Taxonomy is the oldest disciplines of biology. The ideas of taxonomy were there on the earth, right from the beginning of human civilization. Man classified the plants and animals around him, on the basis of their usefulness. The scientific classification of organisms started from the time of Aristotle, a great Greek Philosopher. The credit for classification of plants goes to Theophrastus (370 285 BC), a contemporary of Aristotle.
The term ‘ taxonomy ‘ introduced in 1813, by Augustin Pyramus de Candolle in his book “ Theories Elementaire de la Botanique.” It is a combination of two Greek words Taxis + Nomos, where Taxis means orderly arrangement and nomos means discourse / study. In biological classification, the taxonomic group of any rank is known as a taxon (pl.taxa). Herman J. Lam (1948), proposed the term ‘taxon’.
Till the end of 19 century, the systems of classification proposed were based on external morphological features, mainly of the flower. Such a classical approach known as “Alpha taxonomy”. In later years the information from other disciplines also taken into consideration, particularly in proposing Phylogenetic classification. Various branches like Anatomy, Cytology, Embryology, Chemistry and Palynology, in addition to morphology are taken into consideration. Thus the multidisciplinary, synthetic approach is an attempt to achieve “Omega taxonomy”.
Taaxonomy deals with 3 aspects. 1. Identification 2. Nomenclature and 3. Classification
1. Identification
It is the determination of taxon, whether or not it is similar to the known which has been recorded earlier. It is fulfilled by consulting the botanical literature or material in the herbaria. The botanical literature refers to Flora or Monographs. The plant life in a given geographical area is known as flora and the enumeration, description and means of identification contained in a book form as Flora.
Based on description of the taxa, artificial keys are provided in the Flora. The plants are identified easily, by descriptions and keys. Herbarium is another source of identification. It may be international (Royal Botanical Garden Herbarium – Kew), national (Central National Herbarium – CAL, Kolkata), or regional (Herbarium Hyderabadensis – Osmania University, Hyderabad).
2. Nomenclature
It is concerned with allocation and determination of names, also concerned with construction, application of rules of naming. The vernacular names are not acceptable for usage in botanical literature as there is no universality. The polynomials are in vogue centuries ago is replaced by Binomial nomenclature introduced by Gaspard Bauhin. Later Caroli Linnaei (1753), used binomial nomenclature, more methodically in his book ‘ Species Plantarum’.
3. Classification
Classification is the orderly arrangement of plants. Grouping of like organisms is made. Then the units so made, are named on the basis of hierarchy of categories.
Types of classification
Broadly classificatory types are divided into 3 main types A) Artificial Classification B) Natural Classification C) Phylogenetic Classification
A) Artificial Classification
In this type one / two characters are given importance, on the account of medicinal and commercial considerations. Theophrastus “ the grandfather of modern botany” classified plants into herbs, sub shrubs, shrubs and trees is an example to artificial classification. Linnaeus also made sexual system of classification, in which he recognised 24 classes, mainly on the basis of number, length, union and separation of stamens and carpels.
B) Natural classification
This is the type of classification in which, use of as many characters as possible from natural habitats is made to group the taxa which are similar are placed together. The first scheme of classification based on natural characters was presented by Antoine Laurent de Jussieu of France in 1789. Contemporary to A.L. de Jussieu, Augustin Pyramus de Candolle (1778 – 1841), presented a new classification of plants and put all alike plants together.
The latest, the best and highly recognised natural system of classification was proposed by George Bentham (1800 – 1884) and Joseph Dalton Hooker (1817 – 1911). They classified plants strictly on the basis of natural scheme, and published in their book “ Genera Plantarum.”
C) Phylogenetic classification
This classifications are based on Darwin’s theory of organic evolution in 1859 in the book “ On the origin of Species”. Botanists started working on the concept of evolution regarding the development of a classification system of plants. S. Endlicher and A.W.Eichler, two German botanists, were first to start along this line of thought. Their schemes were later modified and developed by Adolf Englar (1844 – 1930) and Karl Prantl (1849 – 1893), which was published in “Die Naturlichen Pflanzenfamilien.”
Bentham and Hooker’s system of Classification
George Bentham (1800 – 1884) and Joseph Dalton Hooker (1817 – 1911) proposed the most accepted natural system of classification in the three volumed Genera Plantarum in Latin.
Bentham and Hooker’s system of classification is still used and followed in several herbaria of the world. Most of the Indian herbaria are also arranged according to this system of classification. It is the best system to identify plants in the laboratories. The generic descriptions of the plants prepared from their own observations. In all they described 97,205 species belonging to 200 families of flowering plants.
The Bentham and Hooker’s system of classification is clearly derived from the systems of de Jussieu and de Candolle. They divided all Phanerogams or seed plants into Dicotyledons, Gymnosperms and Monocotyledons. Ranales were placed in the beginning and the grasses at the end in this classification. A summary outline of their classification is mentioned below.
A) Dicotyledons ( reticulate venation, 2 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 4 more families.
Order 2. Parietales : Papaveraceae, Capparidaceae, Brassicaceae, Violaceae and5 more families.
Order 3. Polygalineae : Polygaleae and 3 more families.
Order 4. Caryophyllineae : Caryophyllaceae, Portulacaceae and 2 more families.
Order 5. Guttiferales: Guttiferae (Clusiaceae) and 5 more families.
Order 6. Malvales : Malvaceae, Tiliaceae and Sterculiaceae.
Series II : Disciflorae (hypogynous flowers, disc present)
Order 1. Geraniales : Geraniaceae, Rutaceae, Meliaceae and 8 more families.
Order 2. Olacales : Olacineae and 2 more families.
Order 3. Celastrales : Rhamnaceae and 3 more families.
Order 4. Sapindales : Sapindaceae, Anacardiaceae and Sabiaceae.
Series III : Calyciflorae ( perigynous or epigynous flowers, ovary inferior )
Order 1. Rosales: Leguminosae, Rosaceae and 7 more families.
Order 2. Myrtales : Combretaceae, Myrtaceae, Lythraceae and 3 more families.
Order 3. Passiflorales : Cucurbitaceae, Begoniaceae and 5 more families.
Order 4. Ficoidales : Cactaceae and Ficoideae.
Order 5. Umbellales : Umbelliferae and 2 more families.
2. Gamopetalae ( Petals of corolla partially / completely fused )
Series I. Inferae ( inferior ovary )
thOrder 1. Rubiales : Rubiaceae and Caprifoliaceae
Order 2. Asterales : Compositae and 3 more families
Order 3. Campanales : Campanulaceae and 3 more families
Series II. Heteromerae ( ovary superior, androecium of one / two series, carpels more than 2)
Order 1. Ericales : Ericaceae and 5 more families
Order 2. Primulales : Primulaceae and 2 more families
Order 3. Ebenales : Sapotaceae and 2 more families
Series III. Bicarpellatae : ( ovary superior, androecium of 1 series, carpels 2)
Order 1. Gentianales : Oleaceae, Apocyanaceae, Asclepiadaceae and 3 more families
Order 2. Polemoniales : Convolvulaceae, Solanaceae and 3 more families
Order 3. Personales : Scropulariaceae, Pedaliaceae, Bignoniaceae, Acanthaceae and 4 more families.
Order 4. Lamiales : Labiatae, Verbenaceae and 2 more families
3. Monochlamydeae (Petals absent )
Series I. Curvemryeae ( embryo coiled, ovule generally 1) : Amaranthaceae, Chenopodiaceae, Polygonaceae and 4 more families
Series II. Multiovulate aquaticae (ovules many, immersed aquatics): Podostemaceae.
Series III. Multiovulatae terrestris ( ovules many, plants terrestrial ) : Nepenthaceea and 2 more families
Series IV. Microembryeae ( embryo very minute) : Piperaceae and 3 more families
Series V. Daphnales (ovary with 1 carpel and 1 ovule) : Proteaceae and 3 more families
Series VI. Achlamydosporeae (usually inferior ovary, 1 locule with 1-3 ovules) : Loranthaceae, Santalaceae and Balanophoreae
Series VII. Unisexuales ( Flowers unisexual) : Euphorbiaceae, Urticaceae and 7 more families
Series VIII. Ordines anomali (families of uncertain relationship) : Ceratophyllaceae and 3 more families.
B) Gymnospermae (naked seed plants) : Gnetaceae, Coniferae and Cycadaceae
C) Monocotyledons (Parellel venation, one cotyledon and trimerous flowers)
Series I. Microspermae (inferior ovary, minute seeds ): Orchidaceae and 2 more families
Series II. Epigynae (inferior ovary, large seeds) : Iridaceae, Amaryllidaceae and 5 more families
Series III. Coronarieae (superior ovary, coloured perianth) : Liliaceae, Commelinaceae and 6 more families
Series IV. Calycineae (superior ovary, green perianth) : Juncaceae, Palmae and Flagellariaceae
Series V. Nudiflorae (perianth usually absent, superior ovary) : Typhaceae, Araceae and 3 more families
Series VI. Apocarpae (carpels free) : Alismaceae and 2 more families
Series VII. Glumaceae (reduced perianth, bracts large, scaly) : Cyperaceae, Gramineae and 3 more families.
Merits of Bentham and Hooker’s classification
1. It is the first great natural system of classification.
2. It is very easy to follow for all practical purposes, and that’s why Kew Herbarium and several other Herbaria of the world, including India, are arranged according to this system.
3. The classification starts with Ranales (primitive) and ends with Glumaceae (advanced) and the same is in consonance with the present concept. Arber, Parkin and Hutchinson were expressedsimilar view with regard to the primitive flower.
4. In this system the monocots are derived from dicots. Several recent taxonomic findings support this view.
Demerits
1. The position of Gymnosperms in between dicots and monocots is foremost demerit.
2. Several important floral characters have been neglected.
3. Some 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.
4. Advanced families such as Orchidaceae, have been considered primitive by placing them in the beginning.
5. The entire arrangement of monocots is unnatural and unphylogenetic in this system.
Englar & Prantl’s system of classification
Adolf Englar (1844 – 1930) & Karl Prantl (1849 – 1893) of Germany proposed a phylogenetic system of classification. This system of classification was based on Eichler. They published their work in 23 volumes of Die Naturlichen Pflanzenfamilien. This work consists of well illustrations, provides keys and description of all the plant families known to them at that time. They classified all the plants from algae to angiosperms. This system is used in most of the non – British herbaria of the world.
The followers of Englar and Prantl published revised classification in several successive editions of syllabus der Pflanzenfamilien. The twelfth edition of Syllabus, dealing angiosperms, was edited by Melchior in 1964.
The most noteworthy features of Englar & Prantl’s system of classification are that they (i) placed monocots before dicots (ii) considered orchids to be more evolved than grasses, and (iii) considered apetalous and catkin – bearing dicots primitive to the dicots bearing petals and simple unisexual flowers.
Subdivision – Angiospermae
Class 1. Monocotyledoneae
Orders 1. Pandanales (Typhaceae)
2. Helobiae (Alismataceae & 6 more)
3. Triuridales (Triuridaceae)
4. Glumiflorae (Cyperaceae & Gramineae)
5. Principes (Palmae)
6. Synanthae (Cyclanthaceae)
7. Spathiflorae (Araceae, Lemnaceae)
8. Farinosae (Commilinaceae and 12 more)
9. Liliflorae (Juncaceae., Liliaceae, Amaryllidaceae, Iridaceae and 5 more)
10. Scitamineae (Musaceae & 3 more)
11. Microspermae (Orchicdaceae & Burmanniaceae)
Class 2. Dicotyledoneae
Sub class 1. Archichlamydeae
Orders 1. Verticellatae (casuarinaceae)
2. Piperales (Piperaceae & 2 more)
3. Hydrostachyales (Hydrostachyaceae)
4. Salicales (Salicaceae)
5. Garryales (Garryaceae)
6. Myricales (Myricaceae)
7. Balanopsidales (Balanopsidaceae)
8. Leitneriales (Leitneriaceae)
9. Juglandales (Juglandaceae)
10. Julianiales (Julianiaceae)
11. Batidales (Batidaceae)
12. Fagales (Fagaceae, Butolaceae)
13. Urticales (Moraceae, Urticaceae, Ulmaceae)
14. Podostemonales (Podostemonaceae)
15. Proteales (proteaceae)
16. Santanales (Santalaceae, Loranthaceae & 5 more)
17. Aristolochiales (Aristolochiaceae and 2 more)
18. Balanophorales (Balanophoraceae)
19. Polygonales (Polygonaceae)
20. Centrospermae (Chenopodiaceae, Amaranthaceae, Nyctaginaceae and 7 more)
21. Ranales (Ranunculaceae, Magnoliaceae, Annonaceae and 15 more)
22. Rhoeadales (Papavaraceae, Capparaceae, Cruciferae and 4 more)
23. Sarraceniales (3 families)
24. Rosales (Rosaceae, Leguminosae and 15 more)
25. Pandanales (Pandanaceae)
26. Geraniales (Geraniaceae, Rutaceae, Meliaceae, Euphorbiaceae and 17 more)
27. Sapindales (Anacardiaceae and 22 more)
28. Rhamnales (Rhamnaceae and Vitaceae)
29. Malvales (Malvaceae, Tiliaceae, Bombacaceae, sterculiaceae and 3 more)
30. Parietales (Violaceae and 30 more)
31. Opuntiales (Cactaceae)
32. Myrtiflorae (Myrtaceae, Combretaceae and 21 more)
33. Umbelliflorae (Umbelliferae and 2 more)
Subclass 2. Metachlamydeae (Sympetalae)
Orders 1. Diapensiales (Diapensiaceae)
2. Ericales (Ericaceae and 3 more)
3. Primulales (Primulaceae and 2 more)
4. Plumbaginales (Plumbaginaceae)
5. Ebenales (sapotaceae and 6 more)
6. Contortae (Apocynaceae, Asclepiadaceae, Oleaceae and 3 more)
7. Tubiflorae (Convolvulaceae, Borginaceae, Verbenaceae, Labiatae, Solanaceae, Scrophulariaceae, Bignoniaceae, Pedaliaceae, Acanthaceae and 13 more)
8. Plantaginales (Plantaginaceae)
9. Rubiales (Rubiaceae and 4 more)
10. Cucurbitales (Cucurbitaceae)
11. Campanulatae (Campanulaceae, Compositae and 4 more)
Merits of Englar & Prant’s Classification
1. This is a convenient and well known filing system of several herbaria of the world.
2. Polypetalae and Monochlamydae of Bentham & Hooker were merged by Englar & Prantl into single subclass Archichlamydae.
3. This system treated families such as Orchidaceae and Compositae as advanced families.
4. In this system several closely related families (Liliaceae, Juncaceae, Iridaceae and Amaryllidaceae) are treated close to one another.
5. Abundant illustrations are provided along with the description of the families.
6. The system is provided with exhaustive keys of families and orders.
7. The description of each family also contains a summary of its embryology, morphology, anatomyand geographical distribution.
Demerits
1. Monocots have been placed before dicots in this system. But it has been revised in 1964 edition.
2. Naked flowers of Amentiferae have been treated as primitive in this system.
3. Helobiae, consisting of primitive forms, have been placed between two advanced orders Glumiflorae and Pandanales.
4. Araceae are derived from Liliaceae, but Englar & Prantl placed Araceae before Liliaceae.
5. This system fails to recognize the significance of reduction, and becauseof this “ simple” were equated with “primitive” according to Cronquist (1965).
Angiosperm Phylogeny Group (APG)
Angiosperms / flowering plants, traditionally have been divided into 2 primary groups based on the presence of a single cotyledon (Monocots) or two cotyledons (Dicots). This division has accounted for the long recognition of those groups in flowering plant classifications. However phylogenetic analysis based on nuclear, plastid and mitochondrial DNA sequences and morphology do not support this dichotomy. Cladistic analysis of the families of dicotyledons revealed that their diagnostic characters represent simply the plesiomorphic ( = primitive characters within angiosperms. Thus, the dicots represent a paraphyletic group.
On the other hand, the monocots formed a monophyletic group, and this clade (= branch) is given the name Monocots. Within the dicots majority of the species share the characters traditionally attributed to dicots, such as flowers with 4/5 parts and tricolpate pollen, thus forming a monophyletic group. This group of flowering plants had been called eudicots or tricolpates.
During the 1900’s, reconstruction of flowering plant phylogeny took great step forward. Rapidly accumulating DNA sequences, in particular from the plastid gene rbc L, Cp DNA mat K gene, mitochondrial gene atp A and 18 Sr DNA provided new and informative sets of data. At the same time the development of effective PCR techniques made possible to apply cladistic methods of analysis.
This new knowledge of phylogeny revealed relations in conflict with the widely used modern classifications ( Cronquist, 1981, Thorne 1992, Takthajan, 1997) which were based on selected similarities and differences in morphology. It became clear that none of the previous classifications accurately reflected the phylogenetic relationships of the flowering plants.
To solve this problem, a group of flowering plant systematic, calling themselves as the Angiosperm phylogeny Group (APG), proposed a new classification for the families of flowering plants.
This group is composed of B.Bremer (Royal Swedish Academy of Sciences, Sweden), K.Bremer (Uppsala University, Sweden), M.W.Chase (Royal Botanic Gardens, Kew, UK), J.L.Reveal (University of Maryland, Colorado, USA), D.E. Soltis (University of Florida, USA), P.S.Soltis (Florida Museum of Natural History, USA), and Peter F.Stevens (University of Missouri, USA).
The APG system of classification proposed in 1998 (APG I, 1998), comprised 462 families arranged in 40 monophyletic orders under small number of informal monophyletic higher groups : monocots, commelinoids, eudicots, core eudicots, rosids, eurosids I, eurosids II, asteroids, euasterids I and euasterids II.
Judd et al. (1999) presented some modifications in APG – I, by recognising a total of 51 orders and shifting some families to these orders from informal groups.
A recent revision of APG (APG – II, 2003) and continuous up gradation on Angiosperms phylogeny has resulted in considerable refinement.
A broad outline of APG – II (2003) classification is presented below.
Magnoliophyta
Group order
Austrobaileyales
1. Monocots 1. Acorales
2. Alismatales
3. Asparagales
4. Dioscoreales
5. Liliales
6. Pandanales
2. Magnolids 1. Magnoliales
2. Laurales
3. Canellales
4. Piperales
5. Ceratophyllales
3.Commelinids 1. Arecales
2. Poales
3. Commelinales
4. Zingiberales
4. Eudicots 1. Ranunculales
2. Proteales
5.Core Eudicots 1. Gunnerales
2. Caryophyllales
3. Santalales
4. Saxifragales
6.Rosids 1. Crossosomatales
2. Geraniales
3. Myrtales
7.Eurosids I 1. Celastrales
2. Malpighiales
3. Oxalidales
4. Fabales
5. Rosales
6. Cucurbitales
7. Fagales
8.Eurosids II 1. Brassicales
2. Malvales
3. Sapindales
9.Asterids 1. Cornales
2.Ericales
10.Euasterids I 1. Garryales
2. Gentianales
3. Lamiales
4. Solanales
11. Euasterids II 1. Aquifoliales
2. apiales
3. Asterales
4. Dipsacales
Unplaced families : Amborellaceae, Cabombaceae, Chloranthaceae and Nymphaeaceae.
Current concepts of Angiosperm Taxonomy
Morphological characters of plants been used extensively both for producing classification and for identification purposes and still they are indispensable for the taxonomist. However morphological features alone are not adequate in proper assessment of the systematic status of a taxon and its phylogeny. Evidences from other disciplines like cytology, anatomy, embryology, Physiology, palynology, phenology, biochemistry and genetics etc, have been found to be useful in solving some of the taxonomic problems. The impact of the above disciplines on present day taxonomy has changed it from alpha (classical) to omega (modern) taxonomy.
Embryology in taxonomy
Embryology is the study of micro and megasporogenesis, gametophyte development, fertilization and development of endosperm, embryo and seed. Embryological evidences have been used in solving the taxonomical problems at all levels. Role of embryology in solving taxonomic problems was done by German embryologist, Schnarf in 1931. According to Jones and Luchsinger (1987), the embryological characters have proved to be significant help “in determining relationships within families, genera and species”.
Some examples of role of embryology in taxonomy
1. Dicots and monocots: Angiosperms are universally divided into dicotyledons and monocotyledons. This classification is based on number of cotyledons.
2. Caryophyllales: Trinucleate pollen, bitegmic crassinucellate ovules which are campylotropous, seed with peripheral embryo and perisperm with little or no endosperm, are the characters found only in Caryophyllales, widely known as Centrospermae (Cronquist, 1968).
3. Helobiae : the monocotyledonous order, treated as a subclass in some recent systems of classification, by presence of Helobial type of endosperm.
4. Orchidales : the distinguishing feature of the members of this order is presence of undifferentiated embryo and very little or no endosperm.
5. Onagraceae : this family is recognized by the presence of Onagrad type of embryosac.
6. Cyperaceae : in flowering plants, 4 functional microspores developfrom each microspore mother cell (pollen mother cell). But in Cyperaceae, each microspore mother cell gives rise to only one pollen grain. Out of 4 nuclei, formed by meiosis, 3 are cut off on one side and do not form pollen grains.
7. Exocarpus : because of a naked ovule and pollen chamber, exocarpus was removed from Santalaceae of angiosperms and was treated as a member of the family Exocarpaceae near Taxaceae in gymnosperms. But the presence of a typical angiospermic flower, polygonum type of embryosac, cellular embryosac led Ram (1956) to confirm that Exocarpus belongs to the family Santalaceae of angiosperms, not with gymnosperms.
8. Trapa : majority of the taxonomists treat trapa as a genus of Onagraceae, while others consider it to belongs to the family Hydrocaryaceae. But embryological details (polygonum type embryosac, absence of endosperm, well developed suspensor haustorium etc) suggest that Trapa should be treated under an independent family Trapace
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