TAXONOMY OF ANGIOSPERMS – Types of Classification (Artificial, Natural and Phylogenetic), Biosystematics, Binomial nomenclature, International Code of Botanical Nomenclature (ICBN) and Herbaria and their uses (12TH STANDARD BOTANY)
Taxonomy is concerned with the laws governing the classification of plants. The term taxonomy includes two Greek words taxis – arrangement and nomos– laws. Plant taxonomy is otherwise known as systematic botany. Classification, identification, description and naming the plants are the bases of plant taxonomy. The taxonomic knowledge about the plants is based on their form and structure. The knowledge gained through taxonomy is useful in the fields of medicine, agriculture, forestry, etc.
The ultimate aim of classification is to arrange plants in an orderly sequence based upon their similarities. The closely related plants are kept within a group and unrelated plants are kept far apart in separate groups. The other aim of classification is to establish phylogenetic relationships among the different groups of plants. The plants that are closely related show more similarities than differences. The earliest systems of classification were simple and based on one or few characters. They gave importance to vegetative characters. The later systems of classification gave more importance to floral characters because floral characters are more stable and permanent.
Types of classification
The different types of classification proposed by earlier taxonomists can be broadly categorized into three systems– artificial, natural and phylogenetic.
It was based on one or at most only a few superficial characters. In 1753, Carolus Linnaeus of Sweden published his book “ Species Plantarum” wherein he described 7,300 species. He divided the plants into 24 classes based on number, union, length and certain other characters of stamens. Hence, this system is also known as sexual system of classification. In those days, it was an important over other systems of classification. The importance of floral characters was felt by Linnaeus and his classification was more important than others. The main defect of this system is that totally unrelated plants are brought together in a single group and those that are closely related plants are placed in widely separated groups. For example, plants belonging to Zingiberaceae of Monocotyledons and that of Anacardiaceae of Dicotyledons had been placed in one group called Monandria, as these possess only one stamen. Another defect of this system was that no importance was given to either natural or phylogenetic relationships among different groups of plants.
In this system of classification, plants are classified based on their natural affinities. More number of characters are taken into consideration in this system. It is mainly based on all the informations that were available during the time of direct observation of plants. The most important natural system of classification of seed plants was proposed by two British botanists George Bentham and Sir Joseph Dalton Hooker. It helps to determine the relationships between various groups of plants. However, it does not attempt to bring out evolutionary relationships among different groups of plants.
This system is based on evolutionary sequence as well as genetic relationships among different groups of plants. In addition to this, it employs as many taxonomic characters as possible. Charles Darwin’s concept of Origin of Species had given enough stimulus for the creation of phylogenetic system of classification. Adolf Engler (1844-1930) and Karl Prantl (1849-1893) of Germany published a phylogenetic system in their monograph on “Die Naturlichen Pflanzen Familien”. In this system, floral characters such as single whorl of perianth or no perianth and unisexual flowers pollinated by wind were considered as primitive characters when compared to perianth with two whorls, bisexual flowers pollinated by insects. According to them, members of Asteraceae of dicotyledons and Orchidaceae of monocotyledons were highly advanced.
Taxonomy is mainly concerned with the observation of similarities and differences that exist in the morphology of a vast number of plants. But it has now been accepted that in general, morphological characters alone are not the criteria for distinguishing and classifying plants from one another. One has to take into consideration, the characteristics and differences from other disciplines of science such as cytology, genetics, physiology, ecology, phytogeography, phytochemistry, numerical taxonomy, molecular biology, breeding systems and any other available sources for classification.
Biosystematics may be defined as ‘taxonomy of living populations’. In the present day classification of plants, species is taken as basic unit and it is the local breeding population. Numerous disciplines of science thus provide innumerable number of datas of all the characters of the individual or a species. This helps to clear problems concerning those plants that differ in their interrelationship, classification and evolution. It provides sufficient genetic variations that warrants separation so as to recognise them as a separate taxon based on their evolutionary progress.
Variations in a species may be due to several factors such as genetic, ecological, physiological, population dynamic study and many other factors. All the evidences provided by the biosystematist are taken for analysis and considered by the classical taxonomist in order to arrive at any controversial problems that may arise during their phylogenetic classification based on their evolution of species under study.
Aims of biosystematics
Camp and Gily 1943, coined the term ‘biosystematics’. The aims of biosystematics are as follows.
i) To delimit the naturally occurring biotic community of plant species.
ii) To recognise the various groups as separate biosystematic categories such as ecotypes, ecospecies, cenospecies and comparium.
Methods in the study of biosystematics
Three important methods are as follows.
i) It involves thorough sampling analysis of the taxonomic species under study. Its population, cultivation, geographical range, cytology, anatomy, palynology, phytochemistry, chromosomal number and behavior are keenly observed and studied for finding any genetic differences that may arise among different populations.
ii) It includes determination of ability of different populations to interbreed among one another to form a variant species with its vigor and fertility. This will reveal the presence or absence of breeding barriers between taxa at various levels.
iii) It involves the study of similarity of chromosomes in the hybrids during meiosis.
Ecotype is the basic unit in biosystematics, adapted to a particular environment but capable of producing fertile hybrids with other ecotypes. Ecotype is regarded as equivalent to subspecies of classical taxonomy.
Ecospecies is a group of plants comprising one or more ecotypes within the cenospecies, whose members are able to interchange their genes. Ecospecies is regarded as equivalent to species of classical taxonomy.
Cenospecies is a group of plants representing one or more ecospecies of common evolutionary origin. It is regarded as equivalent to subgenus of classical taxonomy. Cenospecies of the same comparium are separated by genetic barriers and all hybrids between them are sterile.
Comparium is composed of one or more cenospecies that are not able to intercross. Complete genetic barriers exist between ifferent comparia.
The informations obtained from the above mentioned studies were compared with the data obtained through comparative morphology and geographical distributions resulted in the recognition and identification of a total variety or species. To conclude, biosystematic study in the contemporary and modern taxonomy plays a vital role in separating and solving some of the problems that may develop in the identification of plants at the level of species. Biosystematist provides all the necessary data in solving the real position of species that was in controversy.
The system of naming the plants on a scientific basis is known as botanical nomenclature. Naming of the plants is useful in assigning their identity and relationship. Before the middle of the eighteenth century, the names of plants were commonly polynomials i.e. they were composed of several words in series constituting more or less the description of the plant. This can be illustrated with the example of Caryophyllum. The name given was Caryophyllum saxatilis folis gramineus umbellatis corymbis meaning Caryophyllum growing on rocks, having grass like leaves with umbellate corymbose inflorescence. Since lengthy names are difficult to remember and use, attempts were made to shorten these names. Carolus Linnaeus suggested a system of binomial nomenclature. Although the binomial system was introduced by Gaspard Bauhin as early as 1623, it had properly been made use by Linnaeus in his book Species Plantarum.
In binomial nomenclature, every species is given a name of two words. For example, the binomial nomenclature of mango tree is Mangifera indica. Here the first word Mangifera refers to the genus and the second word indica to the species. The two words in combination comprise the name of the plant. Thus the binomial is a binary name. Hence, from the days of Linnaeus, two different kinds of plants could not have the same generic and specific names.
International Code of Botanical Nomenclature (ICBN)
In 1930, the fifth International Botanical Congress was held at Cambridge, England to frame rules and regulations for naming plants. The twelfth meeting was held at Leningrad, USSR in July 1975. Based on the resolutions of this meeting, the current system of International Code of Botanical Nomenclature was adapted from 1978.
Some of the salient features of ICBN
1. The generic name is a singular noun. The first letter of generic name is always written in capital. The specific epithet is an adjective and is always written with small letters. It is derived from many sources and may consist of one or two words. eg. Oryza sativa and Oldenlandia albonervia.
2. The name should be short, precise and easy to pronounce. 3. The binomials are printed in italics or underlined. The generic and specific epithets are underlined separately. eg. Abutilon neilgherrense or Abutilon neilgherrense
4. When new names are given to any plant, then the herbarium preparation of the same specimen with its original description is preserved in any recognized herbarium. This specimen is denoted as type specimen. It is to be preserved on herbarium sheet.
5. The person who publishes the description of any plant for the first time or giving a new name to a plant is considered as author. The name of plant should bear the author’s abbreviated name at the end of specific epithet. This is called author citation. Abbreviations were made for eminent taxonomists. The name Linnaeus was abbreviated to L. or Linn., Robert Brown to R.Br. and Sir Joseph Dalton Hooker to Hook. eg. Malva sylvestris Linn.
6. The original description of the plant should accompany the latin translation.
7. If naming the plant is from a source of error, it is regarded as ambiguous name. It is also called nomen ambiguum and is completely ignored from use.
8. If the generic and specific epithets are the same, it is called tautonym. eg. Sassafras sassafras. Such names are not accepted in the system of nomenclature.
Herbaria and their uses
Herbarium is a collection of pressed, dried plant specimens mounted on specified sheets, identified and arranged in the order of an approved and well known system of classification. It also refers to the institution where dried plant specimens are maintained and studied. eg. Herbarium of Botanical Survey of India, Coimbatore.
A twig with leaves, inflorescence or flowers is collected from shrubs and trees. In the case of herbs, the collected plant specimens should contain both vegetative and reproductive parts. They are dried by keeping them between the folds of old newspapers. It is necessary to change these papers at regular intervals, until the plants are well dried. The plant specimens along with their parts are dried in a plant press. It consists of two boards with straps, which help in tightening the newspapers with specimens between the boards.
The dried specimens are pasted on the herbarium sheets of standard size 41 cm X 29 cm. The process of attaching dried and pressed plant specimens on herbarium sheets is known as mounting of specimens. All the mounted specimens are sprayed with fungicide like 0.1% solution of Mercuric chloride. To protect these dried specimens from the attack of insects, pesticides such as naphthalene and carbon disulphide can be used. The heavy parts of plants such as seeds and fruits are kept in packets and attached to the sheets.
When a new name for a species is suggested, it is the rule that plant specimens of the same should necessarily be deposited in a recognized herbarium. These specimens are called type specimens. The name of the family is always based on type genus. These specimens are most valuable part of herbarium and they are handled with special care. They are stored in fire-proof cabinets. If the herbarium specimens are handled with special care, they will be in good condition for a long time. Precautions should be taken against attacks of fungi and insects. It is always better to use chemicals, which can repel the insects from herbarium specimens. The herbarium is always accompanied with a label. It carries the information about the botanical name of the plant, name of the family, habit, place and date of collection and name of the person who collected the specimens.
Importance of herbarium
Herbarium is a source of knowledge about the flora of a region or a locality or a country.
It is a data store in which the information on plants are available.
The type specimens help in the correct identification of plants.
It provides materials for taxonomic and anatomical studies.
Typical pollen characters have been well emphasized in taxonomy.
Morphological characters of the pollen remain unaltered even after storage upto nearly 200 years.
It is very much useful in the study of cytology, structure of DNA, numerical taxonomy, chaemotaxonomy, etc. It acts as a reservoir of gene pool studies. Because of its importance, several herbaria have been established at the national and international centres.