Of course there are many sub-categories of the basic groupings listed below, but these are the general categories.
Any taxonomic group (species, genus, etc.) = taxon (singular); taxa (plural)
Kingdom (Plantae, Fungi, etc.)
An example: Sorbus scopulina var. cascadensis is is the scientific name of Cascade Mountain ash.
In general, each species has a single scientific name, but taxonomy does change, sometimes because of new understanding in systematics (patterns of relationships among species and higher taxa), sometimes because older names are re-discovered (and so have priority). Taxonomy attempts to reflect evidence of evolutionary relationships, but this is not always so easy. New evidence does not necessarily lead to clearer understanding, and sometimes (as you may have suspected) taxonomic changes are often rather arbitrary.
Plant types created by humans
Following the present rules, cultivars are named in a modern language (not Latinized). The name is capitalized, with single quotes, e.g.:
Penstemon fruticosus 'Purple Haze'
Hybrid plants may occur naturally or be created by humans. They usually have the letter "X" somewhere in their name. I have some penstemons in the nursery that I grew from seed and suspect are hybrids. I know what one of the parent species is, but am not sure of the other, so I call them Penstemon X subserratus.
Gender of Names
Rosa nutkana (Nootka rose), Rosa gymnocarpa (bald-hip rose)
But in many genera of trees, gender of names does not agree:
Plant Families, Genera & Species.
Species
Genus
Family
Some plant families have "old" names (often descriptive of flower, fruit or inflorescence shapes) that have no type genus. In at least six cases, the "old" names and the "new" names have both been judged acceptable because the families are so well known and the old names in such common usage. Different authors have their preferences, so you never know which name will be used. Here are the alternate names of some of our most familiar families:
(the following are generally clearest, most useful and are discussed in greater detail)
The scientific name of each species is the entire binomial, that is: Genus + species. And there may be categories below this.
Scientific names at the level of genus or below are "Latinized" & underlined or italicized: Genus (capitalized) species (lower case). Other naturally-occurring sub-categories: subspecies or variety (lower case, italicized):
(There are also the often vague categories of "ecotype" and "forma," but these are not commonly used.)
Some groups of plants are developed by humans. These are usually propagated asexually (by cuttings, tissue culture, etc.) to generate many identical plants from a choice wild plant or hybrid, an unusual individual or a part of a plant with some interesting somatic (=non-sexual) mutation. This is a cultivated variety or cultivar (cv.) and asexual propagation of these plants without paying royalties is illegal if the cultivar is patented!
Arctostaphylos uva-ursi 'Vancouver Jade'
or simply: Arctostaphylos 'Vancouver Jade'
Latin is (or was) a language where objects have a gender - they are either male or female. Most of the time the generic name and specific epithet should agree:
Alnus rubra (red alder), Pinus ponderosa (ponderosa pine), Quercus garryana (Oregon white oak)
Years ago, I was told as a student that the specific epithet of many tree species was feminine because they were believed to house female spirits (dryads, I think) by the ancient Greek botanists, and this tradition was carried along through the centuries. As a result, many trees have a masculine generic name and a feminine specific epithet. I really don't know if this story is true, but it's a good one! And it isn't the only strange tradition in Botany!
In the formal rules of nomenclature set by international agreement, each species must have a type specimen housed at a herbarium and must be validly published to be acknowledged.
This is the most debated taxon in biology, but it continues to resist complete definition. It has classically been defined as a group of individuals or populations who interbreed and produce viable offspring only with each other. But many plants (and animals, too) break these "rules" - plants are often promiscuous and many species hybridize freely. They sometimes have sex with themselves or no sex at all, and still produce viable seed. Sometimes a plain diploid cell (not a gamete or sex cell), if it's in the proper location, will behave as a zygote and begin to form a new embryo in an apparently normal seed and fruit (called apomixis in plants, parthenogenesis in animals). We still don't really know much about what species are or how they generate themselves, but they do seem to express some sort of reality that allows us to assign them to a category or group. Most of the time we identify members of a species by a combination of unique characteristics - an operational definition - and this is just fine. This (sometimes rather tacit) approach to recognition is greatly improved with practice and generally works well!
This a group of species sharing a common evolutionary history identified by one or more uniquely shared characteristics (or perhaps a unique "suite" of characters). It can be pretty arbitrary, but a great many genera seem distinctively real and natural. Some may have subgroups - "Sections."
This is a group encompassing the groups of genera and species below it. Again, it is sometimes a rather arbitrary collection of genera, but many families seem clearly defined - at least intuitively! References such as the Sunset Western Garden Book started to include familily names several years ago - I was delighted - this gave me so much more information about the genus being discussed. Families are worth getting to know and recognize. They are sometimes divided into Sub-families, and some of the really huge families, such as the grasses and sunflowers, are divided into Tribes between the level of Sub-families and Genera. The more recent rules of nomenclature insist that every family will be named after a "type genus" and each will have the ending "-aceae" - Rosa is the type genus for the Rosaceae, Scrophularia for the Scrophulariaceae, Acer for the Aceraceae (in Zoology, families have the ending "-idae" - as in Salmonidae, Hominidae).
Common Name Old Fam. Name New Fam. Name Type Genus
Grasses Gramineae Poaceae
Poa (Bluegrass)
Sunflowers Compositae Asteraceae
Aster (Aster)
Peas/Beans Leguminosae Fabaceae
Faba (Fava Bean)
Mustard/Cabbage Cruciferae Brassicaceae Brassica (Mustard)
Carrots/Celery Umbelliferae Apiaceae Apium (Celery)
Mints Labiatae Lamiaceae Lamium (Henbit)