Lesson 1, Topic 4
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Taxonomy & Systematics

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The term “taxonomy” originates from two words, taxis” meaning arrangement and “nomos” meaning laws. Plant taxonomy deals with the classification of plants according to certain set rules. The term taxonomy was coined by the Swiss botanist A. P. de Candolle in his book “Theories elementaire de la botanique”.

Plant taxonomy can be defined as the branch of botany which deals with characterisation, identification, classification and nomenclature of plants based on their similarities and differences.

The goals of plant taxonomy are:

  1. Characterisation: to describe all the characteristics of the newly identified species
  2. Identification: identify the unknown species based on its characteristics and by comparing with already existing species
  3. Classification: placing and arranging the known species into different groups or taxa according to similarities and dissimilarities
  4. Nomenclature: giving the scientific name according to the convention

Taxonomy and Systematics

The word systematics comes from the word ‘systema’, meaning the systematic arrangement of the organisms. It takes into consideration the evolutionary relationship of the organisms. Plant systematics deals with interrelation between plants and their evolutionary descent. Systematics studies biological diversity and organises the information into a classification.

Organisms are classified on the basis of similarities, closeness or relationship between them. It shows the phylogenetic relationship between different organisms and shows their line of descent. The similarities among individuals show that they might have developed from the common ancestor. It shows the evolutionary pathway of modern living organisms. Closely related organisms are included in a group, which share a common gene pool.

Organisms are categorised into different taxonomic categories according to the similarities and specific features. The different taxonomic categories in their hierarchical order are:

  • Kingdom
  • Phylum
  • Class
  • Order
  • Family
  • Genus
  • Species

The number of common characteristics decreases as we move from species to the kingdom, where species having fundamental similarities and organisms in the same kingdom having least common features.

List of Systems of Plant Taxonomy

The earliest system of classification considered only a few vegetative characters. Modern taxonomic studies have been more elaborate and taken into consideration various morphological, cellular and molecular characteristics, e.g. cellular and reproductive features, mode of nutrition, habitat, evolutionary relationships, etc. along with morphological features.

There are three main types of systems for plant classification. Here is the list of systems of plant taxonomy:

  1. Artificial systems
  2. Natural systems
  3. Phylogenetic systems

1. Artificial systems: Artificial systems were the earliest systems, which attempted to classify organisms based on a few superficial characters.

These were important in the history of biological classification as this was a novel attempt to organise living organisms. The demerit was that it didn’t consider morphological details and the evolutionary relationship. They gave equal importance to vegetative and sexual characters but it is not true. Vegetative characters are greatly influenced by the environment. As a result, the closely related species were kept apart.

Aristotle classified plants more than 2000 years ago on the basis of simple morphological characters into herb, shrub and trees.

Theophrastus in his book “Historia Plantarum or Enquiry into plants” attempted to arrange plants in various groups based on how plants reproduce and its uses. He is called “Father of Botany”.

Carl Linnaeus is known as the “Father of Modern Taxonomy”. In his book “Systema Naturae” (1735), he gave the hierarchical system of classification of the natural world into the plant kingdom, the animal kingdom and mineral kingdom.

He understood the importance of floral characters and classified plants based on the number of stamens present in them. It is also known as the sexual system of classification.

Linnaeus kept on adding new work to his publications. In “Species Plantarum” (1753), he gave a brief description of all the species known to him. He described around 7,300 plant species in it. He divided the plant kingdom into 24 classes based on the structure, union, length and the number of stamens. E.g. Monoandria (1 stamen), Diandria (2 stamens), Polyandria (more than 12 stamens), Monoadelphia (stamens united in a single bundle), Monoecia, Dioecia, Polygamia (polygamous plants), Cryptogamia (flowerless plants), etc.

He gave the Binomial nomenclature system. In “Philosophia Botanica”, he had given rules for naming every species. It is called binomial because each name has two components, genus name and species name, e.g. Solanum melongena (brinjal), Solanum tuberosum (potato) having the same genus but different species name.

2. Natural systems: In this system of classification, more characters were considered while classifying. It was based on the natural similarities of vegetative and floral characters among the organisms. It took into consideration various external and internal features like the anatomy of a cell, types of embryo and phytochemistry.

Bentham and Hooker proposed the most important natural system of classification of flowering plants. They classified plants into Cryptogams (non-flowering plants) and Phanerogams (flowering plants).

Bentham And Hooker Classification

It helped to determine relationships between the various groups of plants but failed to identify phylogenetic relationships among different groups of plants. It wrongly placed gymnosperms between monocotyledons and dicotyledons.

3. Phylogenetic systems: This system is based on evolutionary sequence and genetic relationship. This system was developed after the publication of Darwin’s theory of evolution. Apart from the morphological characteristics found from fossil records, genetic constituents were also considered. It has been widely accepted by biologists all over the world. According to this system, all the organisms belonging to the same taxa originated from the common ancestor.

Phylogenetic System of Classification

Various scientists namely Engler and Prantl, Hutchinson, Takhtajan, Cronquist, Rolf Dahlgren and Robert F Thorne contributed to the phylogenetic system of classification.

The two main phylogenetic systems of classification are:

  • Engler and Prantl system of classification: In this type of system floral characters like single whorl or no perianth, unisexual flowers pollinated by wind were considered primitive characters as compared to two whorls in the perianth and bisexual flowers pollinated by insects. They arranged plants based on the increasing complexity of the flower morphology.
Engler and Prantl System of Classification
  • The plant kingdom was divided into 13 divisions:
    • 11 are Thallophytes
    • 12th Embryophyta Asiphonogama, i.e. plants having embryos but pollen tubes are absent (bryophytes and pteridophytes)
    • 13th Embryophyta Siphonogama, i.e. plants with embryo and pollen tubes (seed plants)
  • Hutchinson’s classification: Hutchinson classified angiosperms into monocotyledons and dicotyledons.
    • Dicotyledons were further divided into two divisions, namely, Lignosae (woody plants) and Herbaoae (herbaceous plants)
    • Monocots were divided into 3 divisions on the basis of flower morphology, namely, Calyciflerae (calyx present), Corolliferae (petaloid perianth) and Glumiflorae (perianth absent)
Hutchinson's Classification

Modern Taxonomic advancements:

With the advent of molecular biology, many techniques to identify genetic materials have been developed. This has equipped us to compare individuals at different taxonomic levels and resolve the difficulties of classifying them even if there is no fossil evidence.

  1. Numerical taxonomy: It is done by using computers and all the observable characters are taken into consideration. Each character is assigned with a code and a number. Hundreds of characters can be considered together and given equal importance.
  2. Cytotaxonomy: it utilises cytological information like chromosome number, shape size, etc. to understand the taxonomy.
  3. Chemotaxonomy: Use of chemical constituents of plants for taxonomic studies is known as chemotaxonomy. Proteins, amino acids, nucleic acids and peptides, starch grains, wax, fat, oil, phenols are studied in chemotaxonomy.

Importance of Plant Taxonomy

  • It gives a detailed overview of various morphological and anatomical structures of a plant species
  • It organises all the information of plants into an orderly fashion
  • It indicates the phylogenetic relationship between species and its ancestry
  • Plant taxonomy enables to identify any unknown species and its place in the classification by comparing with known species
  • Analysis of genetic constituents can be done on the basis of systematics
  • It is used to scientifically name any species, which helps in the uniformity of the name around the world and avoids confusion
  • It helps to understand the biodiversity present at a place
  • It helps in recording all the living species known until now
  • Taxonomy is widely used in agriculture, medicine and forestry