Question

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?

Answer 1

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).

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.