Prior to the 18th century, scientists struggled to find conventions with which to categorize organisms. In the 1730s, the visionary Swedish biologist Carl Linnaeus developed a system of taxonomy (called binomial nomenclature), which assigned Latin names to all living things. This conveniently allowed groups of similar plants and animals to be bundled according to their shared physical characteristics. His work has been adopted and adapted by biological scientists ever since, and is accepted across the globe. A marine biologist working in Helsinki can use this system and be understood by a botanist in Hanoi.
The divisions work like this:
A biological kingdom is split into phyla, then into classes, orders, families, genera, and finally species. Each subcategory grows more specific. A housecat is classified in this system as belonging to the animal kingdom, then the phylum Chordata (animals with spinal chords), then the class Mammalia (all mammals), then the order Carnivora (mammals that eat animals), then the family Felidae (all cats, from lynxes to lions), and finally into the binomial genus and species names Felis catus. Binomial names are always shown in italics to indicate that Latin is being used to name a specific type of organism.
That’s fine for the cat, but what about the eggplant? In the eggplant’s case, the system works like this: Kingdom Plantae (all plants), phylum Angiosperms (all flowering plants), class Asterids (conventional flowers with radial petals), order Solanales, family Solanaceae (shared with nightshade, tomatoes, potatoes, and peppers), and finally the genus/species name Solanum melogena.
Revisions are commonplace, particularly as more organisms are mapped by their genomes, and new evolutionary lineages are discovered.
The Latin name for each plant (and all other living things) contains further information, usually with its roots in Latin or Greek. For instance, all bush and pole beans belong to the genus Phaseolus, and the species vulgaris: Phaseolus vulgaris. The species name “vulgaris” is directly from the Latin, meaning “common.” Runner beans also belong to the genus Phaseolus, but bush beans and runner beans side by side have several striking differences in form, rate of growth, and in the characteristics of their flowers. Runner beans typically have scarlet flowers, so they were named Phaseolus coccineus — coccineus meaning scarlet.
Like the species name “vulgaris,” numerous plants share the Latin species name “officinalis,” which means the one that is most typical of, or best represents, the genus. Another very common species name is from the root sativum, sativus, and sativa, all meaning “cultivated.” A gardener might plant the seeds of Raphanus sativus (radishes), Latuca sativa (lettuce), and Coriandrum sativum (cilantro) all on the same day.
Sunflowers, in all their various sizes and colours and growing forms, are all of the species Helianthus annuus (from helios = sun, and anthos = flower): “Sun-flower that is annual.” The more familiar one becomes with Latin names, the more obviously clever and simple the system appears.
There are all sorts of variations on this system of taxonomy, with subdivisions, super-divisions, groups, clades, and so on, but this is basically how living things are categorized by biologists.
The wild cabbage, in early European farming cultures, migrated as a staple food crop, and strains of it were established in the north, in Scandinavia and the British Isles. Some wild cabbage was cultivated in central Europe and the east, and also in the areas that are now Portugal, Belgium, and Holland. Every time this staple food plant became established among farming cultures, it began to change through selective breeding. The end result of this travel and breeding over centuries is a modern, familiar spectrum of vegetables, often just referred to as “the Brassicas.”
Through selective breeding, the single species Brassica oleracea has now been cultivated to produce common vegetable types that include collards and kale, Chinese broccoli, cauliflower, Romanesco broccoli, broccoflower, cabbage, Brussels sprouts, and conventional broccoli. Botanists apply the Variety name to distinguish between diverse members of a single species. It ranks lower than species, but higher than form, subform, etc…
These variety names are written as Brassica oleracea var. italica (from Italian breeding = broccoli), B. oleracea var. capitata (head-forming = cabbage), B. oleracea var. gemmifera (many tiny heads = Brussels sprouts), B. oleracea var. acephela (headless = kale), and so on.
Of course, kale can just be called kale, and most gardeners will understand one another. And that’s probably all anyone needs. It’s useful to know how the system works though, even if one you never use.