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Sunday, January 08, 2006

Classification of Plants

The most important method of classification has been standardized among botanists for centuries. This method attempts to formulate groupings based upon the evolutionary characteristics inherited genetically among related species. This method uses information from genetics, biochemistry, developmental biology, paleobotany, morphology, anatomy, physiology. Related groups should share evolutionarily-derived characteristics. A natural classification would have very few characteristics that appear to evolve multiple times in related groups. Thus our goal in botanical classification is to arrive at a natural classification that involves the fewest evolutionary steps. If we are successful then groups with the most shared characteristics will naturally be classified closely together.

All living organisms can be divided into a few major kingdoms. The kingdoms commonly recognized include: Archaea and Bacteria (the prokaryotic kingdoms), Protista, Fungi, Plantae, and Animalia. Those organisms that carry out photosynthesis include the cyanobacteria in kingdom Bacteria, the algae in kingdom Protista, and the plants in kingdom Plantae.

Each kingdom is composed of plants of at least some similar organisms. Plantae. for example, holds mosses, ferns, conifers, and flowering plants. Because these groupings are quite distinct from each other, the kingdom is divided further into phyla (singular: phylum). Thus Plantae includes the phyla bryophyta (mosses), pterophyta (ferns), coniferophyta (conifers), and anthophyta (flowering plants). Plants within each phylum share certain important features. For example the anthophyta reproduce by means of flowers including having their ovules enclosed in carpels.

Nevertheless a phylum includes plants with some fairly fundamental differences, so it requires further subdivision into classes. Among the flowering plants we find two major groups: those with a single seed leaf and those with two seed leaves. These differences are accompanied by several other morphological and other differences that divide the flowering plants into the classes: monocotyledonae (monocots) and dicotyledonae (dicots).

Each class is divided into orders, and each order is divided into families, and each family into different genera. Each genus is divided into a range of particular species. Thus the classification is a nested series of categories as follows:

Kingdom --> Phylum --> Class --> Order --> Family --> Genus --> Species

Each species has a scientific name which is a Latin binomial. The binomial is composed of the genus name and the specific epithet (the add-on name of the species). For example the human binomial is Homo sapiens. We are in the genus Homo (meaning self!), and the specific kind of homo that we are is the one that can think rationalize (sapient!). Please note that our name is Homo sapiens whether we speak of one human or the entire population. There is no such thing as a "Homo sapien."

You might wonder why we choose Latin for our binomials. Well, Latin is a dead language; no one speaks it any longer, so its definitions do not change over time! This reduces confusion drastically. For example, a brightly colored plant named a century ago might have been called "gay" if English were the language of binomials. But today that epithet might lead a scientist to wonder whether this meant something about the plants reproductive biology. The definitions in Latin are static. The second reason we choose Latin is that it has become universal among scientists worldwide...preventing much confusion.

"Black-eyed Susan" is an English common name for over 100 species of plants, depending upon which part of the world you come from. Scientists cannot use that name in their research as this would be very confusing. However Rudbeckia hirta is a black-eyed susan about which there is no confusion. If you open a journal written in Chinese characters, right there in the title will likely be a Latin bionomial to help you decide whether to have it translated or not.

Plant binomials are often despised by non-scientists as hard to remember. Yet, they aren't too difficult in many cases if you can simply dissect out the root words. Rosa is the genus name for roses. So Rosa multiflora is a rose that has multiple flowers in each cluster. Rosa grandiflora is a rose with very large flowers. Rosa floribunda produces many flowers over a long summer season...abounding in flowers.

People have modifed some species quite drastically and so having a binomial is sometimes insufficient to describe the specific plant being used. A good example is Brassica oleracea, the mustard of the garden. Europeans long ago prized this plant that grew rapidly in the cool and short summers of norther Europe. To make this plant more diverse as a source of food, they selected from the ancestral (wild) plants plants that showed different characteristics. Through selective breeding they created several different food plants from this single important species. Because these are subdivisions of a natural species, we had to create a subdivision beneath species! This is called variety. Brassica oleracea can now be found in these varieties:

Brassica oleracea capitatacabbage
Brassica oleracea acephalakale
Brassica oleracea gemmiferaBrussels sprouts
Brassica oleracea italicabroccoli
Brassica oleracea botrytiscauliflower
Brassica oleracea caulorapakohlrabi

Of course any perusal of a seed catalog will show that even this is insufficient. The cabbages, for example, exist in dizzying array of different cultivated varieties (cultivar). There is Brassica oleracea capitata 'Late Flat Dutch' and Brassica oleracea capitata 'Copenhagen Market' to name just two examples. You will notice that the cultivar name is given in the "home language" and is put within single-quotes, while the binomial and any botanical variety names are in Latin italics.

Classification of Plants


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