canola, Chinese cabbage, and turnips. The common bean plant is in the genus Phaseolus. Other plants in this genus include lima beans and tepary beans.
➤Species A rank below genus and above variety. “A species is often defined as the largest group of organisms in which two individuals are capable of reproducing fertile offspring, typically using sexual reproduction.” (13)
For example, the scientific species name of cabbage is Brassica oleracea. Other plants classified as Brassica oleracea are broccoli, Brussels sprouts, collard greens, cauliflower, and kale. The species name of the common bean plant is Phaseolus vulgaris.
➤Variety (14, 15) A rank below species. Each variety of a plant species has “some unique characteristic” (15) and “an appearance distinct from other varieties, but will hybridize freely with those other varieties.” (14) “Varieties often occur in nature and most varieties are true to type. That means the seedlings grown from a variety will also have the same unique characteristic of the parent plant.” (15) A plant’s variety is often indicated by the abbreviation “var.” in its full scientific name.
For example, wild cabbage is Brassica oleracea var. oleracea. Head-forming cabbage, a descendent of wild cabbage, is Brassica oleracea var. capitata. Collards and kale are Brassica oleracea var. acephala. Broccoli and cauliflower are Brassica oleracea var. botrytis. The Brussels sprouts plant is Brassica oleracea var. Gemmifera.
Please note, in the realm of cultivated plants, the term “variety” is often used informally and interchangeably with “cultivar” (cultivated variety. See “cultivar.”) To further confuse matters, “variety” is the legal term for cultivar. (14) In any case, seed savers should be aware of a plant’s variety and/or cultivar name if they want to produce more plants true to type, as open-pollinated plants are distinguished from one another at the variety/cultivar level.
➤Cultivar -See “Cultivar versus Variety” (15): A cultivar is a cultivated variety. Cultivars are “selected and cultivated by humans. Some cultivars originate as sports or mutations on plants. Other cultivars could be hybrids of two plants…Cultivars are not necessarily true to type…” (15) Please note: All of the open-pollinated cultivars in the seed library’s collection are true to type.
The terms variety and cultivar are often used imprecisely and interchangeably. Their definitions will depend on usage. (See “variety.”) “In today’s world of horticulture, cultivars are planted and used more than varieties. Yet we often still refer to a type of plant species as a variety instead of what is actually is a cultivar.” (15) Seed savers should be aware of a plant’s variety and/or cultivar name if they want to produce more plants true to type, as open-pollinated plants are distinguished from one another at the variety/cultivar level.
For example, the cultivar name of the ‘Tavera’ bush bean is Phaseolus vulgaris ‘Tavera’. One cabbage cultivar is B. oleracea var. capitata ‘Golden Acre.’
(1) Taylor, Mike. “What are classification, taxonomy, phylogeny, systematics and cladistics?” The Dinosaur FAQ.
(2) “Clade” definition, Google Dictionary
(3) Collins, Allen G., Rob Guralnick, Brian R. Speer, Dave Smith. “An Introduction to Cladistics.” University of California Museum of Paleontology.
(4) “Clade” article, Wikipedia.
(5) “Domain” article, Wikipedia.
(6) “Kingdom (biology)” article, Wikipedia.
(7) “Phylum” article, Wikipedia.
(8) “Division (biology)” article, Wikipedia.
(9) “Class (biology)” article, Wikipedia.
(10) “Order (biology)” article, Wikipedia.
(11) “Family (biology)” article, Wikipedia.
(12) “Genus” article, Wikipedia.
(13) “Species” article, Wikipedia.
(14) “Variety (botany)” article, Wikipedia.
(15) Haynes, Cindy. “Cultivar versus Variety.” Horticulture & Home Pest News.