Plant Classification

Plant Classification


Fig, 1 Carolus Linneaus

Born of Necessity

During the Ages of Discovery, Exploration, and Colonialism, plants and animals from all over the world were brought back to Europe. There was an obvious need to develop some kind of order for all of the material that flooded botanical gardens, universities, and museums. Swedish botanist/zoologist Carolus Linnaeus (1707-1778) (Fig. 1) developed a hierarchical classification system in the mid-1700’s (Systema Naturae 1735) that allowed a better understanding of the relationships and connections among all the plants (and animals).

Modern Classification

The Linnaean system is still used today with modifications. However, its classifications are based on physical traits or features (morphology) which may not coincide with the evolutionary (phylogenetic) relationships. Those evolutionary relationships, found using molecular analyses of DNA, are now a goal of systematic research.


Fig. 2 Levels of Classification

What is a classification hierarchy in botany (taxonomy)? It is a system that organizes or ranks plants in descending levels of specificity. In the inverted pyramid with eight levels (taxa) (Fig. 2), the top level is the general level of Domain that encompasses everything below it, as does each subsequent level. The next level is the plant Kingdom comprising Divisions, each Division comprising Classes, each Class comprising Orders, each Order comprising Families, each Family comprising Genera (sing. Genus), and each Genus comprising Species, the most specific level (taxon) in this hierarchy. (The second part of a scientific name – a binomial (two names) – is called the “specific epithet,” which together with the genus forms the Species name.) In addition, most levels have sub-groups such as sub-class, sub-order, sub-species.

As mentioned above, relatedness is mostly based on physical traits, but much reorganization at all taxonomic levels in the last few decades has been based on DNA sequences, therefore on evolutionary relationships. For those of us who studied plants many decades ago, including to what genus and family a plant belonged, it is necessary to relearn the names of taxa. For example, the Figwort family (Scrophulariaceae) was a large family that has been divided up into several families, including the Broomrape family (Orobanchaceae), the Lopseed family (Phrymaceae), and the Plantain family (Plantaginaceae). Species have also been moved to other genera and sometimes to another family. For example, the Point Reyes bird’s-beak, formerly Cordylanthus maritimus ssp. palustris in the Figwort family, is now Chloropyron maritimum ssp. palustre in the Broomrape family, Cordylanthus being a synonym.

How to Remember It All

Remembering the hierarchy of taxonomy can be daunting. Mnemonic devices can be just as hard to remember. I have always wanted to know who the presidents on Mount Rushmore are and will try to remember “We just like Rushmore!” I do find it easy to recall the mnemonic “Kings do chess on fairly good squares.” ( Kingdom, Division, Class, Order, Family, Genus, Species). Although the International Code of Nomenclature also accepts “Phylum” (so kings can “play” chess), “Division” is preferred in botany. (You may have also heard that “Dear King Philip could only find green socks.”) As of the 1990’s, the mnemonic can become a question: “Do kings do chess on fairly good squares?” because Domain was added as the most inclusive level in the taxonomic hierarchy, indicating that the organisms, i.e., plants in our case, are Eukaryotes having cells with a nucleus containing the genetic material, DNA (Fig. 2).

Domain most general Kingdom Phylum Class Order Family Genus Species most specific Each level is called a taxon. You may want to have students draw an upside down triangle and draw 7 horizontal lines across to divide the pyramid up into 8 sections. Then have the students list the 8 taxonomic categories from top to bottom, starting with Domain at the top in the widest section and ending with species at the bottom in the smallest section.

Practical Use – A Specific Example

Calochortus tiburonensis Tiburon marposa lily Eva Buxton

Fig. 3 Tiburon marposa lily – by Eva Buxton

Starting at the bottom of the inverted pyramid (tip), taxa progress from specific to broader categories. In general, the basic unit is Species – a kind of plant, for example, the Tiburon mariposa lily (Fig. 3). The next level up is Genus, a collection of closely related species such as other mariposa lilies; a Family is a group of related genera such as the Fawn lily, Fritillary, and Clintonia; an Order is a group of closely related families such as the Lily family and False-hellebore family, etc.

Applying the inverted pyramid levels, the Tiburon mariposa lily (Calochortus tiburonensis) would be classified as follows:

Domain – Eukaryota
Kingdom – Plantae
Division – Anthophyta (Angiosperms)
Class – Monocotyledonae (Monocot)
Order – Liliales
Family – Liliaceae
Genus – Calochortus
Species – Calochortus tiburonensis

Take Home

Imagine how about 320,000 known plant species in the world today would be described and information communicated about them, if a way of classifying them, using a universal language – Latin or Latinized forms of names, was not available!

When we go botanizing looking at “flowers” (Division Anthophyta), we usually only are concerned with the last three levels – family, genus, and species. However, we can sometimes differentiate between plant Classes: monocots and eudicots (formerly called dicots). If we are also looking at mosses and ferns, we are dealing with two additional Divisions (Bryophyta and Pteridophyta).

Don’t feel overwhelmed by classification. Remember that you classify every time you go to the grocery store (Order). You go to the meat department (Family) to buy chicken (Genus) and decide on breasts (specific epithet), so you buy chicken breasts (Species), maybe even marinated ones (sub-species)!


Air Plants

Air Plants


Fig. 1 Air plants on powerlines

Fig. 1 Air plants on powerlines

Growing on Air?

We know that plants and animals adapt to their habitats with survival features or behaviors that make life possible in many habitats, from deserts to the arctic. Yet, air plants (Tillandsia) are amazing flowering plants (angiosperms) that live without their roots anchored in soil!

As a tourist in Costa Rica, I kept looking upwards hoping to see a three-toed sloth hanging from a tree branch or a powerline. However, in looking for sloths, I was more likely to see air plants attached to powerlines (Fig. 1). How can a plant survive in such a habitat?

The genus Tillandsia in Bromeliaceae (Pineapple family) includes several hundred species of evergreen, perennial, flowering plants. They are epiphytes, meaning “upon a plant” in Greek. They attach to a substrate, commonly a tree or a rock and also powerlines, but derive no nutrition from the substrate (i.e. they are not parasites). Tillandsia and about half of all known orchids grow epiphytically on “perches” that give them access to light. Tillandsia species are native from the South-Eastern U.S., Mexico, Central America, Caribbean Islands to Central Argentina and are found in many habitats, including forests, mountains, and deserts.

How Air Plants Got Their Generic Name

Erik Tillander (1640-1693) was born in Sweden about 70 years before Linnaeus. He studied botany and medicine at the University of Uppsala, Sweden and the Academy of Turku, Finland. He got his doctorate in Holland in 1670 and shortly thereafter became Professor of Medicine in the Academy of Turku. When Tillander was a student, he got so seasick in a violent storm on a voyage from Turku to Uppsala that he never traveled by boat again. He returned to Turku by walking around the Gulf of Bothnia (northern part of the Baltic Sea), a distance of about 1000 miles. It has been said that he changed his name to Elias Tillandz (‘till lands’ means ‘by land’ in Swedish).

Many decades later, Linnaeus honored the memory of his fellow Uppsala alumni by naming a large genus of American plants capable of growing away from water Tilllandsia after Professor Tillandz because of his fear of water. Linnaeus was known for naming plants to celebrate botanists as well as to insult those that he had quarreled with.

Fig. 2 Cardinal air plant (Tillandsia fasciculata) with broad leaf bases By Usien

Fig. 2 Cardinal air plant (Tillandsia fasciculata) with broad leaf bases By Usien

Some Morphology, Anatomy and Physiology

Air plants have no true roots, but rather a “holdfast” allowing the plant to hold on to a substrate. Water and nutrients must come from the surrounding air, rain, or fog and are absorbed by the leaves instead of through a root system. Some species have a compressed stem axis so that the rosettes of leaves are close together. The bases of the leaves are often flared, overlapping with each other, and forming a funnel or cup that collects and holds rainwater (Fig. 2). The leaves of other air plants hang loosely from their aerial perches (Figs. 3a & 3b).

Scale-like trichomes, minute outgrowths of the epidermis on the leaves, absorb water that is used by the plant in various processes, including photosynthesis. Nutrients in the form of dust are also taken into the leaf by trichomes. Decomposed debris from the surrounding air can accumulate around leaf bases, where it is absorbed by these minute epidermal outgrowths.

Recent studies have shown that in many epiphytic species, bacteria play a great role in fixing atmospheric nitrogen. Tillandsia recurvata (Fig. 3a), a widespread species in North and South America, has been shown to have its leaf surfaces covered by nitrogen-fixing bacteria. Nitrogen-fixing bacteria in root-nodules that produce ammonia and nitrates used by the host plants is common in Fabaceae (the Pea family which includes beans, peas, lupines, and brooms).

Fig. 3a Small ballmoss (Tillandsia recurvata) By Juan Carlos Fonseca Mata Wikipedia

Fig. 3a Small ballmoss (Tillandsia recurvata) By Juan Carlos Fonseca Mata Wikipedia

Fig. 3b Tillandsia sp. By Eva Buxton

Fig. 3b Tillandsia sp. By Eva Buxton

Fig. 4 Blushing bride -Tillandsia ionantha Mokkie

Fig. 4 Blushing bride (Tillandsia ionantha) in bloom with reddish foliage By Mokkie

The flowers are showy in some Tillandsia species (Fig. 4) and inconspicuous in others. The foliage can vary from green to a white, silvery color, and in some species changes to a bright color when the plant is blooming, helping to attract pollinators such as moths and hummingbirds. The seeds have hair-like appendages, so they can be blown away by the wind. Air plants also reproduce vegetatively by growing offsets called “pups” at the base of the plant.




Fig. 5 Spanish moss (Tillandsia usneoides) Wikipedia

Fig. 5 Spanish moss (Tillandsia usneoides) Wikipedia

Spanish Moss

The lacy-looking, grey “stuff” hanging off trees and shrubs in Marin County and often erroneously referred to as Spanish moss is a lichen in the genus Usnea. (A lichen consists of a fungus and an alga in a symbiotic relationship.) Spanish moss (neither from Spain, nor a moss) is an air plant in the genus Tillandsia. Its specific epithet is usneoides (oides means “look like”) because it resembles the lichen Usnea.  My first encounter with Spanish moss (Tillandsia usneoides) (Fig. 5) was in Jacksonville, FL, where it was hanging from the most magnificent oak I have ever seen, a Southern live oak (Quercus virginiana).

Santa gave me a Tillandsia juncea with a flowering stalk. I can’t wait for it to bloom!

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Poinsettia – the Christmas Flower

Poinsettia – the Christmas Flower


tulips w christmas tree

Fig 1. Tulips with Christmas Tree

Fig. 2 Poinsettia (Euphorbia pulcherrima)

Fig. 2 Poinsettia (Euphorbia pulcherrima)

When you leave your homeland, you don’t just leave family, friends, language, familiar places behind – you also leave traditions and trappings associated with them.  For me, one such thing is a planter with 5-6-inch-tall red tulips set in reindeer moss (Cladonia, a lichen) (Fig.1) or moss, perhaps decorated with some small pinecones and a couple of fly agarics (Amanita muscaria) made of papier-mâché.  For the first few decades in this country, I looked for bulbs of short, red tulips year-round, but there were none to be found.  All tulips were – and still are too tall!

Nowadays, like most Americans, I associate Poinsettia (Fig. 2) with Christmas and so do people in Sweden and other countries around the world. At least in California, the plant floods nurseries and box stores even before Thanksgiving.  Poinsettia for most people symbolizes good cheer and wishes for mirth and celebration at Christmas time.  In religious communities, the shape of the red parts of the plants may symbolize the Star of Bethlehem and the red color the blood of Christ.

A Legend
The Legend of the Poinsettia Tomie dePaola

fig. 3 The Legend

According to legend, Poinsettia as the Christmas flower started several centuries ago on Christmas Eve in a small Mexican village. A little girl named Pepita had no gift to present to the Christ child at the Christmas Eve service.  As Pepita walked slowly to the chapel with her cousin Pedro, her heart was filled with sadness rather than joy.  Pedro tried to console her, telling her that even the humblest gift, if given in love, would be acceptable in His eyes.  Not knowing what else to do, Pepita knelt by the roadside and gathered a handful of common weeds and fashioned them into a small bouquet.  Looking at the scraggly bunch of weeds, she felt more saddened and embarrassed than ever by the humbleness of her offering.  She fought back tears as she entered the small chapel.  As she approached the alter, she remembered Pedro’s words: “Even the humblest gift, if given in love, will be acceptable

in His eyes.” She felt her spirit lift as she knelt to lay the bouquet at the foot of the nativity scene.  Suddenly, the bouquet of weeds burst into blooms of brilliant red, and all who saw the flowers were certain they had witnessed a Christmas miracle.  Thomas Anthony “Tomie” dePaola, an American writer and illustrator, offers a retelling of the timeless legend in his children’s book The Legend of the Poinsettia, published in Spanish and English in 2008 (Fig. 3).

Classification, Habitat and Morphology

Poinsettia – also called Christmas star, Lobster plant, Mexican flame-leaf, et al. in English (Julstjӓrna in Swedish!) – is in Euphorbiaceae (Spurge family), one of the largest plant families in the world.  The genus Euphorbia to which Poinsettia belongs (see below) consists of about 2000 species, making it one of the largest genera of flowering plants.  (Euphorbia antiquorum (Malayan tree spurge) is the type species for the genus Euphorbia, described by Linnaeus in 1753.) Euphorbia members all share the feature of having a latex-like sap, more or less poisonous depending on the species.

Poinsettia is a perennial plant, native to southern Mexico and Central America, where it grows as a shrub in mid-elevational, deciduous, tropical forests. Most populations are reported to grow on west-facing slopes in steep canyons.

If the subject of Poinsettia comes up in a conversation, many people immediately declare – “the red parts are not petals but leaves,” which is correct; what surrounds the small flowers in the middle of the plant are large, red bracts, which are modified leaves. The knoblike flowers in the middle of the red bracts are cyathia, the floral characteristic that puts Poinsettia in the genus Euphorbia.  A cyathium in Euphorbia consists of unisexual flowers, both staminate (male) and pistilllate (female) flowers without sepals and petals, borne within a campanulate involucre, a ring of small bracts around the flower cluster.

Some History
Fig. 4 Joel R. Poinsett (1779-1851) Wikipedia

Fig. 4 Joel R. Poinsett (1779-1851) Wikipedia

I will admit that until I started researching the topic of this article, I thought Poinsettia was a generic (genus) name.  The ending “ia” is not uncommon in generic plant names, for example, Begonia, Forsythia, and Magnolia.  Now I know that a German scientist, J.F. Klotzsch, described the plant as a new species in 1834 (Euphorbia pulcherrima) (specific epithet meaning ‘most beautiful’), and that the plant’s common English name is derived from Joel R. Poinsett (1779-1851) (Fig. 4), a physician, diplomat, botanist, the first appointed U.S. Ambassador (Minister) to Mexico, and a U.S. Secretary of War.  Poinsett had found the plant in Taxco in the 1820s, became enchanted by the red “blooms” and sent some cuttings to his home in Greenville, South Carolina, where he later began growing them.   Now there are more than a hundred varieties of Poinsettia, grown in every State in the U.S., with “blooms” in shades of pink, white, yellow, purple, or multicolored.  The red variety is, however, the most popular.

Poinsettia, called Cuetlaxochitl by the Aztecs, was a symbol of purity and cultivated by them long before the European colonization of the Americas.  The red pigment was used as a dye and the milky sap as a medicine to “control fevers.”  After the Spanish conquest during the 17th century, Franciscan friars named the plant with the “bright red flowers” Flores de Noche Buena (Flowers of the Holy Night or Christmas Eve flower), because it bloomed each year during the Christmas season.

A Quandary?

You may be aware that the American Ornithological Society has announced that it will rename all birds currently named for human beings.  The new names will reflect the species’ appearance or habitat, i.e., some trait associated with the actual bird and not with the “colonial explorer” who first identified it.  The Ornithological Society maintains that some of the birds (not all) were named for people who held views considered “repugnant” today.  For example, John James Audubon, the naturalist for whom the Audubon’s shearwater is named, was “an unrepentant slaveholder who opposed emancipation” and Winfield Scott, for whom the Scott’s oriole is named, “led the forced eviction of the Cherokee along what is now known as the Trail of Tears.”

Would American plant societies do the same or are there just too many plants?  Many botanists agree that scientific plant names (the specific epithet) should be descriptive and not include people’s name.  Does a common name (what birders use) or a scientific name of a plant associated with something or someone we consider “repugnant” today warrant change?  Should the genus Claytonia, named in honor of John Clayton, be changed, because he owned slaves to work his tobacco plantation? (Should Sir Francis Drake Boulevard be changed to the Coastal Miwok Trail?) Do we no longer want to call the plant we associate with Christmas ‘Poinsettia,’ because Joel R. Poinsett was a proponent of slavery and owned slaves himself; as Secretary of War oversaw the Trail of Tears; and presided over the continuing suppression and relocation of the Cherokee, Chickasaw, Choctaw, et al.?

Would ‘Christmas Star’ (Swedish translation) ever catch on?