Table of Contents
Linnaean System of Classification
Definition: Plant taxonomy is a system of classification for plants. We use the plant taxonomy developed by Linnaeus (1707-1778). Improving the systems of his predecessors, Linnaeus simplified plant taxonomy through the “binomial” system. Linnaeus’ system uses one Latin name to indicate the genus, and another to indicate the specific epithet. Together, the genus and epithet comprise the “species.” E.g., our plant taxonomy classifies bittersweet nightshade as Solanum dulcamara, where the first Latin name is for the genus (nightshade), and the second is for the specific epithet (bittersweet).
- The species is a subset of the genus.
- The genus begins with a capital; the epithet with lower-case. Both are italicized.
- When we translate from Latin, we reverse the order of the names, putting the epithet before the genus.
- Sometimes you’ll see a third name. In such cases, we’re simply getting more specific, accounting for variation within a species. Most commonly, this third name is a cultivar, and it will appear in quotation marks.
- Sometimes yet another word is added after the genus name and epithet, which is neither italicized nor set off by quotation marks — the name of the person who first described the plant. These names are sometimes abbreviated. When the name is abbreviated as “L,” it stands for “Linnaeus.”
- When you see a genus name, followed by the letter “x,” followed by an epithet, this is an indication that the plant is a cross between two different plant species — a “hybrid.”
In this tutorial, you will be learning about the Linnaean system of classification used in the biological sciences to describe and categorize all living things. The focus is on finding out how humans fit into this system. In addition, you will discover part of the great diversity of life forms and come to understand why some animals are considered to be close to us in their evolutionary history.
How many species are there?
This is not an easy question to answer. About 1 3/4 million have been given scientific names. Nearly 2/3 of these are insects. Estimates of the total number of living species generally range from 10 to 100 million. It is likely the actual number is on the order of 13 to 14 million, with most being insects and microscopic life forms in tropical regions. However, we may never know how many there are because many of them will become extinct before being counted and described.
The tremendous diversity in life today is not new to our planet. The noted paleontologist Stephen Jay Gould estimated that 99% of all plant and animal species that have existed have already become extinct with most leaving no fossils. It is also humbling to realize that humans and other large animals are freakishly rare life forms since 99% of all known animal species are smaller than bumble bees.
Why should we be interested in learning about the diversity of life?
In order to fully understand our own biological evolution, we need to be aware that humans are animals and that we have close relatives in the animal kingdom. Grasping the comparative evolutionary distances between different species is important to this understanding. In addition, it is fun to learn about other kinds of creatures.
When did scientists begin classifying living things?
Before the advent of modern, genetically based evolutionary studies, European and American biology consisted primarily of taxonomy or classification of organisms into different categories based on their physical characteristics. The leading naturalists of the 18th and 19th centuries spent their lives identifying and naming newly discovered plants and animals. However, few of them asked what accounted for the patterns of similarities and differences between the organisms. This basically nonspeculative approach is not surprising since most naturalists two centuries ago held the view that plants and animals (including humans) had been created in their present form and that they have remained unchanged. As a result, it made no sense to ask how organisms have evolved over time. Similarly, it was inconceivable that two animals or plants may have had a common ancestor or that extinct species may have been ancestors of modern ones.
One of the most important 18th-century naturalists was a Swedish botanist and medical doctor named Karl von Linné. He wrote 180 books mainly describing plant species in extreme detail. Since his published writings were mostly in Latin, he is known to the scientific world today as Carolus Linnaeus, which is the Latinized form he chose for his name.
In 1735, Linnaeus published an influential book entitled Systema Naturae in which he outlined his scheme for classifying all known and yet to be discovered organisms according to the greater or lesser extent of their similarities. This Linnaean system of classification was widely accepted by the early 19th century and is still the basic framework for all taxonomy in the biological sciences today.
The Linnaean system uses two Latin name categories, genus, and species, to designate each type of organism. A genus is a higher level category that includes one or more species under it. Such a dual level designation is referred to as a binomial nomenclature or binomen (literally “two names” in Latin). For example, Linnaeus described humans in his system with the binomen Homo sapiens, or “man who is wise”–Homo is our genus and sapiens is our species.
Linnaeus also created higher, more inclusive classification categories. For instance, he placed all monkeys and apes along with humans into the order Primates. His use of the word Primates (from the Latin primus meaning “first”) reflects the human-centered worldview of Western science during the 18th century. It implied that humans were “created” first. However, It also indicated that people are animals.
While the form of the Linnaean classification system remains substantially the same, the reasoning behind it has undergone considerable change. For Linnaeus and his contemporaries, taxonomy served to demonstrate the unchanging order inherent in Biblical creation and was an end in itself. From this perspective, spending a life dedicated to precisely describing and naming organisms was a religious act because it was revealing the great complexity of life created by God.
This static view of nature was overturned in science by the middle of the 19th century by a small number of radical naturalists, most notably Charles Darwin. He provided conclusive evidence that evolution of life forms has occurred. In addition, the proposed natural selection as the mechanism responsible for these changes.
Late in his life, Linnaeus also began to have some doubts about species being unchanging. Crossbreeding resulting in new varieties of plants suggested to him that life forms could change somewhat. However, he stopped short of accepting the evolution of one species into another.
Why do we classify living things today?
Since Darwin’s time, biological classification has come to be understood as reflecting evolutionary distances and relationships between organisms. The creatures of our time have had common ancestors in the past. In a very real sense, they are members of the same family tree.
The great diversity of life is largely a result of branching evolution or adaptive radiation. This is the diversification of a species into different lines as they adapt to new ecological niches and ultimately evolves into distinct species. Natural selection is the principal mechanism driving adaptive radiation.
- Common taxonomic divisions
- Plant names in the binomial system
- Sub-groupings of genus and species
- Monocot or Dicot (chart)
- Examples of taxonomic classification (chart)
One of the most useful classification systems is plant taxonomy. Taxonomy is the science of systematically naming and organizing organisms into similar groups. Plant taxonomy is an old science that uses the gross morphology (physical characteristics such as leaf shape, fruit form, etc.) of plants to separate them into similar groups. Quite often the characteristics that distinguished the plants become a part of their name. For example, Quercus alba is a white oak, named because the underside of the leaf is white.
The science of plant taxonomy is being absorbed into the new science of systematics. Systematics is based on the evolutionary similarities of plants such as chemical make-up and reproductive features. The development of more sophisticated microscopes and laboratory chemical analyses has made this new science possible.
Plant taxonomic classification changes with continuing research, so inconsistencies in nomenclature will be found among different textbooks.
An overview of plant taxonomy helps the gardener understand the basis of many of our cultural practices. For example, fire blight is a disease of the Rose family, therefore it is helpful to recognize members of the Rose family to diagnosis this disease.
Common Taxonomic Divisions
The scientific system of classification has all living things divided into groups called taxa (singular, taxon). Plants are in the Kingdom of Plantae. Other Kingdoms include Fungi, Protista (one-celled organisms including yeasts, bacteria, and protozoans), and Animalia (animals).
The plant kingdom is divided into two groups: broyophytes (including mosses and liverworts) and vascular plants (plants with a vascular system of xylem and phloem).
Vascular plants (sometimes called higher plants) are divided into two subgroups: seedless and seeded. These sub-groups divide into Phyla (plural of phylum). Phylum names end in “phyta”. The seedless phyla include the Pterophyta (ferns). Seeded phyla include Cycadophyta (cycads), Ginkgophyta (ginkgo tree), Coniferophyta (conifers), and Anthophyta (angiosperms).
Angiosperms are divided into two taxa, monocotyledon (monocots) and dicotyledon (dicots). Distinguishing between monocots and dicots is a common practice in landscape management as some of our common herbicides work at the monocot/dicot level. For example, lawn weed sprays (such as 2,4-D and Dicamba) kill dicots (broadleaf plants like dandelions) but not monocots (the grass). Other herbicides will kill monocots but not dicots, allowing the gardener to kill grass (a monocot) in the shrub or flowerbed (dicots).
Additional taxa in descending order include class, order, family, genus, and species.
Families of higher plants are separated from one another by characteristics inherent in their reproductive structures (flowers, fruit, and seed). Families have primary importance in gardening as family members generally share comparable cultural requirements and similar insect and disease problems. Disease management and cultural techniques are often discussed at the family level.
Genera (plural of genus) are groupings whose members have more characteristics in common with each other than they do with other genera within the same family. The similarity of flowers and fruits is the most widely used feature, although roots, stems, buds, and leaves are also used.
Common names typically apply to genera. For example, Acer is the genus of maples, Fraxinus is the ash, and Juniperus is the junipers.
Species generally refer to interbreeding sub-group of genus or groupings of individual plants that adhere to essential identification characteristics but display sufficient variation so as not to be categorized as replicas of one another. The species name is always used in conjunction with the genus.
Plant Names in the Binomial System
Plants are named using a binomial system. The Genus name comes first and is analogous to a person’s last name (family name). The specific epithet name follows the genus name and is a more specific identifier. It would be analogous to a person’s first name.
Following the genus and species is the cultivar or variety name. This is an even more specific identifier, similar to a person’s middle name.
When genus and specific epithet names are written, they should always be underlined or italicized to denote they are Latin words. However, cultivar and variety names are not italicized. The genus name is always capitalized, but the species name is not. A genus and specific epithet was written together denotes a species.
The singular and plural spelling of species is the same. In writing, the abbreviation “sp.” following the genus indicates a single unidentified species and “spp.” indicates multiple species. For example “Acer sp.” would indicate an unidentified species of maple, and “Acer spp.” refers multiple species in the maple genus. The “sp.” or “ spp.” are not underlined or italicized.
Wild and naturally occurring plants are named under the rules of the International Code of Botanical Nomenclature. Cultivated plants are named according to the same principles found in the International Code of Nomenclature of Cultivated Plants.
Genus and species names are universal, being used worldwide. Since taxonomy classifies living organisms, there will be some inconsistencies between books.
On the other hand, common names are often local in use and many times don’t clearly identify the specific plant. For example, Liriodendron tulipfera is known as the Tulip Tree in the north and as Yellow Poplar in the south. Carpinus caroliniana goes by American Hornbeam, Blue Beech, Musclewood, Water Beech, and Ironwood. The European White Lily, Nymphaea alba, has 15 English common names, 44 French common names, 105 German common names, and 81 Dutch common names.
Sub-groupings of Genus and Species
Variety or Subspecies is a sub-grouping of species assigned to individuals displaying unique differences in natural populations. The differences are inheritable and reproduce true-to-type in each generation. Variety is denoted as var and subspecies are denoted as subsp.
For example, cauliflower and cabbage are varieties of the same species Brassica oleracea. The thornless variety of honeylocust would be written Gleditsia triacanthos var. inermis.
A cultivar is a species sub-grouping of cultivated plants (cultivated variety) displaying unique differences and when reproduced by seeds or cuttings retain its distinguishing characteristics.
For example, Early Girl and Big Boy are cultivars of tomatoes. In technical writing, the cultivar name follows the genus and species and is always capitalized and written inside single quotes but not italicized. For example, October Glory Red Maple is Acer rubrum ‘October Glory’.
It is possible to have a cultivar of a variety. For example, Cornus florida var. rubra ‘Cherokee Chief’.
Note: Cultivar names must be enclosed in single quotes when following a species name. Variety and subspecies names must be denoted with var. or subsp. when following a species name. For example, Oenothera macrocarpa subsp. incana. The use of trinomials Gleditsia triacanthos inermis is improper usage on scientific nomenclature.
Strain is a sub-group of the cultivar with specific characteristics, like resistance to a disease or better color. For example “Early Girl VFN” tomato.
Clone is a sub-group of cultivar derived by asexual propagation (cuttings). The offspring have one parent and therefore are identical to the parent because no exchange of genetic materials has occurred.
Line is a sub-group of cultivar propagated by seed.
Form is based on selection by growth habit, not reproducible by seed. For example, Columnar Norway Maple.
Herbarium collections have international codes consisting of one or more letters.
The code for the herbarium at Glasnevin is DBN.
The code for the Trinity College, Dublin herbarium is TCD.
The code for the Royal Botanic Gardens at Kew is K.
Some herbaria have huge collections:
K (Kew) has more than 5 million specimens.
NY (New York Botanical Garden) has about 4 million specimens.
US (US National Herbarium, Smithsonian Institution, Washington) has nearly 4 million specimens.
LE (Komarov Botanical Institute, St. Petersburg) has over 5 million specimens.
In Ireland, DBN has c.500,000 specimens, TCD has c.200,000 and BEL (Ulster Museum) has c.110,000.
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