Unit 3: Systems of Classification
Major Contributors to Systematics
Plant classification has evolved through different stages, from simple artificial systems based on one or two characters to modern phylogenetic systems based on evolutionary relationships.
- Theophrastus: Known as the "Father of Botany," he classified plants into herbs, undershrubs, shrubs, and trees based on their habit.
- Linnaeus: Developed an Artificial System (Sexual System) based primarily on the number and arrangement of stamens.
- de Candolle: Introduced the term "Taxonomy" and emphasized natural affinities between plants.
- Hutchinson: Proposed a phylogenetic system emphasizing the origin of angiosperms from pro-angiosperms and divided them into Lignosae (woody) and Herbaceae (herbaceous).
- Takhtajan and Cronquist: Contributed modern phylogenetic systems that broadly categorized angiosperms into Magnoliopsida (Dicots) and Liliopsida (Monocots).
Bentham and Hooker System of Classification
This is a Natural System of classification published in the three-volume work Genera Plantarum. It is still widely used in herbaria, particularly in India and Commonwealth countries.
Outline of the System
Bentham and Hooker divided seed plants (Phanerogams) into three main classes:
- Dicotyledones: Seeds with two cotyledons; further divided into:
- Polypetalae: Petals free (Series: Thalamiflorae, Disciflorae, Calyciflorae).
- Gamopetalae: Petals fused (Series: Inferae, Heteromerae, Bicarpellatae).
- Monochlamydeae: Perianth usually simple or absent (8 series including Curvembryeae, Multiovulatae aquaticae, etc.).
- Gymnospermae: Seeds naked, not enclosed in an ovary (3 families: Cycadaceae, Coniferae, Gnetaceae).
- Monocotyledones: Seeds with one cotyledon; further divided into 7 series (e.g., Microspermae, Epigynae, Glumaceae).
Engler and Prantl System of Classification
This is a Phylogenetic System published in Die Natürlichen Pflanzenfamilien. It arranges plants based on their supposed evolutionary sequence, from "primitive" to "advanced".
Key Characteristics
- They placed Monocots before Dicots, considering them more primitive.
- They considered wind-pollinated plants with simple flowers (Amentiferae) as primitive.
- Within Dicots, they recognized two subclasses: Archichlamydeae (Apetalae and Polypetalae) and Metachlamydeae (Sympetalae).
Angiosperm Phylogeny Group (APG III) Classification
The APG systems represent a departure from traditional classification. Instead of being the work of one or two individuals, it is a consensus system developed by a large group of researchers using molecular data (DNA sequences).
Distinctive Features of APG III
- It focuses on Monophyly, ensuring that every group includes all descendants of a common ancestor.
- Traditional groups like "Dicots" are recognized as non-monophyletic (paraphyletic) and are replaced by groups like "Eudicots".
- It avoids formal taxonomic ranks like Class or Subclass in many instances, preferring informal names for Clades.
Exam Tips
- Classification Comparison: Be prepared to compare the natural system of Bentham and Hooker with the phylogenetic system of Engler and Prantl.
- Gymnosperms Position: A common exam question is the placement of Gymnosperms between Dicots and Monocots in the Bentham and Hooker system, which is considered its biggest flaw.
- APG III: Highlight the use of molecular evidence as the primary tool for this classification.
Common Pitfalls
- Mixing up the series of Polypetalae vs. Gamopetalae in Bentham and Hooker's system.
- Thinking APG III is based purely on morphology; it is heavily reliant on DNA sequences.