05. General Classification

General background on plant classification.

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Classification Overview

Kingdom, Domain, and Division

All plants are classified in a hierarchy kingdom, domain, division, class, order, family, genus, and species. Trees belong to the following hierarchy:

• Kingdom: Plantae
• Domain (phyla): Eukaryota
• Division: Tracheophyta (Vascular plants)

The Tracheophyta division includes Angiosperms (flowering plants) and Gymnosperms (non-flowering plants). Other major subdivisions of Tracheophyta include Pteridophytes (ferns and their relatives) and Lycophytes (clubmosses).

In traditional taxonomy, most plant, algal, and fungal divisions end in -ophyta, signifying broad evolutionary groups. Examples include Bryophyta (mosses), Tracheophyta (vascular plants), Lycopodiophyta (club mosses), Monilophyta (ferns), Pinophyta (conifers), and Magnoliophyta (flowering plants). Algae divisions include Chlorophyta (green algae), Rhodophyta (red algae), and Phaeophyta (brown algae), while fungi groups such as Myxomycophyta (slime molds) and Zygomycophyta (zygote fungi) also follow this convention. However, not all groups ending in -ophyta are divisions, and some exceptions exist, such as Myxomycophyta, which refers to non-plant slime molds.

Clades

A clade is a group of organisms that includes a common ancestor and all of its descendants, representing a single branch on the tree of life. In evolutionary biology, clades are identified based on shared characteristics that have been inherited from that common ancestor.

Clades are fundamental units in modern classification systems (called phylogenetic or cladistic systems), which organize organisms based on evolutionary relationships. Unlike older systems, which were based primarily on observable traits (like flower structure or leaf arrangement), clades are defined by ancestry and are often determined using genetic data.

A key feature of a clade is that it is monophyletic, meaning it includes an ancestor and all of its descendants, but no unrelated organisms. If a group excludes some descendants or includes unrelated organisms, it would not be considered a true clade.

For example, the clade Eudicots includes all plants that share a common ancestor which was itself a eudicot, along with all of that ancestor’s descendants, and the clade Mammalia includes all mammals, their common ancestor, and all species descended from that ancestor. In contrast, a group like “reptiles” in traditional classification is not a clade because it excludes birds, even though birds evolved from within the group we call reptiles.

In modern phylogenetics, clades increasingly replace formal divisions which leads to more flexible naming conventions beyond -ophyta. A clade does not necessarily correspond to a fixed taxonomic rank like class, order, or family. Clade names often end -ids. Examples include rosids, asterids, fabids (a subgroup of rosids that includes Fabales and Fagales), and malvids (another subgroup of rosids, including Brassicales and Sapindales).

Clade Naming

Clades in biology are named following either traditional taxonomy or phylogenetic nomenclature, depending on the context.

In traditional taxonomy, governed by the International Code of Nomenclature (ICN), clades are named according to their rank, such as family, order, or class. The names are Latinized, with specific endings for each rank. For example, the suffix “-aceae” is used for plant families like Rosaceae. A key species, called the “type species,” often forms the basis for the name of the clade.

In phylogenetic nomenclature, often guided by the PhyloCode, names are given to clades based on evolutionary relationships rather than traditional ranks. These names are intended to reflect shared ancestry and may not correspond to the hierarchical levels used in traditional taxonomy. Clades are defined through node-based, stem-based, or apomorphy-based definitions, such as by referencing the last common ancestor of certain species and all its descendants.

Regardless of the naming system, the use of Latin or Latinized forms is consistent. In traditional taxonomy, priority is given to the earliest validly published name, while in phylogenetic nomenclature, the focus is on defining groups by their evolutionary connections. Both approaches contribute to the classification and naming of organisms based on their relationships and characteristics.

Class, Order, Family

In botanical nomenclature, the suffixes of taxonomic names provide a clue about their rank, and their plural form is typically indicated by these suffixes. After kingdom, domain, and divisions, the next three levels are class, order, family, genus and species and naming within these obeys the following general rules.

Level		Ending
class	\(\leftrightarrow\)	-opsida
orders	\(\leftrightarrow\)	-ales
family	\(\leftrightarrow\)	-aceae

• The names of classes in plants and animals often end in -opsida (for plants) or -ia (for animals). For example Magnoliopsida, the class of flowering plants, Pinopsida, the class that includes conifers, and Liliopsida, the class that includes monocots (like grasses and lilies). From the Greek word opsis meaning appearance or view. (Remember: o for outside, a big group.)

• The names of orders always end in -ales, and this form is already plural, for example, Cupressales, Fagales, Rosales, and Pinales. From the Latin signifying group. (Remember: ales like allez to go, an order.)

• Family names always end in -aceae, which is the plural form of the name based on the type genus. For example, Cupressaceae refers to the cypress family, Fagaceae (oak and beech family), Rosaceae (rose family), and Pinaceae (pine family). A plural from the Latin used for families, referring to belonning to a specific genus. (Remember: c for child, part of your family.)

The book Christenhusz et al. (2020) lists 91 orders and about 450 families. It gives the following list of common endings.

Level	Ending	Example	Used
Division	-ophyta	Equisetophyta	rarely
Subdivision	-ophytina	Equisetophytina	rarely
Class	-opsida	Equisetopsida
Subclass	-idae	Equisetidae
Superorder	-anae	Equisetanae
Order	-ales	Equisetales
Suborder	-ineae	Equisetineae	rarely
Family	-aceae	Equisetaceae
Subfamily	-oideae	Equisetoideae
Tribe	-ideae	Equisetideae
Subtribe	-inae	Equisetinae

Capitalization

In scientific nomenclature, the genus name is always capitalized while the species name is lowercase, and both are italicized (e.g., Quercus robur). For higher taxonomic ranks, such as family, order, class, and above, the names are capitalized but not italicized (e.g., Fabaceae, Pinales). Common names are typically lowercase unless they contain a proper noun (e.g., oak tree, but Douglas fir), though conventions in some fields, like bird species, may capitalize common names.

Italicization is used only for genus and species names, not for higher ranks. Thus, a capitalized Latin word in italics, it is a strong indication that the word refers to a genus.

Angiosperms (Flowering Plants)

Angiosperms are the largest and most diverse group of plants, distinguished by their flowers and enclosed seeds. The two major classes within angiosperms are Monocotyledons (monocots) and Dicotyledons (dicots).

Monocots have a single seed leaf, or cotyledon, in the embryo. They typically show parallel leaf venation and floral parts arranged in multiples of three. Their vascular bundles are scattered in the stem without a distinct arrangement. Common examples of monocots include grasses, such as maize (Zea mays), rice (Oryza sativa), and wheat (Triticum aestivum), as well as flowering plants like lilies and orchids. Although monocots are mostly herbaceous (e.g., grasses, lilies), a few monocots do grow as trees. These trees typically lack the same secondary growth (wood) seen in dicots and often have fibrous, non-woody trunks. The most prominent examples of monocot trees are palms and a few other tropical plants. Examples include: Palms (Arecaceae family) such as Cocos nucifera (coconut palm); Banana Trees (Musa species), although not true trees, bananas are large monocot plants; and Yucca and Dracaena which can grow tree-like forms in certain environments.

Dicots have two seed leaves, or cotyledons, in the embryo. They usually exhibit net-like or reticulated leaf venation and have floral parts in multiples of four or five. In dicots, vascular bundles are arranged in a ring within the stem. This class includes a wide range of plants, from small herbaceous species to large woody trees. Examples of dicots are roses (Rosa), oak trees (Quercus), peas (Pisum sativum), and sunflowers (Helianthus).

What are Dicots?

Dicotyledons (dicots) are not considered a formal taxonomic rank like class, order, or family in modern plant classification. Historically, dicots were treated as a class called Magnoliopsida under older classification systems (such as the Cronquist system). However, with advances in molecular phylogenetics, it became clear that dicots are not a monophyletic group, meaning they do not all share a common ancestor.

As a result, the term “dicot” is now more of a descriptive or informal group rather than a formal taxonomic category. Most dicots are now placed within a larger clade called Eudicots, which represents a more specific evolutionary lineage. So, while dicots used to be treated as a class, in modern classifications they are better understood as part of the broader angiosperm lineage without a direct rank.

Additionally, modern classifications may divide angiosperms into further clades like Eudicots (true dicots) and Magnoliids based on genetic relationships, but monocots and dicots remain the two primary distinctions.

Eudicots vs. Magnoliids

Eudicots and magnoliids differ in their evolutionary history and key morphological traits. Eudicots, the largest group of angiosperms, are considered more advanced and are characterized by tricolpate pollen, which has three furrows or pores. Their floral parts are typically in multiples of four or five, and they exhibit net-like leaf venation. In contrast, magnoliids represent an older, more primitive lineage of flowering plants. They have monocolpate pollen, with a single furrow or pore, and their floral parts are often in multiples of three, sometimes arranged in spirals. Magnoliids tend to be more variable in their floral structures, reflecting their earlier divergence in plant evolution. Examples of eudicots include oaks and roses, while magnoliids include magnolias, avocados, and nutmeg.

Gymnosperms (Non-Flowering Plants)

Gymnosperms are ancient, seed-producing plants that differ from angiosperms in that their seeds are not enclosed within an ovary but are instead exposed, typically on cones. Gymnosperms are classified into several major groups, including Pinopsida (conifers), Cycadopsida (cycads), Ginkgoopsida, and Gnetopsida.

Pinopsida, or conifers, are the most common gymnosperms, characterized by their needle-like or scale-like leaves and their reproductive structures in the form of cones. Many conifers are large, long-living trees. Well-known examples include pine trees (Pinus), cedar trees (Cedrus), including the Cedar of Lebanon, and giant redwoods (Sequoia).

Cycadopsida, or cycads, are an ancient group of tropical and subtropical plants that resemble palms but reproduce via cones. They have large, compound leaves and are slow-growing, often found in regions with poor, sandy soils. Examples include the sago palm (Cycas revoluta) and Zamia integrifolia.

Ginkgoopsida contains only one surviving species, Ginkgo biloba, commonly known as the maidenhair tree. It has fan-shaped leaves and a unique reproductive system, with seeds exposed after the fleshy outer covering decays.

Gnetopsida is a small, peculiar group of gymnosperms that share some characteristics with angiosperms, such as vessel elements in their vascular tissues. This group includes plants like Gnetum, Ephedra, and the strange Welwitschia species, which can live for thousands of years in desert environments.

These groups represent the diversity within gymnosperms, many of which are adapted to harsh environments and can live for centuries or even millennia.

Number of Species per Family

Source: Christenhusz et al. (2020), page 20.

Family	Species	Trees	Common Trees or Examples
Orchidaceae	26,470	No	Vanilla (orchids), Phalaenopsis (moth orchids)
Asteraceae	24,700	No	Helianthus (sunflowers), Taraxacum (dandelions)
Fabaceae	16,020	Yes	Acacia, Robinia pseudoacacia (Black locust), Albizia (Silk tree)
Rubiaceae	13,620	Yes	Coffea (Coffee tree), Cinchona (source of quinine)
Poaceae	11,000	No	Zea mays (maize), Bambusa (bamboo)
Lamiaceae	6,800	No	Mentha (mint), Salvia (sage)
Euphorbiaceae	6,243	Yes	Hevea brasiliensis (Rubber tree), Euphorbia (Spurge)
Myrtaceae	5,828	Yes	Eucalyptus, Syzygium (Rose apple), Pimenta dioica (Allspice)
Cyperaceae	5,500	No	Carex (Sedges), Cyperus papyrus (Papyrus)
Melastomataceae	5,000	No	Tibouchina, Medinilla
Apocynaceae	4,300	Yes	Plumeria (Frangipani), Nerium oleander (Oleander), Alstonia (Devil tree)
Ericaceae	4,250	Yes	Rhododendron, Arbutus menziesii (Pacific madrone), Vaccinium (Blueberries)
Malvaceae	4,225	Yes	Tilia (Linden or Lime tree), Gossypium (Cotton), Hibiscus
Polypodiaceae	4,070	No	Ferns (Polypodium species)
Acanthaceae	4,000	No	Thunbergia, Justicia
Gesneriaceae	3,810	No	Saintpaulia (African violet), Gesneria
Piperaceae	3,700	Yes	Piper nigrum (Black pepper), Piper betle (Betel leaf)
Brassicaceae	3,628	No	Brassica (Cabbage, Mustard), Arabidopsis
Apiaceae	3,575	Yes	Daucus carota (Wild carrot), Heracleum (Cow parsnip), Ferula (Giant fennel)
Bromeliaceae	3,475	Yes	No trees; Ananas comosus (Pineapple), Tillandsia (Air plants)

Next: Tree Classification

References

Christenhusz, M. J., Fay, M. F., & Chase, M. W. (2020). Plants of the world: An illustrated encyclopedia of vascular plants. University of Chicago Press.