B.Sc Botany TOPIC :- PLANT TAXONOMY

PLANT TAXONOMY

The Greek meaning of taxonomy is “arrangement by rules” and of systematics is “to put together”. The term taxonomy was first coined and used by the famous French botanist A.P. de Candolle in 1813 in his book Theorie Elementaire de la Botanique.

Definition:- “science of the classification of organisms according to their resemblances and differences”

BASIC COMPONENTS OF TAXONOMY:-

Classification, identification, description and nomenclature are the four basic components of taxonomy. 

Classification is the arrangement of botanical groups with definite circumscriptions by position and rank according to artificial criteria, phenetic similarities, or phylogenetic relationships.

 Identification is the determination of similarities or dissimilarities between the two elements. Under identification we make a direct comparison of the characteristic features of a specimen with those present in the already existing keys for identification. 

Description is the orderly recording of maximum possible characters of a taxon, individual plant, plant part, or object. 

 Nomenclature is a simple system under which the individual taxonomic groups of plants are scientifically named. 

Classification :-

Andrew Sugden (1984) defined the word “classification” in Longman Illustrated Dictionary of Botany as “the naming of species and their grouping into families, orders, divisions, etc.” Radford (1986) stated that “classification is the arrangement of groups of plants with particular circumscriptions by rank and position according to artificial criteria, phenetic similarities, or phylogenetic relationships.”

In its simplest form, classification is the placement of plants, animals and objects into groups and categories for a clear understanding, proper study and effective organization. 

RANKS OF PLANT CLASSIFICATION:-

Species, genus, family, order, class, and division are the six main ranks of plant classification in an ascending order. Each rank has its subcategories, i.e. towards the higher ranks, subform, form, subvarieties, varieties, and subspecies are the subcategories of species; subsection, section, and subgenus are the subcategories of genus; subtribe, tribe, and subfamily are the subcategories of family; suborder is the subcategory of order; subclass is the subcategory of class; and subdivision is the subcategory of division.

Several species are included within a genus, several genera within a family, several families within an order, several orders within a class, and several classes are included within a division. 

 

TYPES OF CLASSIFICATION:-

(1). Artificial system of classification:-when plants are classify on the basis of one or two 

charaters , so that's artificial system of classification.

Scientist of artificial system of classification :-                                                        BOOK

1.Theophrastus                    Historia plantarum    

 2.Dioscorides                                       De materia medica

( father of medicinal plant -Pharmacognosy )                                              

3.Andrea Cesalpino               De plantis

                                                                    (considered the first textbook of botany) 

4.Gaspard Bauhin                              Pinax theatri botanici

                                                                                 (English, Illustrated exposition of plants)

5.Carl Linnaeus                                   Species Plantarum   

["father of modern taxonomy".]                         

Artificial classification is the most primitive classification.    

             

(2).Natural system of classification :-   The natural system of classification of organisms is according to a relationship based on descent from a common ancestor. In this classification, the organism is classified on the basis of their vegetative characters which are permanent and do not change with the environment. This system of classification avoids the grouping of heterogeneous and unrelated groups of organisms. The first natural system of plant classification was given by Bentham and Hooker.

1. The natural classification is based on more than fewer characters, and mainly totality of characters are considered.

2. It is based on posteriori weighting of the taxonomic characters.

3. Natural Classification is based on diagnostic or hidden characters which show relationships.

4. Natural Classification is an upward classification based on the grouping of related species that resemble each other and forming a hierarchy of higher taxa by grouping similar taxa of the lower rank.

5. This classification helps to find out the missing link and their common ancestors.

6. Natural Classification can establish the relationship among the taxa, so it helps to find out the phylogeny among different taxa.

(3.)     PHYLOGENETIC SYSTEM OF CLASSIFICATION :-

 These systems of classification used as many taxonomic characters as possible in addition to the phylogenetic (evolutionary) interpretations.

It is based on the evolution of life and shows the genetic relationships among organisms. It generates trees called cladograms, which are groups of organisms that include an ancestor species and its descendants.

 Some of the phylogenetic systems of classification were proposed by Eichler (1839–1889), Engler (1844–1930) and Prantl (1849–1893), Bessey (1845–1915), Wettstein (1862–1931), Hallier (1868–1938), Hutchinson (1884–1972), Takhtajan (1980), Cronquist (1981), Dahlgren (1983) and Thorne (1983).

                     

SOME IMPORTANT SYSTEMS OF CLASSIFICATION :-

John Ray (1627–1725):-

An English biologist, John Ray1 treated over 18,000 species in the last edition of his book Methodus Plantarum Nova published in 1703. He developed a classification system on the basis of the form relationships by grouping together the plants that resembled one another. Ray was the first to divide herbs, shrubs and trees into Dicotyledons and Monocotyledons on the basis of the presence of two or one cotyledons. Broadly, he divided the plants as under:-

                                                    

George Bentham (1800–1884) and Joseph Dalton Hooker (1817–1911) :-

The most accepted natural system of classification was proposed by Bentham and Hooker in their Genera Plantarum published during July 1862 and April 1883. Bentham, a self-trained British botanist, and Hooker, the Director of the Royal Botanic Gardens, Kew (England), described all known genera of seed plants in three volumes of their Genera Plantarum, published in Latin.

Bentham and Hooker’s system of classification is still used and followed in several herbaria of the world. In most of the Indian herbaria too, the plants are arranged according to this system of classification. It is supposed to be the best system for the students to identify plants in the laboratories. This is so because Bentham and Hooker prepared the generic descriptions of the plants from their own observations and not by copying from the available literature. Large genera are divided into sections and sub-sections, and the description of genera is complete and accurate.

The Bentham and Hooker’s system of classification is clearly derived from the systems of de Jussieu and de Candolle. Bentham and Hooker divided all Phanerogams or seed plants into Dicotyledons, Gymnosperms and Monocotyledons. Ranales were placed in the beginning and grasses at the end in this classification. A summary outline of their classification is mentioned below: -

(A) Dicotyledons (Reticulate venation; two cotyledons; pentamerous flowers).-

 1. Polypetalae (Corolla of separate petals)   

                   Series I. Thalamiflorae (stamens many; hypogynous; disc absent).

                  Order 1. Ranales: Ranunculaceae, Magnoliaceae, Annonaceae, Nymphaeaceae and four                                         more families.            

                            2. Parietales: Papaveraceae, Capparidaceae, Cruciferae, Violaceae and five more                                           families. 

                            3. Polygalineae: Polygaleae and three more families.

                            4. Caryophyllineae: Caryophyllaceae, Portulacaceae and two more families. 

                            5. Guttiferales: Guttiferae and five more families.

                            6. Malvales: Malvaceae, Tiliaceae and Sterculiaceae. 

       Series II. Disciflorae (stamens hypogynous; disc present). 

 Order 1. Geraniales: Geraniaceae, Rutaceae, Meliaceae, and eight more families.

           2. Olacales: Olacineae, and two more families. 

           3. Celastrales: Rhamnaceae, and three more families.

          4. Sapindales: Sapindaceae, Anacardiaceae and Sabiaceae. 

Series III. Calyciflorae (stamens perigynous or epigynous; ovary generally inferior). 

           Order 1. Rosales: Leguminosae, Rosaceae, and seven more families. 

                    2. Myrtales: Combretaceae, Myrtaceae, Lythraceae, and three more families. 

                   3. Passiflorales: Cucurbitaceae, Begoniaceae, and five more families. 

                   4. Ficoidales: Cactaceae, Ficoideae. 

                   5. Umbellales: Umbelliferae, and two more families. 

2. Gamopetalae (petals of corolla are partially or completely fused). 

        Series I. Inferae (inferior ovary). 

          Order 1. Rubiales: Rubiaceae and Caprifoliaceae. 

                    2. Asterales: Compositae, and three more families. 

                    3. Campanales: Campanulaceae, and three more families. 

      Series II. Heteromerae (ovary superior; androecium of one or two series; carpels more than two). 

           Order 1. Ericales: Ericaceae, and five more families.

                     2. Primulales: Primulaceae, and two more families. 

                     3. Ebenales: Sapotaceae, and two more families.

 Series III. Bicarpellatae (ovary superior; androecium of one series; carpels two).

 Order 1. Gentianales: Oleaceae, Apocynaceae, Asclepiadaceae, and three more families. 

            2. Polemoniales: Convolvulaceae, Solanaceae, and three more families.

            3. Personales: Scrophulariaceae, Pedaliaceae, Bignoniaceae, Acanthaceae, and four more                              families. 

           4. Lamiales: Labiatae, Verbenaceae, and two more families. 

3. Monochlamydeae (Petals absent).

                             Series I. Curvembryeae (embryo coiled, ovule generally one): Amaranthaceae,                                                        Chenopodiaceae,  Polygonaceae, and four more families 

                        Series II. Multiovulatae aquaticae (ovules many; immersed aquatics): Podostemaceae. 

                     Series III. Multiovulatae terrestris (ovules many; plants terrestrial): Nepenthaceae, and                                               two   more families. 

                 Series IV. Microembryeae (embryo very minute): Piperaceae, and three more families.

               Series V. Daphnales (ovary with one carpel and one ovule): Proteaceae, and three more                                           families.          

            Series VI. Achlamydosporeae (usually inferior ovary; one locule, with 1–3 ovules):                                                   Loranthaceae,   Santalaceae and Balanophoreae. 

       Series VII. Unisexuales (flowers unisexual): Euphorbiaceae, Urticaceae, and seven more families.  

   Series VIII. Ordines anomali (families of uncertain relationship): Ceratophyllaceae, and three more                             families. 

(B) Gymnospermae (naked-seeded plants): Gnetaceae, Coniferae, Cycadaceae

(C) Monocotyledons (parallel venation; one cotyledon; trimerous flowers).

                          Series I. Microspermae (inferior ovary; minute seeds): Orchidaceae, and two more                                                families.                          

                      Series II. Epigynae (inferior ovary; large seeds): Iridaceae, Amaryllidaceae, and five                                                       more  families.

                  Series III. Coronarieae (superior ovary; coloured perianth): Liliaceae, Commelinaceae, and                                       six more families. 

              Series IV. Calycineae (superior ovary; green perianth): Juncaceae, Palmae, Flagellariaceae. 

          Series V. Nudiflorae (perianth usually absent; superior ovary): Typhaceae, Araceae, and three                                 more families. 

     Series VI. Apocarpae (carpels free): Alismaceae, and two more families.

 Series VII. Glumaceae (reduced perianth; bracts large, scaly): Cyperaceae, Gramineae, and three more                      families.

Merits of the System of Bentham and Hooker :-

 1. It is the first great natural system of classification. 

 2. It is very easy to follow for all practical purposes, and that is why Kew Herbarium and several other       herbaria of the world, including India, are arranged according to this system. 

 3. This system was never planned by Bentham and Hooker on the basis of phylogeny, although the             theory of organic evolution was already announced by Darwin and Wallace in 1859. So this system         should not be criticized on the basis of phylogeny.

 4. Ranales have been given a primitive position in this system. Recent taxonomic findings also                     indicate  that Ranales are the most primitive living angiosperms. 

 5. In this system the monocots are derived from dicots. Several recent taxonomic findings support this view. 

Demerits of the System of Bentham and Hooker:-

 1. The position of gymnosperms in between dicots and monocots in this system is its foremost                   demerit. This arrangement is made without considering the affinities among these groups.

 2. Several important floral characters have been neglected in this system. 

3. In this system some of the closely related families have been separated and placed under different           orders (cohorts). In the same way, a number of unrelated families have been grouped nearer. Few           examples are undermentioned:-

     (i) All the families of Series Curvembrae are related to Caryophyllaceae of series Thalamiflorae of                Polypetalae. 

    (ii) Podostemaceae of Series Multiovulatae aquaticae of Monochlamydeae deserves a placing under             Rosales of Series Calyciflorae of Polypetalae.

    (iii) Nepenthaceae (of Series Multiovulatae-terrestris of Monochlamydeae) is related more to family             Saracenniaceae (of orders Parietales of Thalamiflorae).

    (iv) Laurineae of Series Daphnales is related closely with Magnoliaceae of Ranales.

 4. Advanced  families, such as Orchidaceae, have been considered primitive in this system by placing         them in the beginning.

 5. The entire arrangement of monocots is unnatural and unphylogenetic in this system.