13.5 Principles of Classification

While developing a system of classification of organisms, certain basic principles are observed. Some of these are as follows:

(i) Morphological criteria:

Morphology forms the primary basis for classifying organisms into various taxonomic groups or taxa. In earlier artificial systems, only one or a few morphological characters were taken into consideration (e.g. plants were classified into herbs, shrubs, trees, climbers, etc. on the basis of their habit). The sexual system proposed by Linnaeus was based mainly on the characteristics of stamens and carpels.

Later on, in the natural systems of classification (e.g. Bentham and Hooker's system of classification of plants), a large number of morphological characters were taken into consideration. As a result, classification of plant groups was more satisfactory and their arrangement was showing natural relationships with each other.

The similarities in the morphological characters are used for grouping the plants together. Because, these similarities indicate their relationships. On the other hand, differences or dissimilarities of characters are used for separating the plant groups from each other. Plant groups with greater differences are considered to be unrelated or distantly related.

For example, all flowering plants with ovules enclosed in an ovary cavity are grouped together as Division - Angiosperms whereas, the angiosperms are further classified into two classes: Dicotyledons and Monocotyledons, on the basis of differences of the characters of root system, leaf venation, flower symmetry and number of cotyledons in the embryo.

(ii) Phylogenetic considerations :

In the more recent systems of classification of plants, a greater emphasis is given on the phylogenetic arrangement of plant groups, an arrangement which is based on the evolutionary sequence of the plant groups. These systems also reflect on the genetic similarities of the plants. Some of the phylogenetic systems of classification of plants are the ones proposed by Engler and Prantle (1887-1899), Bessey (1915), Hutchinson (1926 and 1934), etc.

However, none of these or any other systems is a perfect phylogenetic system. This is because, our present knowledge of the evolutionary history of plant groups is very fragmentary and incomplete. At best, the present day systems can be described as the judicious combination of both natural and phylogenetic systems.

Modern taxonomy takes into consideration data available from all disciplines of botany for classification of plants. This helps immensely in establishing inter-relationships of various plant groups. As a result, taxonomic arrangement becomes more authentic and convincing.

(iii) Chemical taxonomy or chemotaxonomy:

is a comparatively recent discipline. Chemotaxonomy is the application of phyto-chemical data to the problems of systematic botany.

The presence and distribution of various chemical compounds in plants serve as taxonomic evidences. Nearly 33 different groups of chemical compounds have been found to be of taxonomic significance.

(iv) Numerical taxonomy :

Application of numerical methods (data) in the classification of taxonomic units is called numerical taxonomy.

Edgar Anderson (1949) was the first to make use of numerical taxonomy in the classification of flowering plants. It involves exhaustive quantitative estimation of taxonomic characters from all parts of the plant as well as from all stages in the life cycle. The numerical data thus collected for various plant groups is tabulated systematically. Computers are used for this purpose.

The main objective of numerical taxonomy is to clarify and illustrate degrees of relationship or similarity in an objective manner. This branch is becoming an indispensable aid in modern systematics.

Table of Contents

13.0 - Introduction
13.1 Concepts of Species
13.2 Taxa and Categories
13.3 Systematic Hierarchies
13.4 Binomial Nomenclature
13.5 Principles of Classification

Chapter 14