Classifying Plants:

Plant classification is one of the older pastimes in botany. Every culture, society, and religion has taken it as its duty to name and organize the plants in its area into some manageable arrangement. The arrangement at first was meant to facilitate communication between people about the plants under discussion.

The earliest organizations, naturally, were based upon the uses of plants by people. Obviously some plants were useful as vegetables, others were fruits, some make great spices for other foods, some were a good source of sugar, or perhaps starch. Oils could be extracted from others. Some were good for spinning fibers into thread for clothing. Yet others were ornamental.

But as is true of any classification scheme based upon human uses, there are problems of several sorts. The uses of plants by one civilization might not be the same as those by another. Even among individuals in the same society, one person's fruit is another's animal fodder. Moreover there is the conundrum of what to do with plants that present several different uses. George W. Carver demonstrated many uses for peanuts. Henry Ford produced an automobile with body parts made of soybean products. But the worst problem with human-use classification is that it puts related organisms in different classification categories. There is no relationship between the classification system and the evolutionary pathway through which the diversity was obtained.

Other early attempts at classification focused upon the form of the plant. Thus one category would be the herbs; here we use herb in its botanical sense meaning non-woody plants. A second category would be woody plants. These plants have secondary xylem and would need to be subdivided into several sub-categories of form: shrubs, vines, and trees. Each of these categories would have to be subdivided into deciduous and evergreen. Of course whether an evergreen vine is closer related to an evergreen tree or to a deciduous vine is a very good question. An of course there are both woody and herbaceous vines. It is sounding messy. Also, we find that a natural grouping have members in all of these "unnatural" categories.

Humans have also noticed that the seasonality of plants varies and forms another way to divide up the plants into categories. Thus in one category we have annual plants which are planted in the spring, flower that summer, and die in the fall. Then we have the perennial plants which are planted in one year, grow vegetatively in the first year, overwinter, and then flower in each year thereafter. Nurseries like to sell annuals because they know customers will come back next year to replace them all. They sell an entire flat of annuals for a small price. Perennials are planted, and then never replaced, so nurseries place a premium price upon them. However, there is a real "gotcha" among the perennials in some nurseries, however...biennials act like perennials in the first and second year, but die all the way to the ground at the end of that second year. You will have to replace those. Many seed catalogs are divided into annual and perennial categories. However even a casual gardener will be able to find plants of the same kind in both the annual and perennial categories. Certain species of a genus may be perennial, and other species of the same genus may be annual. It is not a natural breakdown.

Another way of dividing the plants is obvious in nursery catalogs: by climate. Tropical plants originate in areas of the world between the tropics of Cancer and Capricorn. This part of the world is very warm year-round with frost never occurring in the year. This is explained as the sun passes directly overhead on at least one day each year, and may do so for much of the year. Tropical plants generally cannot withstand any kind of frost. Subtropical plants are slightly more hardy and can tolerate light frost but not heavy freezes. Hardy plants can tolerate frozen periods of various durations and temperatures. The hardy plants are classified on the basis of hardiness zones. In North America these range from Zone 1 (extremely cold and long winters) to Zone 10 (no frost at all). Subtropical plants survive in zones 8-10. Some hardy plants can only survive in the warmer zones (7-10) while others are tough enough to survive in zones 1 and 2! In nursery catalogs the coldest zone for each plant is noted and gardeners are well-advised to heed those. Connecticut is zone 6 except for the northwest corner which is zone 5. Along the shore one might experiment with some zone 7 plants if a sheltered spot is available. However, again, close relatives can be tropical plants and extremely hardy plants. This is not a natural classification scheme. It is useful but not based upon genetics and evolution, the cornerstones of biology!

Botanical Classification

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. This division was named in a male-dominant culture that focused its attentions upon monarchs...sorry ladies! 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:


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.



This page © Ross E. Koning 1994.



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