Unit 2: Introduction to Plant Taxonomy

Definition and Components of Plant Taxonomy

Taxonomy: (From Greek taxis = arrangement, nomos = law) It is the science of identifying, naming (nomenclature), and classifying organisms.

The components of taxonomy (also called systematics) are:

1. Identification: The process of determining if an unknown plant belongs to a known group, often using taxonomic keys. It is the practical side of taxonomy.
2. Classification: The process of grouping plants into categories (taxa) based on shared characteristics (e.g., morphological, molecular).
3. Nomenclature: The formal system of naming plants according to the rules of the ICN (International Code of Nomenclature).

Taxonomic Literature

This is the body of published work that documents taxonomic research. Key types include:

• Flora: A book or work that describes the plants found in a specific geographic region (e.g., "Flora of Assam").
• Monograph: A comprehensive, global study of a specific taxonomic group (e.g., a monograph on the genus Rosa). It includes all known species, keys, descriptions, distribution, etc.
• Revision: A similar, but less comprehensive study than a monograph, often focusing on a specific region or part of a group.
• Journals: Academic publications where new species are described and revisions are published (e.g., Taxon, Kew Bulletin).

Herbarium

Definition: A herbarium is a "library" of dried, pressed, and preserved plant specimens, arranged in a systematic order for study and reference.

Herbarium Preparation (Step-by-Step)

1. Collection: Collect a complete specimen (with flowers/fruits, leaves, stem, and root if possible). Record data in a field notebook (location, date, habit, collector's name).
2. Pressing: Arrange the specimen in a plant press (two wooden frames) between sheets of blotting paper or newspaper to flatten it and remove moisture.
3. Drying: Change the blotters every 1-2 days until the specimen is completely dry and stiff.
4. Poisoning (Optional but recommended): Treat the specimen with a fungicide/pesticide (e.g., mercuric chloride, now more commonly freezing) to prevent fungal and insect damage.
5. Mounting: Glue or stitch the dried specimen onto a standard-sized, archival herbarium sheet (approx. 42 x 29 cm).
6. Labeling: Attach a herbarium label to the bottom-right corner. This label is critical and contains all the field data (Botanical name, family, location, date, collector, etc.).

Role of Herbarium

• Reference Center: Provides a permanent record for identifying and comparing plants.
• Research: Source of data for taxonomy, anatomy, distribution, and even DNA.
• Type Specimens: Houses "type" specimens, which are the original specimens used to define a new species.
• Conservation: Documents the flora of a region and can show changes over time.

Major Herbaria

• Major World Herbaria:
  • Royal Botanic Gardens, Kew (K): London, UK. One of the largest in the world.
  • Muséum National d'Histoire Naturelle (P): Paris, France.
• Central National Herbarium (CAL): Located at the A JC Bose Indian Botanic Garden, Shibpur/Kolkata. The largest herbarium in India.

Botanical Gardens

Definition: Large, protected areas where plants (especially trees, shrubs, and ornamentals) are cultivated for scientific, educational, and aesthetic purposes.

Role of Botanical Gardens

• Ex-situ Conservation: Conserving rare and endangered species outside their natural habitat.
• Research: Growing plants for taxonomic, horticultural, and acclimatization studies.
• Education: Public display and education about plant diversity.
• Arboretum: A section of a botanical garden dedicated to growing trees.

Major Botanical Gardens

• Major World Gardens: Royal Botanic Gardens, Kew (UK); Padua Botanical Garden (Italy - one of the oldest).
• Major Indian Garden: A JC Bose Indian Botanic Garden, Shibpur, Kolkata. Famous for the Great Banyan Tree.

Flora

A Flora is a work that lists and describes the plant species of a specific geographical area. It typically includes taxonomic keys, descriptions of families, genera, and species, and information on their distribution and ecology.

• Flora of Assam: Several works contribute to this, notably Flora of Assam by Kanjilal et al. It is a vital resource for understanding the rich plant diversity of the region.
• Flora of British India: This is a monumental, 7-volume work by Sir J.D. Hooker (published 1872-1897). It remains the foundational reference for the taxonomy of the Indian subcontinent.

Taxonomic Keys

A taxonomic key is a device used to identify an unknown organism. It works by presenting a series of choices about the specimen's characteristics. By making the correct choice at each step, you are led to the name of the organism.

The most common type is the dichotomous key (dicho = two, tomos = to cut).

• Indented Key: Each pair of contrasting choices (a "couplet") is indented, and the subsequent choices are further indented. This is visually easy to follow.
• Bracketed (or Parallel) Key: The two choices of a couplet are always listed together, numbered (e.g., 1a, 1b). This is more space-efficient.

Taxonomic Evidences

While classical taxonomy relies on morphology, modern taxonomy uses evidence from many other fields (a "polyphasic" approach).

• Morphology: (As seen in Unit 1). The primary source of data.
• Palynology: The study of pollen and spores. The shape, size, and ornamentation of pollen grains are often unique to families or genera and are highly useful in classification.
• Cytology: The study of cells, especially chromosomes. The number, size, and structure of chromosomes (karyotype) are powerful taxonomic characters.
• Phytochemistry: The study of plant chemicals (secondary metabolites). The presence or absence of specific compounds (like alkaloids, flavonoids) can show relationships (e.g., the mustard oil in Brassicaceae).
• Molecular Data: The most powerful modern evidence. This involves comparing DNA sequences (e.g., of specific genes like rbcL or matK) or protein structures. This is the basis for phylogenetic classification (e.g., the APG system).

Taxonomic Hierarchy

This is the "Linnaean" system of classification, which organizes organisms into a series of nested ranks (taxa).

• Kingdom (e.g., Plantae)
• Phylum (Division) (e.g., Magnoliophyta)
• Class (e.g., Magnoliopsida)
• Order (e.g., Asterales)
• Family (e.g., Asteraceae)
• Genus (e.g., Helianthus)
• Species (e.g., Helianthus annuus)