Introduction

There are 1.7 million species of organisms, among which 1.2 million are animal and 0.5 million are plant species. Due to such high number of species it is important to classify them and study them in groups.

Classification : Grouping or categorizing organisms on the basis of their similarities and dissimilarities for the convenience of study is called as classification.

Taxonomy (A. P. de Candolle) : The study which includes identification, nomenclature and classification.

According to some scientists taxonomy and systematics are synonymous terms but according to others, systematics is a wider term.

Systematics : It includes taxonomy and phylogeny. Thus systematics can be defined as study of diversity of organisms and evolutionary relationship among them.

Systematics of plants is called as systematic botany.

Father of systematic botany is Carolus Linnaeus (Also regarded as father of modern taxonomy)

Classical Systematics : All the artificial and natural systems of classification belong to this group. According to this concept species is considered to be static, constant, fixed, immutable entity.

Only morphological characters are taken into account called as -taxonomy.

New Systematics (or Biosystematics or Neo Systematics)

The concept of new systematics was given by Julian Huxley (1940). It takes into consideration all types of  characteristics like morphological, cytological, physiological ecological, biochemical and genetical. Thus it is called as integrated taxonomy also named as -taxonomy by Turril.

Numerical Taxonomy (Phenetic classification)

When mathematical techniques are applied to assess similarity and  disparity through diagrams and using mathematical coefficients. This is called numerical taxonomy.

The family tree based upon phenetic classification is called as dendrogram.

Chemotaxonomy : When we make use of the results of chemical analysis or tests of plant parts and products in study of taxonomy is called as chemotaxonomy.

Nomenclature : The science of giving names to living beings is called nomenclature.

Polynomial Nomenclature

Before 1750 (medieval periods), scholars used to add a series of descriptive words to designate a particular species. This can be illustrated with the example of Caryophyllum. The name given was Caryophyllum saxatilis folis gramineus umbellatus corymbia meaning Caryophyllum growing on rocks having grass-like leaves and umbellate corymb arrangement of flowers. Such long names can not be easily remembered.

Binomial nomenclature

The scientific or technical names were developed by Linnaeus (Philosophia Botanica, 1751). The technical names recognized internationally are the ones given by Linnaeus in the 10th edition of his book Systema Naturae published in 1758. The system developed by Linnaeus is known as Binomial Nomenclature. Binomial nomenclature is the system of scientific naming using ‘genus’ as the first part and ‘species’ as the second part, e.g., Mangifera indica (mango), Apis mellifera (honey bee), etc.

Trinomial nomenclature

Occasionally, three words are also used for naming the organisms, especially the animals. These include generic, specific and sub-species part, for example, the modern man is called Homo sapiens sapiens. Other examples are Puccinia graminis tritici, Acacia nilotica indica, etc.

Rules of binomial nomenclature

• The five codes of nomenclature are: International Code of Botanical Nomenclature (ICBN) and International Code of Zoological Nomenclature (ICZN). The names of bacteria and viruses are decided by International Code for Nomenclature of Bacteria (ICNB) and International Code of Viral Nomenclature (ICVN). Similarly, there is a separate International code of Nomenclature for Cultivated Plants (ICNCP). The important rules are
• A scientific name consists of two words, first genus and second species. They should not have less than three letters and more than twelve letters.
• The generic name is always written first, which is like a noun having its first letter in capital form. The generic name is always unique for a living organism.
• The specific name is written after the generic name, which is like an adjective having its first letter in small form. It can be single or compound (e.g., Hibiscus rosa sinensis).
• The gender of the specific name follows the gender of the generic name, e.g., Mangifera indica, Tamarindus indica.
• The biological or scientific name is always printed in italics whereas it is underlined while handwritten. This is done to make the scientific name distinct from the text.
• The two-word scientific names are generally followed by the name of the discoverer or author. The author’s name can be full or abbreviated (i.e., Mangifera indica L., Homo sapiens Linn., Cycas circinalis Linnaeus). Author’s name is not italicised.
• The names are generally derived from Latin language since Latin is a dead language and, therefore, it will not change in form or spelling with the passage of time. The names that are derived from languages other than Latin are latinized (e.g., Pinus roxburghii, Riccia himalayensis, Spirogyra indica).
• No names are recognized prior to those used by Linnaeus in 1758 in the 10th Edition of Systema Naturae.
• The names of sub families and families should be based on the name of the type genus e.g., family Graminae is changed to Poaceae, Compositae changed to Asteraceae.
• When a species is transferred or revised, the name of the original author is retained but in parenthesis, e.g., Syzygium cumuni (Linn.) Skeels, Albizzia lebbeck (Linn.) Benth.
• In case an organism has been given more than one name, the earlier legitimate one is recognized to be valid (law of priority).

Taxonomic hierarchy

• The main aim of a taxonomic study  is to assign organism an appropriate place in a systematic framework of classification. This framework is called taxonomic hierarchy by which the taxonomic groups are arranged in definite order, from higher to lower categories, depending upon their relative dimensions.  
• It is also called Linnaean hierarchy because it was first proposed by Linnaeus. Linnaeus first used only five categories-Class, Order, Genus, Species and Variety. The last one was discarded and three added so that now there are seven obligate categories i.e., Kingdom, Division or Phylum, Class, Order, Family, Genus and Species. The botanists use Division in place of Phylum as a category in the classification of plant kingdom. In order to make taxonomic position of species more precise, certain subcategories and super categories have been added to this list and they are called intermediate categories e.g., sub-kingdom, super-phylum or super-division, sub-division, super-class, sub-class, super order, suborder, superfamily, subfamily, tribe, subspecies, variety, etc.
• Both in animals and plant kingdoms, the lowest category is Species and highest is the Kingdom. The placement of group of individuals or organisms in species, genus and upto phyla or divisions is determined by the similarities in their characters and relationships. The categories in the hierarchy are thus in ascending order. As we go from the lowest rank Species towards Kingdom, the number of similar characters decreases.

Taxon (Gk. Taxis : arrangement)

• The word taxon signify a taxonomic group of any rank which represents the real biological organisms included in a category like Maize (species), Roses (genus), grasses (family), conifers (order), dicots (class), seed plants (division) etc. The term was introduced by Adolf Meyer (1926) for animal groups. Mayr (1964) defined taxon to be a taxonomic group of any rank that is sufficiently distinct to be worthy of being assigned to a definite category.

For example, Bryophyta is a taxon while division is a category. Similarly Zea mays is a taxon while species is a category. While category represents an abstract term, taxon represents the real organisms.