Taxonomy is an important branch of botany. It deals with the study of morphology, cytology, genetics, physiology etc. in botany. It also deals with the identification nomenclature and classification of plants.
In taxonomy description of morphological characters are given.
Aims of Taxonomy
Aims of taxonomy are—
(a) It gives worlds flora through identification and description.
(b) Taxonomy gives method of identification and communication of plants.
(c) It provides collection and preservation of plants for future use.
(d) It provides insight into the diversity and evolutionary progress among different taxos.
Functions of Taxonomy : The basic functions of taxonomy are
(a) Identification (b) Nomenclature and (c) Classification.
(a) Identification : It provides the identification of plants whether the plant is entirely new or old already known to the scientific world.
(b) Nomenclature : Taxonomy gives the correct scientific name of a plant which has been identified. Common names by which a plant is known is changed to a correct scientific name. In naming the plants binomial nomenclature is used. There is an international code of botanical nomenclature which is used every where. In ICBN—
(i) Every plant has one correct scientific name.
(ii) The name should be in Latin language.
(iii) Binomial nomenclature is used for naming the plants.
(iv) The name should be underlined or written in italics.
(v) Author’s name should be given.
(c) Classification : Classification of plants to specific groups is done on the basis of similarities and disimilarties. The classification should reflect the evolutionary trends.
Types of Classification : Different types of classification have been proposed by several taxonomists. All these classifications are based on different criteria. The different types of classification are—
1. Artificial systems.
2. Natural systems.
3. Phylogenetic systems.
1. Artificial Systems : Artificial systems are based on one or few comparable characters like morphology, nutritional habits etc.
2. Natural Systems : In Natural systems the plants are grouped on the basis of their natural relationships taking into consideration all possible morphological characters. It is believed that all species are created by God and they do not undergo any change.
3. Phylogenetic System : These systems were proposed after the publication of ‘Origin of species’ and the announcement of ‘theory of evolution’ by Charles Darwin. Hence they are called ‘Post-Darwinian classifications’. They reflect the genetic and evolutionary relationships among the taxa and show them in the form of a phylogenetic tree. Classifications made by Eichler, Engler and Prantle, Rendle, Bessey, Hutchinson and others come under this category. More recent phylogenetic classifications made by Takhtajan (1980), Cronquist (1981), Goldberg (1986) and Thorne (1992) have used data from different branches of Botany.
Units of Classification : Different units are used in different systems of classification. Irrespective of its rank in the sequence every unit is called ‘taxon’.
The basic unit of any classification is species. All the identical plants in all respects are regarded as one species. Different species with related characters are grouped into one ‘genus’. Different genera with common characters are grouped into a ‘family’. Different families with similar characters are grouped into an ‘order’. Different orders which are related to each other are grouped into a ‘series’. A group of related series is considered as a ‘class’. Different classes are grouped into a ‘division’. A group of divisions constitute the ‘kingdom’ which is the largest taxon occupying the top most position in the taxonomic hierarchy. Every unit of classification can be subdivided (e.g. sub-class, sub-order etc.). Takhtajan (1980) created a super-order in between class and order.
For example—the classification of cotton or Gossypium herbaceum is as follows :
Kingdom Plant kingdom
Division Phanerogams
(spermatophytes)
Sub-division Angiosperms
Class Dicotyledons
Sub-class Polypetalae
Series Thalamiflorae
Order Malvales
Family Malvaceae
Genus Gossypium
Species Herbaceum
Bentham and Hooker’s System of Classification : George Bentham (1800-1884) and Sir Joseph Dalton Hooker (1817-1911) were the two great British taxonomists. They published a natural system of classification in three volumes of ‘Genera plantarum’ during 1862-1883 in Latin language. Both were the famous scientists of the Royal Botanical Gardens, Kew, England. Subsequently J.D. Hooker became the director of it. Hooker was also responsible for starting Botanical Survey of India (B.S.I.) and published seven volumes of ‘Flora of British India’.
Bentham and Hooker’s classification was based on the system proposed by de Candolle. They identified and described 97,205 species and grouped them into 202 natural orders (present families). Of these 165 belonged to Dicotyledons, 3 belong to Gymnosperms and 34 belong to Monocotyledons. This system is used in all the commonwealth countries even today.
Bentham and Hooker divided the flowering plants into three classes; namely—dicotyledonae, gymnos-permae, and monocotyledonae. Each class was further divided into different sub-classes. The sub-classes were divided into series and each series comprises of different cohorts (present orders) and each cohort was divided into different natural orders (families).
Class Dicotyledonae : The Dicotyledons are characterised by tap root, reticulate venation and two cotyledons. On the basis of the number of whorls in the perianth and the condition of petals, the dicotyledons are divided into three sub-classes namely polypetalae, gamopetalae and monochlamydeae.
1. Sub class—Polypetalae : In polypetalae, the perianth is in two whorls and the petals are free. It is divided into three series on the basis of the nature of thalamus; namely—Thalamiflorae, Disciflorae and Calyciflorae. Gradual evolution of the flower from hypogynous to epigynous condition is clearly reflected in the arrangement of these series.
(i) Series—Thalamiflorae : In this series the thalamus is elongated, conical or convex. It consists of six cohorts (present orders) starting with Ranales and ending with Malvales. The family Malvaceae of the present study belongs to the order Malvales.
(ii) Series—Disciflorae : In this series the thalamus is disc shaped. It has four cohorts from Geraniales to Sapindales.
(iii) Series—Calyciflorae : It this series the thalamus is cup shaped and it includes five cohorts beginning with Rosales and ending with Umbellales. The families Fabaceae and Caesalpiniaceae belonging to the order Rosales are included in our syllabus.
2. Subclass—Gamopetalae : In this the perianth is in two whorls. The petals are fused and the stamens are epipetalous. Based on the nature of the ovary and merosity of the flower it is divided into three series; namely—Inferae, Heteromerae and Bicarpellatae.
(i) Series—Inferae : This series have inferior ovary. There are three cohorts in this series namely Rubiales, Asterales and Campanales.
(ii) Series—Heteromerae : This series has superior ovary and more than two carpels. It has three cohorts—Ericales, Primulales and Ebenales.
(iii) Series—Bicarpellatae : This series have the superior ovary and the number of carpels is two. It includes four cohorts.
3. Sun class—Monochla-mydeae: This has been created on the basis of a single character, the perianth, which is not divisible into calyx and corolla. This resulted in unnatural grouping of families. There are eight series starting with Curvembryae and ending with Ordines anomali. The series is not divided into cohorts and the families are directly included under each series.
II. Class Gymnospermae
This class was divided into three families—Cycadaceae. Coniferaceae and Gnetaceae. Placing gymnosperms which are distinct by having naked seeds in between dicots and monocots is not appropriate.
Merits and Demerits of the System—
Merits
(i) A natural system with practical utility.
(ii) It is being adopted in many Gerbaria.
(iii) Gymnosperms are identified as a separate group.
(iv) Monocolyledon—a highly evolved group is placed at the end.
(v) Creating a new series discifforae.
(vi) Arranging the polypetalous families from hypogynous condition to epigynous condition.
For identification of plants this is the best system.
Demerits
(i) Anomalus position of Gymnosperms in between dicots and monocots.
(ii) Artificial grouping of monochlamydeas.
(iii) No division up to order level of monochlamdeas and monocotyledons.
(iv) Placement of highly advanced families is improper.
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