Plant Taxonomy

About Plant Taxonomy

Plant taxonomy is the science that finds, describes, classifies, identifies, and names plants. It thus is one of the main branches of taxonomy.

Plant taxonomy is closely allied to plant systematics, and there is no sharp boundary between the two. In practice, "plant systematics" is involved with relationships between plants and their evolution, especially at the higher levels, whereas "plant taxonomy" deals with the actual handling of plant specimens. The precise relationship between taxonomy and systematics, however, has changed along with the goals and methods employed.

Plant taxonomy is well known for being turbulent, and traditionally there is no really close agreement on circumscription and placement of taxa.

Taxonomy is the method by which scientists, conservationists, and naturalists classify and organize the vast diversity of living things on this planet in an effort to understand the evolutionary relationships between them. Modern taxonomy originated in the mid-1700s when Swedish-born Carolus Linnaeus (also known as Carl Linnaeus or Carl von Linné) published his multi-volume Systema naturae, outlining his new and revolutionary method for classifying and, especially, naming living organisms.

Prior to Linnaeus, all described species were given long, complex names that provided much more information than was needed and were clumsy to use. Linnaeus took a different approach: he reduced every single described species to a two-part, Latinized name known as the “binomial” name. Thus, through the Linnaean system a species such as the dog rose changed from long, unwieldy names such as Rosa sylvestris inodora seu canina  and Rosa sylvestra alba cum rubore, folio glabro to the shorter, easier to use Rosa canina. This facilitated the naming of species that, with the massive influx of new specimens from newly explored regions of Africa, Asia, and the Americas, was in need of a more efficient and usable system.

Although trained in the field of medicine, botany and classification were the true passions of Linnaeus and he actively explored northern Europe and described and named hundreds of new plant species during his lifetime. As well, Linnaeus spent a great deal of time describing and naming new plant specimens that were sent to him from around the world by other botanists, including from the newly explored regions of the New World. Linnaeus classified this multitude of new plant species based upon their reproductive structures, a method which is still largely in use today. In fact, the majority of the species described by Linnaeus are still recognized today, indicating how far ahead of his time he truly was. Although somewhat rudimentary by today’s standards, Linnaeus’ methods of describing species in such a way as to represent the relationships between them changed the face of taxonomy and allowed biologists to better understand the complex natural world around us.

How Do We Classify Plants?
Plants, and indeed all organisms, are classified in a hierarchical system that attempts to illustrate the evolutionary relationships between the various groupings within the hierarchy. This concept of relatedness forms the backbone of modern classification schemes. Scientists who attempt to classify organisms and place them within an evolutionary framework are called Taxonomists, the most famous of which would be Linnaeus himself.At the broadest level, all organisms on the planet are classified into 5 Kingdoms: Animalia (animals), Plantae (plants, some multicellular algae), Fungi (fungi), Monera (prokaryotic bacteria), and Protista (eukaryotic bacteria, most algae, etc.), representing the most ancient branches of the evolutionary “tree of life.” Organisms in any given Kingdom may be separated from organisms in any other Kingdom by many hundreds of millions, if not billions, of years of evolution. Historically, all organisms known were grouped into only two Kingdoms: organisms that had finite growth, moved, and ate were grouped into the Kingdom Animalia, while organisms that had indefinite growth, didn’t move, and didn’t eat were grouped into the Kingdom Plantae. Of course, as science progressed, it became increasingly evident that such a simplistic approach to taxonomy was ineffective and many species were found that did not fit either grouping particularly well. The proposal to move to an eight-Kingdom system suggests that our current classification system, with its five Kingdoms, may yet change again as our understanding of the diversity of organisms around us continues to grow.

Within each Kingdom, the organisms are grouped into several Phyla (sing. Phylum), also known as Divisions, which represent smaller groupings of more recognizable forms. Although the Kingdom Animalia contains a large number of Phyla (such as chordates [including vertebrates], echinoderms, annelids, arthropods, etc.), Kingdom Plantae contains only ten. The Phylum Bryophyta (mosses, liverworts, hornworts), the most primitive of all true plants, differs from other plant Phyla in that it is non-vascular, meaning that it lacks water-conducting tissues which bring water from the roots of the plant up into the crown, and that the gametophyte (vegetative) generation predominates over the sporophyte (reproductive) generation. The Phyla Psilophyta (whisk ferns), Lycopodiophyta (club-mosses, spike-mosses, quillworts), Equisetophyta (horsetails), and Polypodiophyta (true ferns), including all vascular plants that reproduce using spores, also form an ancient, though largely artificial, grouping and are often referred to as Pteridophytes. The Phyla Cycadophyta (cycads), Ginkgophyta (ginkgo), Gnetophyta (vessel-bearing gymnosperms), and Coniferophyta (conifers) form a second primitive grouping of vascular plants, known as Gymnosperms, which are characterized by the presence of naked seeds (the literal translation of “gymno-sperm”). The final Phylum, Magnoliophyta, contains all of the vascular, flowering plants that are considered to be the most advanced and recently-evolved plants occurring on the planet today.

Within each Phylum, the organisms involved are grouped into progressively smaller, more refined groupings of similar individuals. Below Phylum, organisms are grouped into Classes, Orders, and Families, the latter being the largest-order taxonomic grouping that is commonly used by amateur botanists. As an example, the Phylum Magnoliophyta is split into 2 well-known Classes: Magnoliopsida (Dicotyledons) and Liliopsida (Monocotyledons) based on a variety of features from leaf venation and flower structure to growth form, root structure, and seed structure, each class with its subsequent Orders and Families. Each family is further divided into Genera (sing. Genus) representing organisms with similar morphology, structure, reproductive organs, and, perhaps most importantly, evolutionary history. These genera represent groupings that many of us are most familiar with, such as Rhododendron, Rosa, Chrysanthemum, etc. and are designed to illustrate that the individual organisms grouped within the same genus are very closely related to each other. In fact, the genus is the taxonomic grouping that represents the closest relationship between organisms which, at the smallest taxonomic level, are called Species. Each individual species is given a specific name that, when combined with the generic name, produces the two-term “binomial” naming system that Linnaeus pioneered. For example, within the genus Rosa are a variety of species such as acicularis, nutkatensis, and woodsii. Through the binomial naming system, these species become Rosa acicularis, R. nutkatensis and R.woodsii (the generic name is shortened to the first initial when listing several species in the same genus).


Of course, as with many scientific theories or strategies, there are problems with this system in the way it is currently applied and as a result it is in a continual state of flux, especially at the lower levels of the hierarchy. Even at the highest level (Kingdom), several groups are still cause for debate among taxonomists as to their placement. For example, how do we classify lichens? Lichens were originally placed within the Kingdom Plantae until further research showed that what we call “lichens” are actually a symbiotic relationship between certain species of fungi and certain species of algae. The two species, which can often survive independent of each other, combine to form a third plant-like “species” of organism called “lichen” that differs greatly from either of its two parent species yet functions as its own reproductive, evolutionary organism (thus meeting the criteria for a “species”). Currently lichens are included within the Kingdom Fungi since the fungal partner is the driving force behind the union (essentially “cultivating” its algal partner in order to produce its own nourishment) but this treatment still does not really fit with traditional taxonomy.

Another example of how nature continually confounds attempts to classify it is the vast array of plant-like organisms grouped under the term “algae.” The confusion results from the fact that most algae are unicellular or, if multicellular, composed of a single or very few cell types amassed together to function as a larger individual. So, do we classify multicellular algae based on the characteristics of the single cell (Protista) or as an independent multicellular organism (Plantae)? Most algae are currently placed within the Kingdom Protista despite their often plant-like appearance, with only a few of the multi-cellular forms remaining within the Kingdom Plantae. This treatment is not followed by all authors, however, as some retain all of the algae as a subkingdom within the Kingdom Plantae. Regardless of the treatment, it is obvious that the great diversity within the group “algae,” as well as its unusual morphological and cellular characteristics, is a hindrance to botanists who attempt to classify them within our current taxonomic systems.

What is a “Species”?
At the lowest level of the classification hierarchy is the “species”, a human-derived concept that, to this day, is still not completely understood by scientists.  The general consensus in past decades has been that a “species” is a group of similar individuals which can reproduce successfully with each other while at the same time being reproductively isolated from other similar species (known as the “Biological Species Concept”). This interpretation worked reasonably well when it was first proposed, but the more we learn about ecological systems the more apparent it becomes that nature is by no means so simple. The evolutionary process is a continuum whereby a portion of the population of one entity gradually becomes more and more distinctive and discrete, eventually reaching a state in which it is reproductively isolated from its parent “species.” The infinite range of variation between the two ends of this evolutionary process means that many populations are difficult to assign to either a parent species or a new, independent species.

A newer species concept, known as the “Phylogenetic Species Concept”, attempts to give specific status to any identifiable populations that have a unique evolutionary history and differ collectively in some characteristics from other populations. This system, which places more weight on the evolutionary process and genetic differences between populations, naturally results in a far greater number of recognizable species than the more conservative Biological Species Concept. In truth, however, neither of these widely accepted concepts appears to fully represent the extraordinary complexities of the natural world, and perhaps the most effective current method of species classification is a combination of both systems.

Subspecific Taxonomy
Another method used by taxonomists to deal with the variation within species is the use of “infraspecific” or “subspecific” taxonomy. Many species are not uniform in appearance throughout their distribution, and by assigning subspecies and varietal names to the identifiable populations scientists are able to catalogue and name this variation.

Populations that are approaching species status are typically categorized as subspecies (often written as “ssp.” or “subsp.”), especially when these forms have discrete geographic distributions. For example, in the species Salix reticulata (net-leaved willow) individuals occurring throughout the mountain ranges of the interior of the province with hairy capsules and a strong net-like pattern of venation on the leaves are named S. reticulata ssp.reticulata, while the populations on the Queen Charlotte Islands that have hairless capsules and a weaker net-like venation pattern on the leaves are known as S. reticulata ssp.glabellicarpa. These two subspecies have different geographic ranges and represent evolutionary lines that are fairly well defined, but are similar enough to be classed within the same species.

Identification and classification

Two goals of plant taxonomy are the identification and classification of plants. The distinction between these two goals is important and often overlooked.

Plant identification is the determination of the identity of an unknown plant by comparison with previously collected specimens or with the aid of books or identification manuals. The process of identification connects the specimen with a published name. Once a plant specimen has been identified, its name and properties are known.

Plant classification is the placing of known plants into groups or categories to show some relationship. Scientific classification follows a system of rules that standardizes the results, and groups successive categories into a hierarchy. For example, the family to which the lilies belong is classified as follows:
Kingdom: Plantae
Division: Magnoliophyta
Class: Liliopsida
Order: Liliales
Family: Liliaceae
Genera : ... ...

The classification of plants results in an organized system for the naming and cataloging of future specimens, and ideally reflects scientific ideas about plant inter-relationships.

Classification systems

Bessey system

Charles Edwin Bessey (1845 - 1915)
The goal was to organize flowering plants in a scheme that reflected evolutionary relationships. The main difference between this and Engler and Prantl's work is that Bessey's system had evolution as its central theme from the very beginning. Most modern systems of classifications, including Cronquist's are modifications of Bessey's 'intuitive approach'.

A system of plant taxonomy, the Bessey system was published in

Charles E. Bessey (1915). "The phylogenetic taxonomy of flowering plants". Annals of the Missouri Botanical Garden (Missouri Botanical Garden Press) 2 (1/2): 109–164. doi:10.2307/2990030. JSTOR 2990030. available online at "Botanicus.org" (PDF). Missouri Botanical Garden. Retrieved 2007-08-16. Bessey considered Spermatophyta as having a polyphyletic origin, being composed by three different phyla, of which he only treated Anthophyta (syn.: Angiosperms).

With some modifications, most modern classifications - for example, those of Cronquist (1981, 1983, 1988), Takhtajan (1969, 1980, 1983, 1991), Stebbins (1974), R. Dahlgren (1975, 1980, 1983; R. Dahlgren et al. 1981; R. Dahlgren and Rasmussen 1983; R. Dahlgren and Bremer 1985; G. Dahlgren 1989), and Thorne (1976, 1981, 1983, 1992) - follow the Bessey tradition.

Natural System of Plant Classification according to Charles Bessey:

• A phylogenetic system of organizing flowering plants with the use of genetic techniques.
• Besse's theory was based off of evolution.
• 'Besse's cactus' was based on his 28 rules, which show the evolutionary trends in angiosperm.
• The most direct path represents the actual evolutionary history of a given group.
• The most primitive, original flower was believed to have had many separate petals, stamens, and carpels.
• As they evolved the parts of the flowers either fused together, reduced, or became absent.
• The simple structures are not primitive, but have become simple as a result of a reduction from more complex parts.
• An example of the primitive flower would be the Ranunculus, buttercup flowers.
• A more advanced type of flowering plant would be the catkin inflorescences.

Melchior system

This is the named Melchior system, "a reference in all taxonomic courses", detailing the taxonomic system of the Angiospermae according to A. Engler's Syllabus der Pflanzenfamilien 1964 (also known as "modified or updated" Engler system).

Heinrich Gustav Adolf Engler (Adolf Engler) (1844–1930) and Karl Anton Eugen Prantl (1849-1893) made the Engler and Prantl or Phylogenetic System, published in Die Natürlichen Pflanzenfamilien 1887-1915, where the plants were sorted by the basis of complexity of floral morphology. Characters like a perianth with one whorl, unisexual flowers and pollination by wind were considered primitive as compared to perianth with two whorls, bisexual flowers and pollination by insects. This was the first major Phylogenetic Classification and that gave a slightly changed August Wilhelm Eichler system. They dealt with the primitive groups as well.  It is in line with Adolphe-Théodore Brongniart's 1843 work.

Engler's taxonomic work was also published in Das Pflanzenreich 1900-1968 and Syllabus der Pflanzenfamilien 1924. It contained more than Angiospermae; even green algae, which most taxonomists do not include in Plantae, and it is still one of the most complete works on all plants and related.


The Angiospermae part of the system was slightly changed by H. Melchior in 1964, and after that, it contained 62 ordos with 343 families.

Main groups in 1964:
Divisio Embryophyta
 Subdivision Angiospermae
  Classis Monocotyledoneae
  Classis Dicotyledoneae
   Subclasses Archychlamydeae
   Subclasseis Sympetalae

Heinrich Gustav Adolf Engler was born in Sagan, Prussia (now Żagań), Poland. He obtained his Ph.D. from the University of Breslau (now Wrocław, Poland) in 1866, and worked there for some years. Then he became the custodian of botanical collections of the Botanische Institute of Munich. In 1878, he got professorship at the University of Kiel. In 1884, he went back to Breslau as director of the Botanical Garden. From 1889 to 1921 he was professor at the University of Berlin and director of the Berlin-Dahlem Botanical Garden.

Many plants are named in his honour, such as Englerastrum, Englerella, Engleria, Englerina, Englerocharis, Englerodaphne, Englerodendron and Englerophytum.

The collaborators in orders (and some families) were the following:

• Hans Melchior in Casuarinales, Juglandales, Balanopales, Leitneriales, Salicales, Fagales, Urticales, Didiereaceae, Piperales, Aristolochiales, Guttiferales, Sarraceniales, Papaverales, Hydrostachyales, Podostemonales, Julianiales, Violales, Cucurbitales, Myrtiflorae, Umbelliflorae, Primulales, Tubiflorae, Plantaginales, Liliiflorae p. p., Spathiflorae and Microspermae.
• G. Buchheim in Proteales, Cactales, Magnoliales and Ranunculales.
• W. Schultze-Motel in Santalales, Balanophorales, Medusandrales, Rhamnales, Malvales, Diapensiales, Ericales and Cyperales.
• Th. Eckardt in Polygonales, Centrospermae, Batales, Plumbaginales, Helobiae, Triuridales and Pandanales.
• G. K. Schultze-Menz in Rosales.
• H. Scholz in Geraniales, Rutales, Sapindales and Celastrales.
• G. Wagenitz in Thymelaeales, Ebenales, Oleales, Gentianales, Dipsacales and Campanulales.
• U. Hamann in Cyanastraceae, Pontederiaceae, Philydraceae, Juncales, Bromeliales and Commelinales.
• E. Potztal in Graminales, Principes, Synanthae and Scitamineae.