4.1: Plant Taxonomy

Learning objectives

By the end of this section you will be able to:

• List the seven levels of the plant classification system.
• Recognize the taxonomic diversity of common foods in your diet.
• Summarize the contribution of Linneaus to binomial nomenclature.
• Understand how the two-part scientific naming system works and its applications.

Plant taxonomy

Here are some introductory definitions:

Taxonomy (or systematics): The science of classifying organisms.

Classification: A grouping of plants according to shared qualities or characteristics.

Plant taxonomy: A hierarchical classification system based on morphological (see below) and phylogenetic (see below) similarities among plants.

Nomenclature: A formal system of names attached to taxonomic groupings.

Hierarchy: A system of grouping in which each classification is a subset of a superior grouping, and may contain subordinate categories. As an example: The landmass of the United States (used here as the highest or most inclusive level of classification) is partitioned into states (a middle level of classification). States, in turn, are partitioned into counties (the lowest level in this hierarchy). Counties are subsets of states, which are in turn subsets of the nation. This hierarchical type of grouping system is used in plant taxonomy.

Morphology: The appearance (shape and structure) of a plant. Plant taxonomy is a hierarchy primarily based on grouping together plants that exhibit structural (phenotypic) similarities.

Phylogeny: Ancestral, evolutionary relationships among plants. While plant taxonomy has historically been based on plant morphology, these relationships are currently being verified and expanded using new molecular genetic technologies that uncover genetic similarities through comparisons of shared DNA sequences. In general, plants sharing more DNA are considered more similar from an evolutionary standpoint, and considered to have diverged from each other more recently in evolutionary time than plants that share less DNA.

Taxonomy in the pantry: Classification exercise

“Food on shelf” by Jaranda, CC BY-SA 2.0

To start becoming familiar with taxonomic categories, go to your fridge, cupboard, or pantry and choose a variety of fruits, vegetables, and grains. These might include cans of mushrooms, green peas, black-eyed peas, chickpeas, butter beans, and sweet corn; bags of pine nuts and coconut, and perhaps a banana that was getting a bit too brown.

Think about the many ways in which you could group these foods. You might, for instance, categorize a food by whether it is canned or fresh, by size, manufacturer, or color, or by the meal in which you would typically eat it.

Or you could apply a biological, hierarchical classification system, categorizing them by the morphology and phylogeny of the plant on which they grew. The plant systematics hierarchy we will use in this course is as follows, from highest (most inclusive) to lowest level:

• Kingdom
• Division (or Phylum, although Phylum is more commonly associated with animal taxonomy)
• Class
• Order
• Family
• Genus
• Specific epithet (usually a species name)

Memorize this hierarchy, so it rolls off your tongue like a multiplication table.

Now apply this taxonomic system to your foods. An easy way to do this is to search for each food on the U.S. Department of Agriculture’s site USDA Plants Database. If you type “tomato” into the search bar, select “Common Name” from the dropdown menu, and click “go,” you’ll see all the plants with “tomato” in their common name. Click on Solanum lycopersicum L. (garden tomato) and you’ll get this entry, the description for the common garden tomato. Scroll down to see the “Classification” section, which lists the taxonomic classification and includes Kingdom, Division, Class, Order, Family, Genus, and Specific epithet. Notice that this database has finer divisions of hierarchy than you are required to know, including Subkingdom, Superdivision, and Subclass.

Here’s a look at how you can use the USDA Plants Database to find the full classification of plants, like tomato. Try it out!

You can use the information in the database to classify your foods. For products with several ingredients, pick one from the ingredient list, such as wheat in crackers or tomato in spaghetti sauce.

While you can sometimes find this info on Wikipedia, be aware that Wikipedia is not always reliable and you’ll want to cross-reference with other sources. If you enter “tomato” into the Wikipedia search bar you’ll get this page. The right sidebar includes the taxonomic classification. “Unranked” is used instead of Division and Class, which means there is some disagreement on whether those names are the correct Division or Class names. You might also see several hierarchical terms listed as “Clade,” rather than the proper terms. If you can’t find complete information on Wikipedia, use the USDA site.

For the foods in our hypothetical pantry — mushrooms, green peas, black-eyed peas, chickpeas, butter beans, sweet corn, pine nuts, coconut, and banana — we can divide them into the following Kingdoms: Plantae and Fungi.

We can separate the products within the Plantae kingdom into two Divisions:

• Pinophyta: the pine nuts, which come from a conifer
• Magnoliophyta: everything else in this kingdom, which come from flowering plants

The cans and bags in the Magnoliophyta division can be separated into these Classes:

• Liliopsida (Monocotyledons — one embryonic leaf in the seed, parallel leaf veins, and petals and sepals in multiples of three): corn, coconut, and banana
• Magnoliopsida (Dicotyledons — two embryonic leaves in seed, and branched leaf veins): green peas, black-eyed peas, chickpeas, and butter beans

In comparison to the other levels, Order is a relatively arbitrary set of classifications that were created in part to make subsequent classifications more manageable. Order will be addressed in the section on phylogeny.

Next, the products can be subdivided by Family.

“Public Flower Garden in downtown Seattle” by FallenPegasus, CC BY-NC 2.0

Depending on details of the particular plant classification system used, there are approximately 230 plant families. Families are often based on types and organization of flower parts and fruit type, including the number of petals, sepals, stamens, and pistils, and the location of the ovary relative to petals. In this website from the University of California Cooperative Extension (optional) the authors identify many of the characteristics used to group plants into families.

Among our food examples, the Family hierarchy includes:

• Arecaceae (coconut, which comes from a palm tree)
• Poaceae (corn, which is a grass)
• Musaceae (banana)
• Fabaceae (the three peas and the butter beans, which are legumes)

Genus and Specific Epithet are the last two classifications. The pairing of genus and specific epithet to name a plant is called binomial nomenclature. Tthe first letter of the genus is capitalized, and the entire binomial is either underlined or written in italics.

Watch this video for an explanation of plant taxonomy.

Review questions

Think about these questions and be able to discuss the answers or know how to find the them using the resources provided:

1. Where are blue spruce trees (a conifer) taxonomically separated from lilies (flowering plants) at the Kingdom or Division level? (Hint: Use the online resources that were provided in this section.)
2. Are lilies (monocots) separated from beans (dicots) at the Class or Order level?
3. At which taxonomic level are flowering plants separated into different classifications based on flower and fruit characteristics?

Example taxonomy tree

Now that you have the names for each of your plants, you can organize them into a taxonomic tree that more clearly shows their relationships to one another. Below is an example tree based on some of the foods found in our hypothetical pantry:

Image credit: Tom Michaels

Now, try it yourself. Determine which plants you want to use, look them up on the USDA Plants Database, write down the Kingdom, Division, Class, and so on for each, and begin constructing the tree to show relationships and points of divergence. In this example, the mushrooms diverge from everything else at the Kingdom level, pine nuts diverge from the other three at the Division level, corn is in a different Class than pea and chickpea, and pea and chickpea diverge at Genus.

The point of this exercise is for you to understand that relationships among plants are known, and are categorized in a sophisticated taxonomic system. Some of the plants we commonly eat have close relationships, like the various plants in the Solenaceae family (tomato, eggplant, potato), but others are much more distant.

Linneaus and plant taxonomy

Binomial nomenclature

Carolus Linnaeus (1707–1778), a Swedish professor, is widely recognized for developing the binomial nomenclature for plants. Binomial nomenclature is a scientific classification in which each organism is given two names. In his 1753 book Species Plantarum (kinds of plants), Linnaeus employed this system to describe a great number of plants using Latin polynomials. The first word of the polynomial became the genus, and a marginal note describing the plant became the specific epithet. Several years ago we celebrated Linnaeus’ 300th birthday, and you can find a long set of links about him from a simple Google search.

A proper binomial, in addition to the Genus and specific epithet, also includes the initials of the naming authority — the person who proposed the accepted name. Previous naming authorities might also be listed in parentheses. For example:

• Phaseolus vulgaris L. — common bean. The “L” stands for Linnaeus.
• Phaseolus acutifolius A. Gray — tepary bean. The authority for this one is A. Gray.

Interspecific hybrids (hybrids formed from crossing two different species) may be designated with an “x” separating the two constituent species; the “x” can be read as shorthand for “crossed with” — for example, Phaseolus vulgaris L. x Phaseolus acutifolius A. Gray. They might also be given a new name incorporating an “x” to show that the plant is the result of an interspecific cross: Fragaria chiloensis x Fragaria virginiana = Fragaria x ananassa (cultivated strawberry)

Notice, from these examples of interspecific crosses, that the ability to cross and to have fertile offspring isn’t a firm definition of species. It is generally true that breeding is restricted to within-species boundaries, but there are exceptions.

While some plant names have been updated to reflect the most recent knowledge about their morphology and phylogeny, their older names might still be in common use in some settings. Coleus, for example, has the following binomials, all for the same plant:

• Ocimum scutellarioides L.
• Plectranthus scutellarioides (L.) R. Br. (Notice that “L” is now in parentheses, showing that Linneaus was the earliest naming authority, but that his original name for the plant has now been superseded.)
• Coleus scutellarioides (L.) Benth.
• Coleus blumei Benth.

Important notes about binomial naming conventions:

• The Genus is always capitalized and either italicized or underlined.
• The specific epithet is lowercase and either italicized or underlined.
• The naming authority is capitalized and often abbreviated; if the species has been renamed, the first authority is in parentheses.
• An “x” between the Genus and specific epithet denotes an interspecific cross.
• A “x” before the Genus denotes an intergeneric cross.

Future of plant taxonomy and systematics

Taxonomy might first seem an old and dull science, sorting plants into a database using a system developed by someone born more than 300 years ago. But plant exploration experiments and the discovery of previously unknown species can take researchers to the far corners of the world, and taxonomy is important in classifying and naming these new discoveries. Also, for already discovered species, there is continual discussion about the real relationships among these plants and others and whether currently classified plants should be reclassified based on new information. With advances in molecular genetics through techniques that reveal a plant’s DNA sequence, for example, taxonomy is moving more and more toward a phylogenetic basis, based on evolutionary relationships established through DNA similarities and differences instead of solely on morphological characteristics (features about the plant that you can see).

• Traditional taxonomy relies on morphological phenotype (the appearance of the plant).
• Molecular taxonomy relies on genotype (the particular combination of alleles of each gene in the organism).

For more information, check out this Wikipedia article about molecular phylogenetics.

The Angiosperm Phylogeny Group (APG) is a group of taxonomists who are working together to modify flowering plant taxonomy using molecular systematics. The APG’s work is focused at the taxonomic level of Order and, to some extent, Family. While Order has long been a fairly arbitrary categorization, it may now be based more on molecular relationships.

The utility of classification goes beyond the satisfaction of good organization. Classification can inform us of new or lesser-studied plants that share valuable characteristics with plants already familiar to us. Now we have tools and knowledge that give us increasing control over the transfer of DNA among plants. Plant breeders can use insights from taxonomists to identify DNA sequences in related plants that might provide new sources of resistance to disease and insects, or new quality attributes, if transferred to food crops.

Review questions

An interactive H5P element has been excluded from this version of the text. You can view it online here:
https://open.lib.umn.edu/horticulture/?p=141#h5p-5