CLASSIFICATION |
The Binomial SystemWe are beginning a review of the diversity of plant life. First we need to look at how plants are classified. |
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Plants fit into a system of classification common to all organisms, the binomial system, which was devised by Linnaeus. It is surprising how many plants and other organisms Linnaeus got to know. Many binomials are followed by the abbreviation L., indicating that Linnaeus described and named them. This is Linnaeus' favorite plant Linnaea borealis |
The (Latin) binomial name is an unambiguous identifier. It consists of a genus name (with capital first letter) and a specific epithet (entirely in lower case) and is printed in italic or underlined. This is all it takes to identify an organism but we may put organisms together in larger groups for convenience. |
Kingdom |
Plantae |
Plantae |
Division |
Anthophyta |
Anthophyta |
Class |
Dicotyledones |
Monocotyledones |
Family |
Boraginaceae |
Liliaceae |
Genus |
Mertensia |
Scilla |
Specific epithet |
virginica |
non- scripta |
Common name |
So a species is defined by the genus name and specific epithet along with the authority (such as L. for Linnaeus) who first gave a botanical description of the type specimen. Classification becomes complicated for cultivated plants since we often need to distinguish particular cultivars and many of these plants are hybrids of two or more wild species. If you see an X before the specific epithet such as Dendranthema X grandiflora for Chrysanthemum this tells you that the plant is a hybrid. It is more likely that species within a genus will be able to hybridize, but occasionally intergeneric hybrids are possible, such as X Cupressocyparis leylandii derived from a cross between Cupressus and Chamaecyparis species. Basis of ClassificationAbout 250,000 species of flowering plants are known. Species were first defined in terms of their appearance. And the first criterion for including a plant in a species should be that it looks similar to other members of the species. Plants are very variable in vegetative features, whereas flowers tend to be much less variable at least in basic structure. This is why flower features have been used so extensively in classification. We have come to use further criteria as we try to make our classification more "natural": |
Members of a species should: |
be able to interbreed and produce viable seed |
The GenusFurther problems in classification arise as we move above the species level. A genus may include only one species, or a number that are similar enough to group together according to the same criteria that we use for species. Leaf shape and general appearance may be quite variable (although we do not often find a woody and a herbaceous plant in the same genus). The basic pattern of flower structure will stay the same even if there are variations in color, size and other details. Although intergeneric hybrids occur occasionally, hybrids between species in the same genus are much more common. Some people argue that if two species can be crossed to produce viable seed, then they belong in the same genus. The FamilyThe most commonly used taxonomic grouping above the genus is the family. We will look at some of the 300 or so families within the flowering plants. Again there may be one or many genera in a family. In the Anthophyta, members of a family tend to share some important feature of flower structure as in the Asteraceae, or Araceae, although the variation can sometimes be bewildering as in the Rosaceae. Orders and classesFamilies can be grouped into orders such as the Geraniales which include the Geraniaceae (Geranium family), Linaceae (flax family) and Oxalidaceae (Sorrel family). However, orders are not much used as a taxonomic unit. On the other hand the two classes within the anthophyta, monocotyledones (monocots) and dicotyledones (dicots) are frequently mentioned. DivisionsAbove the family kingdoms are broken into divisions. In this course we will look at most of the divisions of the plant kingdom: |
Division |
Common name |
Covered |
Hepaticophyta |
yes |
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Anthoceropyta |
hornworts |
no |
Bryophyta |
yes |
|
Psilophyta |
yes |
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Lycophyta |
yes |
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Sphenophyta |
yes |
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Pterophyta |
yes |
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Coniferophyta |
yes |
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Cycadophyta |
yes |
|
Ginkgophyta |
Maidenhair tree |
yes |
Gnetophyta |
-- |
no |
Anthophyta |
yes |
Sometimes confusion arises because a plant or group of plants lacks one of the features that we expect: it has no chlorophyll or is aquatic. If everything else about it is plant-like, we assume that it lost its other plant-like features in the course of evolution, so we still include it in the kingdom. This is Beech Drops (Epifagus virginiana) a common parasitic flowering plant that lives on the roots of beech trees in Ohio. It is brown rather than green because it lacks chlorophyll, but everything else about it is plant-like. |
Copyright © Michael Knee |