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Tuesday - June 17, 2014

From: Denton, TX
Region: Southwest
Topic: General Botany
Title: Taxonomy question concerning the Family Commelinaceae
Answered by: Nan Hampton

QUESTION:

Hi I have a question. Many people refer to plants differently, I have always used the Genus and species and rarely the family name..it is very confusing .. when a professional uses a name that is a combo of a common name that is not commonly used and the family name never even getting to the genus, I find it very frustrating. Am I wrong in your way of thinking as well? Example Recent article about dayflower family (commelina erecta) it is in fact genus tradescantia.. in the article he refers to commelina erecta in parenthesis and calls it dayflower and then later in his article he referes to it as commelinaceae in parentheses and then spiderwort as common family name. and I find other sites refering to commelina as the genus? It is not it is the family C O F G S ... can you shed some light on this difference? Are in fact dayflower, spiderwort,tradescantia and commelina all the same plant?

ANSWER:

Yes, it is just a bit confusing!  The most precise way to refer to a plant is by its binomial botanical name.  This is made of the genus and the specific epithet.   The two together give you the species name.  There is an excellent explanation of botanical nomenclature online from Utah State University.

To start, let's consider the species named Commelina erecta (Whitemouth dayflower).  The genus is Commelina, the specific epithet is erecta, the common name is whitemouth dayflower (or widow's tears or dayflower or white-mouth dayflower—there may be more common names and we'll talk about that later).   It is in the Family Commelinaceae, the spiderwort family.  Here is the taxonomic breakdown for it:

Kingdom — Plantae – Plant

  Sub-Kingdom — Tracheobionta – Vascular plants

    Superdivision — Spermatophyta – Seed plants

      Division — Magnoliophyta – Flowering plants

        Class — Liliopsida – Monocotyledons

          Subclass — Commelinidae

            Order — Commelinales

              Family — Commelinaceae – Spiderwort family

                Genus — Commelina L. – dayflower

                   Species — Commelina erecta L. – whitemouth dayflower

There are 10 genera in Family Commelinaceae—one of those genera is Tradescantia with 33 species shown in the USDA Plants Database.

If we look at the species called Tradescantia gigantea (Giant spiderwort), it's taxonomic profile is exactly the same as Commelina erecta except for the genus and specific epithet designations:

Kingdom — Plantae – Plant

  Sub-Kingdom — Tracheobionta – Vascular plants

    Superdivision — Spermatophyta – Seed plants

      Division — Magnoliophyta – Flowering plants

        Class — Liliopsida – Monocotyledons

          Subclass — Commelinidae

            Order — Commelinales

              Family — Commelinaceae – Spiderwort family

                Genus — Tradescantia L. – dayflower

                   Species — Tradescantia gigantea Rose – giant spiderwort

Another plant in the Family Commelinaceae that resembles the two plants above is Tinantia anomala (False dayflower).  In our Native Plant Database we show 27 species that are included in the Family Commelinaceae

Now to complicate things a bit more, Family Tradescantiaceae (Spiderwort family) is an unaccepted synonym of Family Commelinaceae (Spiderwort family).  Family Commelinaceae is the name accepted as correct by the rules of the International Code of Nomenclature for algae, fungi and plants (ICN).  The Integrated Taxonomic Information Service (ITIS) maintains a database of accepted and unaccepted names that includes scientific names of plants, animals, fungi and microbes. 

The "authorities" sometimes make changes (based on new evidence) in the names that have been used in the past.   Here are a few examples:

Family Poaceae (Grass Family) was Family Graminaceae or Family Gramineae .

Family Asteraceae (Aster Family) was once Family Compositae.

Styphnolobium affine (Eve's necklace) until a few year's ago was known as Sophora affinis (Eve's necklace).

By the way, referring to a plant by its common name is the least precise and potentially confusing way to refer to it.  For instance, consider plants called "hemlock".   In our Native Plant Database you can find Cicuta maculata (Spotted water hemlock), Family Apiaceae (Carrot Family) and Tsuga canadensis (Eastern hemlock) in Family Pinaceae (Pine Family).  One is a herbaceous plant and the other is a tree.  Not only are they in different genera, they are in different families.  There are many more instances of a common name referring to two or more completely different plants.

To my way of thinking, the best way to refer to a plant is by its binomial designation—its genus and specific epithet which makes up its species name.  You can mention one of its common names along with that and there are times you do want to mention its family name just to show its relationship to other plants that might be of interest.

 

From the Image Gallery


Whitemouth dayflower
Commelina erecta

Whitemouth dayflower
Commelina erecta

Giant spiderwort
Tradescantia gigantea

Giant spiderwort
Tradescantia gigantea

False dayflower
Tinantia anomala

False dayflower
Tinantia anomala

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