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What exactly does it mean when a plant has a scientific name that specifies a vairety, for example Nothofagus solandri var. cliffortioides, or when the name includes an "x", as in Populus maximowiczii Henry x trichocarpa, Populus xcanadensis, or "Crataegus x macrocarpa"?

All of these species are in the USDA Plants database (of plants that occur in the US). There are > 6000 occurences of the string " var. " ~500 occurances of " ssp. " and only a handful occurences of " x " in the database,

  • Does the x indicate that a plant is a hybrid, and are the varied uses indicate a different meaning?
  • Does the use of " var " mean that the plant is a variety?
    • Does it mean that each of these species has been cultivated and/or bred by a human?
  • Does "ssp" mean that the plant is distinct yet compatible with others in the species?
  • is there a way to identify a that a plant is cultivated based on its scientific name?
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2 Answers 2

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Let me clarify my answer since it is lower quality than people may like. To answer the question, my friend is a horticulist and has given a more detailed answer.

Subspecies is the most generic, taxonomically-defined term one rank order lower than species. The subspecies (either an individual subspecies, or collective group of subspecies) are defined to be genetically or morphologically distinct among other subspecies belonging to the same species, yet still produce viable offspring from interbreeding. For example, every type of dog is a sub-species of Canis familiaris, and all dogs are capable of breeding with each other. Subspecies, abbreviated Ssp, may refer to an individual subspecies type, or a collective group of related subspecies when a distinct subset is difficult to define or unknown.

There are a variety of terms that are analogous to subspecies, when the strict definition does not apply, or a different term has been used historically. For example, a varietal is the botanical term, and refers to a named subspecies. For example, grapes are a family of species, divided into the various subspecies, and a varietal is a specific subspecies that we refer to by a more common or familiar name, such as Riesling, Chardonnay, etc. (Grapes are a slightly different case in which most grapes are produced from hybrids or crosses for reasons of withstanding environmental conditions avoiding insect prey and fungal infections.) Microbiology has yet another distinction, called a strain. Strains are often genetically different, and yet all belong to a species. For example, several E. coli strains exist. In contrast, viruses are also given strain/subspecies names which don't technically fit the ICZN definition of interbreeding. Cultivar is a similar term, also from the field of botany. Cultivar is specifically applied to plants that are grown for some agricultural benefit, and have been bred or altered by humans for some reason.

In short, there is a zoological definition of subspecies set out in ICZN. This definition requires viable interbreeding among subspecies belonging to the same species. The term applies to individuals, or groups, as the context dictates. Analogous terms of subspecies are applied (adapted?) to other realms of biology, such as botany, microbiology and virology, but the definitions are slightly different, but consistent within the branch of biology (eg, viruses can't breed so they don't fit the ICZN definition, but the term applies). Ssp can also be qualitatively used to describe an unknown subset of a species (I've seen this used in microbiology mostly).

Crosses/hybrids are indicated with the "x" in the nomenclature. Since I found this confusing, I'm copying from Wikipedia.

From a taxonomic perspective, hybrid refers to offspring resulting from the interbreeding between two animals or plants of different species.

  1. Hybrids between different subspecies within a species (such as between the Bengal tiger and Siberian tiger) are known as intra-specific hybrids. Hybrids between different species within the same genus (such as between lions and tigers) are sometimes known as interspecific hybrids or crosses. Hybrids between different genera (such as between sheep and goats) are known as intergeneric hybrids.

  2. The second type of hybrid consists of crosses between populations, breeds or cultivars within a single species. This meaning is often used in plant and animal breeding, where hybrids are commonly produced and selected because they have desirable characteristics not found or inconsistently present in the parent individuals or populations. This flow of genetic material between populations or races is often called hybridization.

There are some horticultural examples of crosses in which an example of interspecific and intergeneric hybrids, and others, are shown.

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  • $\begingroup$ I know what words are being abbreviated, and what they generally mean. I am wondering if the uses have clear definitions and consistent use $\endgroup$ Apr 26, 2012 at 0:19
  • $\begingroup$ @David - Ah, that was not clear in your question. The International Code of Zoological Nomenclature is as definitive a naming system that you will find. The definitions are clear, however not everyone uses the system consistently (particularly older literature). $\endgroup$
    – user560
    Apr 26, 2012 at 2:46
  • $\begingroup$ are there definitions of "var.", "ssp", and "x" available? are they used consistently in the ICZN approved list of plant names? $\endgroup$ Apr 26, 2012 at 2:57
  • $\begingroup$ You'll have to find a copy and look through it, and for a specific (sub)species of interest read the literature to see how it is referenced. $\endgroup$
    – user560
    Apr 26, 2012 at 20:58
  • $\begingroup$ I don't dislike your answer, and the recent changes did provide additional information, but it doesn't answer my questions, For example: is the term varietal used only for cultivated plants? i.e. does use of "var" indicate that a species has been cultivated? Also, is there meaning to the different uses of "x", e.g. "Genus x species" vs "Genus species x otherspecies" vs "Genus xspecies"? Finally, is there a biological definition of 'subspecies' - e.g. can subspecies interbreed? Does subspecies have a consistent biological definition, or is it just used to describe distinct populations? $\endgroup$ May 2, 2012 at 23:27
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Does the use of "var", "x", and/or "ssp" in a scientific name provide specific information?

Yes, those terms denote the rank of the taxon in question.

Does the use of " var " mean that the plant is a variety?

Yes, the use of "variety" or "var." in a taxon name indicates it has the rank of variety. There is also "subspecies", "subsp.", or "ssp." to indicate the rank of subspecies.1

Does [the use of "var."] mean that each of these [taxa] have been cultivated and/or bred by a human?

No. Subspecies and varieties are wild taxa found in nature (mostly, some common crops are called species, such as Zea mays subsp. mays).

Does "ssp" mean that the plant is distinct yet compatible with others in the species?

Probably. This is complicated because it gets into species concepts, but some kind of distinctiveness is required, and the biological species concept (where a species is basically defined as being able to interbreed) is popular.

Does the x indicate that a plant is a hybrid, and are the varied uses indicate a different meaning?

Yes. The multiplication sign × is used to indicate a taxon of hybrid origin. Its different placement can indicate whether the taxon is an intergeneric or interspecific cross. If the × glyph is not available, a lower case x is used (though, really, this should be rare these days). The × should occur before either the genus name or before the specific epithet (e.g., the species Populus maximowiczii is just a regular species name that happens to contain the letter "x").

Is there a way to identify a that a plant is cultivated based on its scientific name?

Yes. Names that include quotes (specifically single quotes), non-italicized text, or the addition sign + could all be indications that the plant is a named cultivar (or group, grex, or graft-chimaera).


Since the question references the USDA plants database, I'll limit the answer to plants. Similar rules are applied to animals and microorganisms.

Each entity being referenced is a taxon (plural taxa), that is, a group of populations of organisms that are seen by taxonomists as forming a unit. Taxa are hierarchically ranked, with the highest being kingdom2, such as Plantae (the green plants), the "basic" rank being species, such as Populus maximowiczii, and—most relevant here—ranks below species, such as subspecies and variety.

The rules for how (wild) plant taxa are named are defined by the International Code of Nomenclature for algae, fungi, and plants (the Code). There are similar codes for animals, prokaryotes, viruses, and, interestingly, cultivated plants.

While the Code recognizes that there are an indefinite number of ranks, it gives names for many of them from kingdom down to forma (grouping them as principle or secondary ranks). It also specifies that more ranks can be created by adding a "super" or "sub" prefix to the rank name to indicate a higher or lower level rank, respectively.

Species is the lowest principle rank, subspecies is below species, and variety is below subspecies (and, for completeness, there is forma below variety).

Strictly, since subspecies and variety are different ranks, a taxon could be classified as a variety that itself is a part of a subspecies. In practice this is rare, usually a taxon is assigned to one or the other.3

Besides denoting a different rank, what is the actual difference between a subspecies and variety? It turns out that the biological definition of these terms is basically up to the taxonomist doing the work. Just like there's no set definition of species, there is no set definition for intraspecific taxa. Some taxonomists might basically rank all intraspecific taxa as subspecies or as varieties, depending on their philosophy. Those that use both terms need some recognition that variety is a narrower term than subspecies, but what that looks like operationally is up to the person/group doing the work.4

(However, one should follow the resource they are using. If it's listed as variety in USDA, use variety. It's incorrect to just decide you don't like it and change it to subspecies [for example]; that would require a research publication.)

Species, either members of the same genus or of different genera can sometimes interbreed and form a natural population. One could refer to these by simply listing the hybrid parents in a hybrid formula, such as Agrostis stolonifera L. × Polypogon monspeliensis (L.) Desf.5 One could also formally name the resulting hybrid. In this case the hybrid nature would be noted by placing the × before either the genus or species name. The above intergeneric cross between Agrostis and Polypogon has been given the name ×Agropogon lutosus C.E.Hubb. The name Populus ×canadensis refers to the interspecific cross between P. nigra and P. deltoides.6

The basic unit of cultivated plants is the cultivar, which is an assemblage of plants selected (by humans) for a specific character, and that remains stable through appropriate propagation. The naming of cultivars is governed by its own code. This code only recognizes the terms subspecies, variety, or form, as they apply in the (wild) plant Code. (Note that other groups, such as the Association of Official Seed Certifying Agencies, do consider cultivar and variety to be synonyms.)

An example cultivar name is Clematis alpina 'Ruby'. Note the use of the correct botanical name, the cultivar epithet in plain text, and the use of single quotes. See other examples here.


1The Code recommends the abbreviation "subsp." for subspecies, but "ssp." is very commonly used.

2Kingdom is the highest rank recognized by the Code; there are generally recognized ranks that are higher, such as superkingdom and domain.

3For example, take the taxon Saxifraga aizoon var. aizoon subvar. brevifolia f. multicaulis subf. surculosa Engl. & Irmsch. This has been fully classified to a particular variety, subvariety, forma, and subforma. To refer to this taxon, one would almost always just write Saxifraga aizoon subf. surculosa, and that is in fact it's name as defined by the Code. One would only spell the whole thing out if they needed to give the full classification. Since no other plant can have the same name, there can be no other subforma within Saxifraga aizoon with the epithet surculosa, even if it were to belong to another variety, forma, etc.

4I could include forma in this paragraph, too, but this seems to have greatly fallen out of fashion as a useful rank.

5I've also included the author abbreviations here. They have their own set of rules. Just know they're not technically part of the name, but often included to help avoid ambiguity.

6This could be written as Populus ×canadensis or Populus × canadensis. The × glyph is not actually part of the name, and can be placed for best readability.

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