2. Plant Classification
A. History
Linnaeus made the first classification of plants. This is still the basis of the taxonomy together with his nomenclature. He ordered the species in genus’s and families. Later the families were ordered in order, classes and divisions. Total orderings of plants have been made by Takthajan (1980), Dahlgren (1983), Thorne (1983, 1992), Cronquist (1981, 1988) and Stebbins. These classifications have many similarities, but they also differ in many respects in respect to the orders, classes and divisions. Recently a new classification has been developed, the APG classification, by the "Angiosperm Phylogeny Group". This is based on DNA analysis and is seen as the most reliable.
B. Members
What has to be classified as members of groups are species. Cronquist defines a species as (1988): "Species are the smallest groups that are consistently and persistently distinct, and distinguished by ordinary means."
C. Qualities
Many qualities have been used in the development of botanical taxonomy. They reflect the development of science as a whole. The most useful tools in the study of systematics in botany have been:
1 Anatomy: 1800s
a. Morphology, anatomy: This is the oldest criterion for classification and for a long time has been almost the sole criterion. A good example is the "umbrella" form of the flowers of the family Apiaceae (former Umbelliferae). Taste, smell and effects on living crea- tures were used as additional information.
b. Embryology: microsporogensis, megasporogensis. gametophytes, fertilization, endosperm, embryo, seed coats 2 Chromosome numbers: 1920's
a. Cytology, palynology, paleobotany, ecology
3 Comparative garden studies: 1940's
4 Palynology: 1950's
5 Phenetics: 1960's
6 Biochemical systematics: 1970's: chemosystematics
7 Cladistics: 1980's
8 Chloroplast DNA and RNA: 1990's: genetic sequencing data
D. Development of taxonomy
As expressed above taxonomy has had its own evolution. This evolution has been influenced mostly by new insights and by new kinds of evidence. One can distinguish:
1. Phenetic taxonomy: Present nature of organisms
Before evolutionary theories organisms were placed in "buckets" by common attributes, relying upon appearance (phenotype). A key developer in our present system was Carolus Linnaeus (1707-1778). In modern times, this method is made more quantitative through computer analysis of many features.
2. Phylogenetic taxonomy: Evolutionary relationships of organisms
This system arose in the late nineteenth century with evolutionary theory. Here groupings are made by presumed ancestry.
3. Cladistics (from Greek klados branch) taxonomy: Branching order by which organisms have derived from an ancestral stem. This scheme arose in the 1970's to use a more objective approach based on DNA, proteins, etc. Overall similarities are not important.
This classification is based on various recently published phylogenetic analyses, and it is equivalent to "An ordinal classification for the families of flowering plants", as submitted for publication in the Annals of the Missouri Botanical Garden by the "Angiosperm Phylogeny Group".
Several families are deliberately not classified to order to avoid undesirable non-monophyletic taxa as well as redundant monofa- milial taxa. Note that the Linnaean categories of order and family are only to be conceived as a convenient reference to hierarchical relationships. Groups of the same rank are not comparable units unless they are sister groups.
E. Evolution
The taxonomy reflects the evolution of the plants. This is expressed in the tree like form of the taxonomy. It can also been seen in the circular form, the center being the oldest part of the evolution and the more eccentric parts the recent developments of the plants in the evolution.
The qualities that characterize late evolution are (Besseyan principles):
1. Woody to non-woody; perennial to annual; terrestrial to aquatic or epiphytic, saprophytic and parasitic conditions
2. Cylindric to scattered vascular bundle(s)
3. Alternate leaves to opposite or whorled; simple leaves to compound leaves
4. Bisexual flowers to unisexual in most instances
5. Perianth parts many and spirally arranged to few and whorled with actinomorphic (regular) to zygomorphic (irregular) flowers
6. Separate perianth parts, poorly differentiated in to sepals and petals, to fused perianth parts, sharply differentiated; petalous to apetalous
7. Many separate stamens to few united stamens
8. Hypogyny to perigyny and epigyny (e.g. superior to inferior)
9. Numerous and separate carpels to few and fused carpels
10. Pollen grains with one pore to grains with two or more pores
11. Small embryo in abundant endosperm in arillate seeds to large embryo without endosperm in non-arillate seeds
Plant seminar, Part 1, 22-23 March, © HAU
512. Axile placentation to free-central placentation
13. Single fruits to aggregate fruits; capsules to drupe or berry
14. Pro-anthocyanin compounds to anthocyanin compounds
In summary: angiosperm evolution has been one of reduction in number, fusion and specialization of parts coupled with changes in symmetry justification: evolution is the cause of a natural system of classification; natural classification will represent the hierarchy of evolution. Classifications are based on features that seem to reflect common ancestry.
F. Use
It is typical that the majority of the old classifications are confirmed by new criteria of classification. Although there are changes in the higher groupings, the majority of families and groupings are the same for the newest APG classification as it was with that of Linnaeus. This is a confirmation of the idea that groups are not made by change, that they are not just arbitrary or artificial. An artificial order would lead to many equivalent orders instead of one generally accepted order.
Taxonomy is useful, because it has predictive value.
a. Qualities of a group can be applied to an unfamiliar plant that is a member of the group.
b. Qualities of a few members of a group can lead to some good guesses about the group as a whole.