Classification Systems and taxonomy branches

Plant Kingdom

The broad classification of living organisms was proposed by Whittaker in 1969.  He suggested a five-kingdomclassification, consisting of Monera, Protista, Fungi, Animalia, and Plantae. In this chapter, we will furtherstudy the classification within Kingdom Plantae or the plant kingdom. Kingdom Plantae is further classified into algae, Bryophytes, Pteridophytes, Gymnosperms, and Angiosperms.

Classification systems and taxonomy branches

Plant taxonomy can be defined as a branch of botanythat dealswith the characterization, identification, classification, and nomenclature of the plants on the basis ofsimilarities and differences.

The purpose of plant taxonomy is:

1. Identification:  To identify the unknown species based on its characteristics and compare it with already existing species.

2. Characterization:  To provide a description of all the characteristics of the newly identified plant species.

3. Classification:  A suitable placement of the known species into different groups according to similarities and differences.

4. Nomenclature: Providing a suitable scientific name to the convention.

Only a few vegetative features were considered in the first classification system. Along with morphological traits, modern taxonomy studies have become more comprehensive and taken into account numerous morphological, cellular, and molecular factors, such as cellular and reproductive aspects, mode of nutrition, habitat, evolutionary relationships, and so on.

Plant classification systems are divided into three categories.

(I) Artificial system of classification: it was the earliest system that tried to classify organisms based on a few superficial characteristics. Ithowever did not consider morphological details and the evolutionary relationship among the organisms.  This system gives equal importance to vegetative and sexual characters.  Vegetative characters are greatly influenced by their surroundings.  Therefore the closely related species could not be properly classified.More than 2000 years ago, Aristotlewas able to classify plants into herbs, shrubs, and trees, solely on the basis of morphology characters. Carl Linnaeus in his book Systema Naturaegives the hierarchical system of classification for the plant kingdom and animal kingdom. He proposed Binomial Nomenclature and the rules for naming every species.

(II) Natural system of classification:  In this system, more traits were taken into account, in order to characterize. It was based on the natural similarities of vegetative and floral characteristics among organisms it considers various external and internal features like the anatomy of the plant, cell types of an embryo, and phytochemistry.

Bentham and Hooker’s system of classification categorized plants into cryptogams (non-flowering plants) and Phanerogams(flowering plants). It helped to determine relationships between the various groups of plants and failed to identify the phylogenetic relationship among different groups as it incorrectly placed gymnosperms between monocotyledons and dicotyledons.

(III) The phylogenetic system of classification: This system is based on evolutionary sequences and genetic links. This system was developed after the publication of Darwin's theory of evolution. Apart from the morphological characteristics found inthe fossil record, genetic components were also taken into consideration.This system of classification is most widely accepted by biologists throughout the world. According to this system, all the organisms occupying the same taxa originated from a common ancestor.  Various taxonomists like Englar and Prantl, Hutchinson, Takhtajan, Robert F Thorn, etc. have contributed to the phylogenetic system of classification.

Numerical taxonomy is based on all observable characteristics present in a species. A number of codes are assigned to all the characters and the data is then processed in such a way that each character is given equal importance and at the same time, the hundreds of characters can be considered. It can be easily carried out using a computer.

Cytotaxonomy is based on cytological information like chromosome number cell structure and behavior. Chemotaxonomy exploits the chemical constituents of the plant to resolve confusion related to classification. All these techniques are widely used by taxonomistsnowadays.