PLANT TAXONOMY

Taxonomy consist of two words "Taxis" means arrangement and "nomos" means law and together we will called it law of arrangement or classification. The word taxonomy was proposed by A.P.de. Candolle in his book "Theories elementaire de la botanique" (Theory of elementary botany).

Taxonomy includes study of following points :

1. Identification : A process by which an organism is recognised  from the other already known organisms and is assigned to a particular taxonomic group is called identification.
2. Nomenclature : Naming of organisms according to international scientific rules is called nomenclature.
3. Classification : A process by which any organism is grouped into convenient categories on the basis of some easily observable characters.

In modern taxonomical studies, taxonomists use external and internal structure structure, along with the structure of cell, development process and ecological information of organisms. The number of species that are known and described range between 1.7 to 1.8 million. This refers to biodiversity or the number and types of organism present on earth but till many species are un-identified, so the practical significance of taxonomy is identification of unknown organism.

Types of taxonomy :

1. Cytotaxonomy : The use of cytological characters of plants in classification or in solving taxonomic problems is called cytotaxonomy. It is based on cytological information like chromosome number, structure and behaviour etc.
2. Chemotaxonomy : It is based on the chemical constituents of plants. The basic chemical compounds used in chemotaxonomy are alkaloids, carotenoids, tannins, polysaccharide, nucleic acids, fatty acids, amino acids, aromatic compounds etc.

Nomenclature :

(1) Binomial system : Given by Carolus Linnaeus

Linnaeus used this nomenclature system for the first time on large scale and proposed scientific name of all the plants and animals. Linnaeus is the founder of binomial system. Linnaeus proposed scientific names of plants in his book "Species plantarum". It was published on 1 May 1753. So this was the initiation of binomial system for plants. So any name proposed for plants before this date is not accepted today. Linnaeus proposed scientific name of animals in 10th edition of his book "Systema naturae". This 10th edition of Systema naturae was published on 1 August 1758. So initiation of binomial system for animals is believed to be started on 1 August 1758.

Nomenclature of plants and animals are given under the govern of some international rules. Collection of rules regarding scientific / nomenclature of plants is known as ICBN (International code of botanical Nomenclature). ICBN was first accepted in 1961.

Main rules of ICBN :

1. According to binomial system name of any species consists of two components or words i.e., Generic name (name of genus) and specific epithet. e.g. Solanum tuberosum (potato) Here Solanum is generic name and tuberosum is specific epithet.
2. In plant nomenclature tautonyms are not valid i.e., generic name and specific epithet should not be same in plants. e.g. Mangifera mangifera is not a valid name and we know that the correct one is Mangifera indica (Mango). But tautonyms are valid in animal nomenclature (ICZN - International code of zoological nomenclature) e.g. Naja naja (Indian cobra), Rattus rattus (Rat).
3. First letter of generic name should be in capital letter and first letter of specific epithet should be in small letter. e.g. Mangifera indica.
4. When written with free hand or typed, then generic name and specific epithet should be separately underlined but during printing name should be in italics to indicate their latin origin.
5. Name of scientist who proposed nomenclature should be written in short after the specific epithet. e.g. Mangifera indica Linn.
6. Name of scientist should be neither underlined nor in italics, but written in Roman letters (simple alphabets).
7. Scientific names should be derived from Latin (usually) or Greek languages because they are dead languages.
8. Type specimen (Herbarium Sheet) of newly discovered plant should be placed in herbarium.

(2) Trinomial system :

According to this system name of any organism is composed of three words i.e., Generic name, Specific epithet and Subspecific epithet (Name of variety). e.g. Brassica oleracea botrytis (Cauliflower), Brassica oleracea capitata (Cabbage), Brassica oleracea caulorapa (Knol-Khol) here botrytis, capitata and caulorapa are the different variety of Brassica oleracea.

Biological Classification :

The art of identifying distinctions among organisms and placing them into groups that reflect their most significant features and relationship is called biological classification. The purpose of biological classification is to organize the vast number of known organisms into categories that could be named, remembered and studied.

Types of biological classification :

(1) Practical classification : In this type of classification, plants are classified on the basis of their economic importance or human use. This classification system is the earliest system. e.g. oil yielding plants - coconut, walnut, soyabean, fibre yielding plants - jute, cotton, Medicinal plants - Rauwolfia, cinchona, eucalyptus. In this classification, any one plant can be a member of more than one group. e.g. Turmeric is multi uses plants, it gives both medicines and spices.

(2) Artificial classification : In this type of classification plants are classified on the basis of one or two morphological characters, i.e. over all morphology is not considered. Classification proposed by Linnaeus is artificial. In the book "Genera Plantarum" Linnaeus classified the plant kingdom into 24 classes on the basis of stamen so, Linnaeus classification is also called as sexual classification. Linnaeus divided phanerogams (gymnosperm and angiosperms) into 23 classes starting with class monandria with a single stamen (e.g. Canna) and plants with twenty or more stamens attached with calyx were assigned to class Icosandria. He included other plants such as algae, fungi, mosses (bryophytes) and ferns (pteridophytes) in a separate class called cryptogamia.

(3) Natural classification : In this type, plants are classified on the basis of their complete morphological characters of (stem, root, leaves, flowers etc.). Natural classification is believed to be best classification, because it represents the natural similarities and dissimilarities of plants i.e. it represents the inter relationship among plants. In this classification, the plants belonging to the same group shows many similarities, while in artificial classification, the plants belonging to the same group shows only, 1 or 2 similar characters. They have many dissimilarities.

Natural classification is of two types :

• Natural non phylogenetic : In this classification, the phylogeny of the plant is not considered i.e. only the morphology of the plant is considered.
• Natural phylogenetic : In this classification, both morphology and phylogeny are considered. In phylogenetic classification, the plants are arranged on the basis of their evolution.

(4) Phenetic classification or Numerical classification : In it plants are classified on the basis of numbers of similarities and dissimilarities. This classification is easily carried out by using computers and it is based on all observable characteristics. In this classification number and codes are assigned to all the characters and the data are prepared and then processed. Those organism which have maximum similarities are placed in same group. In this way each character is given equal importance and at the same time hundreds of characters can be considered. In this classification importance to any one character is not given, all characters have same importance. While in natural classification floral (reproductive) characters have more importance than vegetative (root, stem and leaves) characters.