Chapter 16

Plant Classification (Systematics)I. Introduction:Plant classification or taxonomy is the science of grouping plants on the basis of their use or shared characteristics. Two ways to name and describe plants:

1. Scientific Names: allow organisms to be identified anywhere, no matter what language is spoken. Each organism has a unique name.

2. Common Names: problems with that system

- same plant was given many different common names

- different but closely related species may have the same common name

- sometimes the same common name was used for two different plants in two countries speaking different languages (in Europe)

II. Development of the Binomial System of Nomenculature

1. Theophrastus (3rd Century) in his Historia Plantarum classified 500 plants into trees, shrubs and herbs and used leaf characteristics in classification.

2. De Materia Medica of the Greek physician Discorides, in the first century, is the first herbal which described 600 medicinal plants. This herbal remained in use by physicians of the Western world for over 1500 years.

3. By the end of the 17th century, John Ray divided more than 18,000 kinds of flowering plants into monocotyledons and dicotyledons.

4. In the 18th century A.D., fruit and flower structure were used as a criteria for classification. Descriptive Latin phrase names were used to describe plants. The first word of the phrase indicated a particular genus to which a group of plants belonged. e.g. Mentha floribus spicatis, foliis oblongis serratis means spearmint with flowers in a spike, leaf oblong, saw-toothed.

5. Carolus Linnaeus (1707-1778)

- published his two-volume book "Species Plantarum" in 1753 introduced the species which combined one to many specific kinds of plants called species into one genus.

- This is known as the Binomial System of Nomenculature which is applied to all living organisms in the time being. the binomial name is followed by the authority for the name. e.g. Mentha spicata L., the L. stands for Linnaeus.

- Linnaeus' book included 7,500 kinds of plants.

- He organized plants into 24 classes based on the features of the stamens, including the number of stamens per flower, whether or not they were fused together, and whether or not they occurred on the same flower as the carpels.

6. The International Code for Botanical Nomenculature

a) First meeting of European and American botanists in Paris (1867) to standardize rules for naming and classifying plants.

b) The International Code of Botanical Nomenclature

- recognizes Linnaeus’ Species Plantarum as starting point for botanical nomenclature

- sets rules for naming and classifying.

III. Development of the Kingdom Concept

1. Two Kingdoms:

• Kingdom Plantae: nonmotile, photosynthetic autotrophs• Kingdom Animalia: motile, ingestive heterotrophs

Two problems for the two kingdom system:

1. Euglenoids: are single-celled, motile by flagella and ingestive resembling animals yet they have chloroplasts that can engage in photosynthesis

2. Slime Molds: while feeding look and behave like amoebas (resembling animals) and but during reproduction, they become stationary and develop fungus-like reproductive bodies.

2. Third Kingdom:

• Kingdom Protoctista (Protista): proposed by J. Hogg and Ernst Haeckel (1860's)

• Contained simple organisms that did not develop complex tissue (e.g. algae, fungi and sponges)

3. Fourth Kingdom:

• In 1938 H.F. Copeland proposed dividing the protoctista and developed the fourth kingdom, Kingdom Monera.• It included unicellular prokaryotic (no nuclear membrane) protists leaving behind algae and fungi with protista.

4. Five Kingdoms:

• Proposed by R.H. Whittaker (1969)• Divided complex forms of organisms depending on the mode of nutrition into

Kingdom Plantae: photosynthetic organismsKingdom Animalia: ingest solid foodKingdom Fungi: absorb nutrients in solution

• Divided protists (single cells) on basis of the presence or absence of nuclear membrane into

Kingdom Protista: eukaryotic single cellsKingdom Monera: prokaryotic single cells

5. Other Kindoms:

• In 1980's Carl Woes, a microbiologist has presented arguments that favour the division of the Monera into two kingdoms, archaebacteria (primitive bacteria) and eubacteria.

• James Lake proposed further dividing the archaebacteria into two kingdoms.

• Slime molds still do not fit into the five kingdom concept because they have no cell walls during their active state but develop walls when they reproduce.

IV. Classification of Major Groups

A. Categories (each higher category contains one or more of the immediately subordinate categories):

. Kingdom

. Division

. Class

. Order

. Family

. Genus

. Species

B. Viruses: do not possess the attributes of living organisms, do not fit into the classification scheme

C. Lichens: are an association of an alga and a fungus, do not fit into the classification scheme

D. Example of Classification: The Onion (Allium cepa)

1. Kingdom: Plantae

2. Division: Magnoliophyta

3. Class: Liliopsida

4. Order: Liliales

5. Family: Liliaceae

6. Genus: Allium

7. Species: Allium cepa L.

E. Role of Taxonomists

• Identifying, naming and classifying organisms• Construction of keys for identification. Keys should be dichotomous, presenting mutually exclusive alternatives

V. Keys to Major Groups of Organisms (see book pages 294-296)

VI. Cladistics

Cladistics is a system of examining natural relationships among organisms based on common features. The relationships are represented by straight line diagrams called cladograms.