Botanical Plants Classification (Part 1)

Botanical Plants Classification

(Part 1)

Classification

          Classification is the arrangement of plants into groups having common characteristics. These groups are arranged into a system. Similarly, species of flowering plants are placed in a Genus; similar genera are grouped into Families; families with common features are arranged into Orders; orders into Classes and classes into Divisions.

The Aim of Botanical Classification

The aim of the classification is to place the plant into a hierarchy of ranks or categories such as species, genera, families, and so on. In addition to expressing’ relationship based on common features, classification serves as a filing and information retrieval system and allows easier reference to organisms comprising the filing system, i.e., it provides an idea about the sequence of evolution of plants from simple to more complex and from more primitive to more advanced types.

The Criteria of the Botanical Classification

          The Criteria of the Classification are the Characters on which the classification is based. The characters of an organism are all the features or attributes (leaf width, stamen number, corolla length, locule number, placentation, etc.) possessed by the organism that maybe compared, measured, counted, describe or otherwise assessed. This means that differences, similarities, and discontinuities between plants and taxa are reflected in their characters. The characters of a taxon are determined by observing or analyzing samples of individuals and recording the observations, or by conducting controlled experiments.

Certain characters which are used in description, delimitation, or identification are called Diagnostic Characters, whereas the characters of constant nature, which are used to “help define a group, are termed as Synthetic Characters. A character may be Qualitative Character when it refers to such things as flower color, odour, leaf shape, etc. Or Quantitative Character when it expresses the features that can be counted or measured such as size, length, breadth, etc.

Characters are important as these:

1) Provide information for construction of taxonomic system;

2) Supply characters for construction of keys for identification;

3) Furnish features useful in the description and delimitation of taxa;

4) Enable the scientists to use the predictive value of classification.

Morphological and anatomical characters are used for the purpose of classification.

Units of Botanical  Classification

          Following are the units used by plant taxonomies.

Species

          The fundamental category forming the base of plant classification is the Species. In taxonomic practice, a group of individual plants that is fundamentally alike is generally treated as species. Ideally, a species should be separated by distinct morphological differences from other species in order to have a practical classification. However, it is sometimes difficult to delimit a species precisely.

A species shows quite a large number of variations within its population. To provide recognition to these variations Infra-Specific Categories, i.e., categories below the rank of species, are used. These are Sub-Species, Variety and Form. The categories of sub-species and variety are applied to populations of species in various stages of differentiation and the category of form is generally used to recognize and describe sporadic variations in single morphological features such as occasional white-flowered plants in a normally purple flowered species.

Genus

           The species are grouped together into an inclusive group Genus, whose species have more characteristics common with each other than with the species of other genera within the same family. Genera therefore, are aggregates of closely related species. The species together. A genus may consist of one species and is known as Monotypic; or many species may be present in a genus, e.g., Senecio of Compositae contains 2000 to 3000 species.

Family

            Genera with common characteristics are grouped together into a more inclusive group, the Family. Both reproductive and vegetative features are used to characterize families. The reproductive characters provide more characters for definitions of families than vegetative characters as these are little affected by environment. The characters usually used to delimit families are: ovary position, kinds of pistils and stamens, carpel number, fruit type, symmetry of flower, leaf arrangement, leaf morphology, and habit.

Order

          Next higher category is the Order which includes one or more families. Orders are characterized by an aggregate of characters.

Division

           Similarly, the orders are arranged into Classes, the classes into Sub-Divisions, and sub-division into a Division.

Systems of Classifications

 Systems of Classification

         Various system of classifications, based upon varying characteristics were proposed by the botanists. These systems of classifications are grouped into following four categories:

  1. Artificial Systems

        When the plants are classified for the sake of convenience, using some arbitrary or at least easily observable characters, often irrespective of their affinity. The classification is called Artificial and the system of classification is termed as Artificial System. Early systems of classification were mostly artificial, for example, Linnaeus used only one character, the (Cannaceae) which are similar in one respect only. Similarly, unlike plants such as Cacti and Cherries were placed together. 

  1. Natural Systems

         By the end of 1700s, most botanists realized that there were Natural Affinities among plants. Also, the theory of evolution by Charles Darwin proposed that present day plants have descended from those existing in ancient past through a series of modifications in response to changing environmental conditions. This also suggest that all plants existing, today are related to each other. Therefore, closely related plants should naturally be grouped together, Such, a system of classification is called Natural System. Such a system helps in identification of plants also. This system superseded artificial systems and most of the Pre-Darwanian systems of classification are natural systems including that of Bentham & Hooker’s system of classification. These systems were based upon morphological and reproductive characters. 

  1. Phylogenetic Systems

           The systems of classification that try to reflect evolution are said to be Phylogenetic. As the impact of evolutionary theory become apparent, taxonomists started using evolutionary concepts into their classification. However, there was no abrupt departure from previous systems. The taxonomists tried to arrange the natural groups of plants in an evolutionary sequence from simple to the most complex. One of the advantages of phylogenetic system is that it is a rich source of information as the identity of a plant is based upon its affinities and evolutionary relationships. Most of the post-Darwanian systems are phylogenetic including that of Engler and Prantll’s system of classification.

  1. Modern Systems

           At present, the classification of plant kindom continues to be modified as new information become available. These systems of classification are called Modern Systems. In these systems, modern information from the fields of paleobotany, biochemistry, anatomy, karyology, etc. have been used in classifying the flowering plants. Electron microscopy also helped in formulation of modern systems. Systems of classification proposed by Robert Thorne (1968), Armen Takhtajan (1980), Arthur Cronquist (1981) and Rolf Dahlgren (1981) are modern systems.

History of Classifications

 Theophrastus

Theophrastus (372-287 BC), Father of Botany, made a pioneering attempt to place his knowledge of plants on scientific footing. He arranged the plants in several groups. He named and described some 500 plants in his book De Historia Plantarum.

Linnaeus

           Linnaeus is regarded as a father as a modern taxonomy as classification and nomenclature took root in his time practically. Pre-Linnaeus period is governed by classification system of Greeks which were considered as Herbalists, because they classified the plants on the basis of the habit or according to their economic utility. Major change in the classification of plants took place in the middle of eighteenth century Carl van Linnaeus, a Swedish botanist proposed a system of classification based on characters of stamens. He also used binomial nomenclature in his system. Linnaeus classified the plants into 24 classes starting with Monandria (number of stamen one, e.g., Canna), Diandria, Tri-Tetra, Penta-, Hexa-, Hepta-, Octand-, Ennean-, Deca-, Dodecamdria (stamens 12-19, e.g., Euphorbia) Icosandria (stamens – 20 or more, perigynous, e.g., Cactus). Polygamia (flowers polygamodioecious, e.g., Ficus) and Crytogamia (flowers concealed or absent, e.g., Equisetum). Linnaeus system was artificial and resulted in quite unnatural grouping of plants. Linnaeus himself realized this defect and tried to evolve a natural system but he did not live long enough to complete his work.

Bernard de Jussieu

          Bernard de Jussieu improved upon the Linnaeus system and laid down the foundation of a natural system. His work was published by his nephew Anto Laurent de jussieu. They classified the plants into orders, now families and these orders were grouped into 15 classes. They recognised difference between Monocots and Dicots and separated Cryptogams (acotyledons) from seed plants.

Augustin Pyrame de Condolle

          Augustin Pyrame de Condolle (A.P. de Condolle) improved Jussieu’s system. He was of the view that the morphological characters should be the basis of classification rather the physiological processes. He classified the plants into 213 orders (now families). He started describing all the known species of the plants. His work was completed by his son and grandson.

Robert Brown

Robert brown, distinguished Gymnosperms as naked seeded plants from the Angiosperms in which the seeds are enclosed in a vessel.

Stephen Endlicher

          Stephen Endlicher, distinguished Gymnosperms as naked seeded plants from the Angiosperms in which the seeds are enclosed in a vessel.

(i)     Thallophytes, containing Algae, Fungi and Lichens.

(ii)     Carmophytes, containing mosses, ferns and seed plants.

Bentham & Hooker

           Bentham & Hooker, two botanists, proposed a natural system based on the Condelle’s system. They divided the seed plants into 202 orders (now families) grouped into cohorts (now orders).

At the same time, Wilhelm Hofmeister, with the help of compound microscope discovered the phenomenon of alternation of generation in the lower plants. Darwin’s theory of evolution and discovery of the phenomenon of alternation of generation paved the way for initiation of phylogenetic period in the system of classification.

Wilhelm Eichler

           Wilhelm Eichler first introduced phylogenetic trends in classification.

He classified the plants into two major groups:

 (i) Cryptogamae: Further divided into three major divisions:

(1) Thallophyta with two classes:

(a) Algae (b) Fungi.

(2) Bryophyta with two classes:

(a) Hepaticeae (b) Musci.

(3) Pteridophyta with three classes:

(a) Equisetineae (b) Lycopodineae and (c) Filicineae.

(ii) Phanerogamae: Further divided into two divisions:

(1) Gymnospermae, and (2) Angiospermae: divided into two classes:

(a) Monocotyleae (b) Dicotyleae; further divided into two sub-classes:

(i) Choripetalae (ii) Sympetalae.

Adolph Engler & Karl Prantll

Adolph Engler in association with Karl Prantll classified the whole plant kingdom on the basis of phylogenetic relations. This system is an elaboration of Eichler’s system.

Charles Edwin Bessey modified Bentham & Hooker’s system, making it a phylogenetic one, on the basis of certain principles called Bessey’s Dicta. He renamed cohorts as orders and the orders as families.

Persons like John Hutchinson, Alfred Barton Rendle, Oswald Tippo, Arthur J. Eames and I. W. Bailey researched in various fields and their work helped in classification. Presently the classification of plants continues to be modified as new information become available.

Modern systems of classification have been proposed by Robert Thorne (1968), Armen Takhtajan (1980), Arthur Consquist (1981), Rolf Dahlgren (1977, 1980, 1981) has attempted to develop phylogenetic classification based upon evolutionary Principles using newly available informations from the field of embryology, chemistry and anatomy.

Bentham & Hooker’s system of classification

George Bentham and sir joseph Dalton Hooker presented their system in a three volume work in Latin, entitled Genera Plantarum between 1862 and 1883. The system of classification proposed by Bentham and Hooker is a natural system. It has been retained in British and Indian herberia. Bentham and Hooker’s system of classification is a slight modification of de Condolle’s system and is a non-phylogenetic natural system. The flowering plants are divided mainly on the basis of many constant superficial characters neglecting many floral characters. Hence, many closely related families are kept apart and unrelated families are put together.

All seed plants were classified into 3 classes, 3 sub-classes, 21 series, 25 cohorts and 202 orders. Spermatophytes (seed plants) are divided into Dicotyledons, Gymnosperms and Monocotyledons. The origin of the angiosperms is not established and position of the gymnosperms is anamalous, i.e., in between dicotyledons and monocotyledons. Arboreous and herbaceous habits are not considered as important in the classification of angiosperms. The dicotyledons are placed before monocotyledons and probably dicotyledons are considered to be more primitive than monocotyledons. Monocotyledons are divided into 7 Series beginning with Microspermae and ending in Glumaceae. These series include 34 Natural orders. 5 cohorts and 33 Orders; (c) Monochlamydeae, which is classified into 8 Series, consisting of 33 Natural Orders.

A broad outline of Bentham & Hooker’s system of classification is as follow:

Plant Kingdom

Division: Angispermae

Class: Dicotyledons

A-Sub class: Polypetalae           

Corolla of separate petals.

Series l: Thalamiflorae

Stamens hypogynous and usually many. These series include 6 cohorts and 33 orders.

Series ll: Disciflorae

Hypogynous disc often present. Stamens definite as many as twice the number of petals.

This series include 4 cohorts and 23 orders.

Series lll: Clayciflorae

Sepals united, stamens peri or epigynous. This series include 5 cohorts and 27 orders.

  1. Sub class: Gamopetalae

Corolla of united petals.

Series l: Inferae

Stamens as many as petals and alternating with them; ovary inferior.

This series include 3 cohorts and 9 orders.

Series ll: Heteromerae

Stamens as many as corolla lobes and opposite them, or many; ovary superior or inferior, carpels more than 2, generally isomerous with corolla lobes.

Series lll: Bicarpellatae

Stamens as many as corolla lobes and alternating with the or fewe, ovary mostly bicarpellary and superior.

This series includes 5 cohorts and 27 orders.

  1. Sub Class: Monochlamydeae

Perianth 1-2 seriate, mostly sepaloid, minute or absent.

Series l: Curvyembryeae

Seeds mostly with endosperm; embryo curved, lateral or peripheral; ovary mostly one ovuled.

This series include 7 natural orders.

Series ll: Multiovulatae aquaticae

Immersed aquatic herbs; ovary syncarpous, many ovuled.

This series include a single natural order.

Series lll: Multiovulatae terrestris

Terrestrial herbs; ovary syncarpous, many ovules.

This series include 3 natural orders.

Series lV: Microembrayeae

Carpels 1-2 ovuled; ovules with copious emdosperm and minute embryo.

This series include 4 natural orders.

Series V: Daphnales

Ovary monocarpellary, rarely syncarpous and 2-4 ovules; plants woody or herbaceous.

This series include 3 natural orders

Series Vl: Achlamydosporeae

Ovary unilocular, 1-3 ovuled; seeds endospermic, without testa. This series include 3 natural orders.

Series Vll: Unisexuals

Flowers strictly unisexual or polygamous.

This series include 9 natural orders.

Series Vlll: Ordines anomaly

This series include 4 natural orders.

Class: Monocotyledons

Series l: Microspermae

Inner perianth petaloid, ovary inferior; seeds minute and many.

This series include 3 natural orders.

Series ll: Epigynae

Inner perianth petaloid, ovary inferior, ovules large and few to many.

This series include 7 natural orders.

Series lll: Corenarieae

Inner perianth petaloid, ovary free.

This series include 8 natural orders.

Series lV: Calycinae

Inner perianth sepaloid, rigid or herbaceous.

This series include 3 natural orders.

Series V: Nudiflorae

Perianth absent or reduced.

This series include 5 natural orders.

Series Vl: Apocarpae

Perianth 1-2 seriate or absent: carpel solitary or gynoecium apocarpous.

This series include 3 natural orders.

Series Vll: Glumaceae

Flowers in dense inflorescence subtended by bracts or glumes: perianth reduced, glumaceous or absent: ovary or locules single ovule.

This series include 5 natural orders.

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