CLASSIFICATION

The Binomial System

We are beginning a review of the diversity of plant life. First we need to look at how plants are classified.

Both of these plants could be called bluebells but the one on the left is Scilla non-scripta and is related to lilies and hyacinths, whereas on the right is Mertensia virginica which is related to bugloss and lungwort.

Above Hypericum calycinum (family Hypericaceae) is called"Rose of Sharon" in England.
Right Hibiscus syriacus (family Malvaceae) is called"Rose of Sharon" in North America

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

Virginia bluebell

English Bluebell

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 Classification

About 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
be physiologically similar
occupy similar habitats

The Genus

Further 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 Family

The 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 classes

Families 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.

Divisions

Above 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

liverworts

yes

Anthoceropyta

hornworts

no

Bryophyta

mosses

yes

Psilophyta

whisk ferns

yes

Lycophyta

clubmosses

yes

Sphenophyta

horsetails

yes

Pterophyta

ferns

yes

Coniferophyta

conifers

yes

Cycadophyta

cycads

yes

Ginkgophyta

Maidenhair tree

yes

Gnetophyta

--

no

Anthophyta

flowering plants

yes

As we shall see, within each division there is something distinctive about the mode of reproduction and the morphology of the plants.

Kingdoms

In addition to plants we will look at selected Protista, Bacteria and Fungi that share some plant-like features and some of which are called plants by many people.

We are using a traditional classification of organisms into five kingdoms and the one kingdom that we do not look at is Animalia. If you look through other textbooks you may see organisms move around. At one time multicellular algae were included in Plantae, whereas they are now grouped with unicellular organisms in the Protista. Also many fungus-like organisms have been moved into the Protista. Raven's "Biology of Plants" divides the bacteria into two kingdoms: Archaebacteria, an unusual group that may have been the ancestor of other kingdoms and Eubacteria which includes most of today's bacteria.

This illustrates the fact that ideas about classification keep changing from the grand scale of kingdoms down to species and even sub-species. In recent years molecular characters have increasingly been used to decide taxonomic relationships and disputes. When we discuss"plant like" organisms it will be useful to have a list of plant features in mind for comparison. All members of the kingdom Plantae have:

• eukaryotic cells (containing vacuoles, chloroplasts and other organelles)
• cell walls composed of cellulose and pectin
• differentiated cell types
• photosynthesis, (utilizing chlorophylls a and b, and storing starch)
• adaptation for life on the land
• sexual reproduction through an alternation of generations

Prokaryotes have various mechanisms for gene exchange but all eukaryotes have some method of sexual reproduction involving cellular fusion. This entails that at some time there must be a stage of segregation - otherwise we would go on increasing our chromosome number at every generation.

Given that a fusion of haploid cells (gametes) leads to a diploid cell (zygote) which at some time undergoes meiosis to give rise to more haploid cells, there are two obvious ways that organisms could develop:

• (a) exist normally as haploid cells (single or multicellular), occasionally fuse to form a zygote which immediately undergoes meiosis to begin a new haploid generation or:
• (b) exist normally as diploid cells which occasionally undergo meiosis, followed more or less immediately by fusion to form a zygote which begins a new diploid generation

There is however a third way which plants have adopted which combines both of these systems in an alternation of generations:

• (c) the organism can exist in both haploid and diploid states; the zygote grows into a diploid sporophyte in which some cells undergo meiosis to produce spores; the spores germinate and grow into a haploid gametophyte that produces male and/or female gametes; the gametes fuse to produce a new zygote

Alternation of Generations

Alternation of generations occurs throughout the plant kingdom right through to flowering plants and it is impossible to understand plant reproduction without grasping this concept.

Some algae show this feature particularly well. For example in Ulva lactuca (Sea lettuce) the haploid and diploid generations are well developed and look identical until they produce gametes or spores, respectively:

Life cycle of Ulva lactuca

Exceptions to the rule

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
The Ohio State University
All rights reserved.