PHYLOGENETIC SYSTEM OF CLASSIFICATION

Phylogenetic System of Plant Classification | Botany

List of six eminent botanists who contributed to the phylogenetic system of plant classification:- 1. Adolf Engler (1844-1930) 2. John Hutchinson (1884-1972) 3. Armen Takhtajan (1911) 4. Arthur Cronquist (1919-1992) 5. Rolf Dahlgren (1932-1987) 6. Robert F. Thorne (1920).

Botanist # 1. Adolf Engler (1844-1930):

The best known and widely accepted phylo­genetic system is that by Adolf Engler, Professor of Botany, University of Berlin. In 1892, he pub­lished a system of classification mainly based on August Wilhelm Eichler in the book ‘Syllabus der Vorlesungen’ as a guide to study the plants avai­lable in the Breslau Botanic Garden.

During 1887-1915, Engler and his associate Karl Prantl made a monographic work, the “Die naturlichen Pflanzenfamilien” in 20 volumes, including all the known genera of plants from algae to the phanerogams along with key to identify the plants.

Engler, in collaboration with Gilg, and later with Diels, published the works in a single volume ‘Syllabus der Pflanzenfamilien’. After his death, the book was revised by followers in several editions and the latest (12th) one in 2 volumes in 1954 and 1964.

The system of Engler has been widely used in the American and Europian continents. Engler divided the plant kingdom into thir­teen (13) Divisions.

The thirteenth (13) Division is the Embryo­phyta Siphonogama (the seed-bearing plants i.e., Spermatophyta). It is divided into two Sub­divisions, Gymnospermae and Angiospermae. The Angiospermae is divided into two Classes — Monocotyledonae and Dicotyledonae. The Class Monocotyledonae is divided directly into 11 Orders.

On the other hand, the Class Dicotyledonae is divided into two Subclasses — Archichlamydeae i.e., lower dicotyledons, and Metachlamydeae or Sympetalae i.e., higher dicotyledons. The Archichlamydeae is further divided into 33 Orders, and Metachlamydeae into 11 Orders. The Orders are divided into Suborders, Families, Genera and finally into SpeciES

In this system, the Plant Kingdom contains 309 families. The Class Monocotyledonae starts with the family Typhaceae and ends in Orchidaceae, while the class Dicotyledonae starts with the family Casuarinaceae and ends in Compositae.

In this system, Engler considered that in Embryophyta Siphonogama the flower without perianth is the primitive one. Thus, plants like Oak, Willow etc., with woody stem and uni­sexual apetalous flowers (Amentiferae), are treated as primitive Dicotyledons.

The main distinctive features of Engler’s sys­tem that separate it from that of Bentham and Hooker’s system are:

1. The Polypetalae and Monochlamydeae of Bentham and Hooker are amalgama­ted and placed into a single group (Subclasses) Archichlamydeae.

2. The families of the flowering plants are arranged in ascending order with the increasing complexity of the flowers (mainly on floral envelope).

3. Monocotyledons are placed before Dicotyledons.

4. The term Natural order has been replaced by Family.

5. The term Series or Cohort has been replaced by Order.

Merits and Demerits:

Merits:

1. The entire Plant Kingdom was broadly treated with excellent illustrations, and phylogenetic arrangement of many groups of plants was made.

2. The amalgamation of Polypetalae and Monochlamydeae into Archichlamy­deae is justified.

3. Consideration and placing of Orchidaceae at the end of Monocotyledons and Compositae at the end of Dicoty­ledons are justified — since they are most highly evolved.

4. Juncaceae, Amaryllidaceae and Iridaceae are placed judiciously nearer to Liliaceae.

Demerits:

1. The placement of Amentiferae and Centrospermae almost at the beginning of Dicotyledones, even before Ranales, are not justified.

2. The assemblage of all sympetalous mem­bers under Metachlamydeae increased the distance of closely related orders.

3. The placing of Monocotyledons before Dicotyledons is not appropriate, because it is generally agreed that monocots have arisen from dicoty­ledons by reduction.

4. The placing of the order Helobiae between the advanced orders Pandanales and Glumiflorae is questionable. Araceae was placed much earlier than Liliaceae, from which it has been derived.

5. Fossil evidences gave little support to this system.

Botanist # 2. John Hutchinson (1884-1972):

John Hutchinson was a British botanist asso­ciated with Royal Botanic Gardens, Kew, England. He developed and proposed his system based on Bentham and Hooker and also on Bessey. His phylogenetic system first appeared as “The Families of Flowering Plants” in two volumes.

The first volume contains Dicotyledons (published in 1926) and second volume contains Monocotyledons (published in 1934). He made several revisions in different years. The final revi­sion of “The Families of Flowering Plants” was made just before his death on 2nd September 1972 and the 3rd i.e., the final edition, was pub­lished in 1973.

The following principles were adopted by Hutchinson to classify the flowering plants:

1. Evolution takes place in both upward and downward direction.

2. During evolution all organs do not evolve at the same time.

3. Generally, evolution has been consistent.

4. Trees and shrubs are more primitive than herbs in a group like genus or family.

5. Trees and shrubs are primitive than climbers.

6. Perennials are older than annuals and biennials.

7. Terrestrial angiosperms are primitive than aquatic angiosperms.

8. Dicotyledonous plants are primitive than monocotyledonous plants.

9. Spiral arrangement of vegetative and floral members are primitive than cyclic arrange­ments.

10. Normally, simple leaves are more primitive than compound leaves.

11. Bisexual plants are primitive than unisexual plants and monoecious plants are primitive than dioecious plants.

12. Solitary flowers are primitive than flowers on inflorescence.

13. Types of aestivation gradually evolved from contorted to imbricate to valvate.

14. Polymerous flowers precede oligomerous flowers.

15. Polypetalous flowers are more primitive than gamopetalous flowers.

16. Flowers with petals are more primitive than apetalous flowers.

17. Actinomorphic flowers are more primitive than zygomorphic flowers.

18. Hypogyny is considered as more primitive from which perigyny and epigyny gradually evolved.

19. Apocarpous pistil is more primitive than syncarpous pistil.

20. Polycarpy is more primitive than gynoecium with few carpels.

21. Flowers with many stamens are primitive than flowers with few stamens.

22. Flowers with separate anthers are primitive than flowers with fused anthers and/fila­ments.

23. Endospermic seeds with small embryo is primitive than non-endospermic one with a large embryo.

24. Single fruits are primitive than aggregate fruits.

He divided the Phylum Angiospermae into two Subphyla Dicotyledones and Monocotyledones. The Dicotyledones are further divided into two divisions — Lignosae (arboreal) and Herbaceae (herbaceous).

The Lignosae includes, fundamentally, the woody representatives derived from Magnoliales and Herbaceae includes most of the predominantly herba­ceous families derived from Ranales. The subphylum Monocotyledones are divided into three divisions — Calyciferae, Corolliferae and Glumiflorae.

1. The division Lignosae was further divided into 54 orders beginning with Magnoliales and ending in Verbenales.

2. The division Herbaceae was divided into 28 orders beginning with Ranales and ending in Lamiales.

3. The division Calyciferae was divided into 12 orders beginning with Butamales and ending in Zingiberales.

4. The division Corolliferae was divided into 14 orders beginning with Liliales and ending in Orchidales.

5. The division Glumiflorae was divided into 3 orders beginning with Juncales and ending in Graminales.

So in the latest system of Hutchinson, the Dicotyledones consists of 83 orders and 349 families and Monocotyledones consists of 29 orders and 69 families.

Merits and Demerits Merits:

1. Hutchinson proposed the monophyletic origin of angiosperms from some hypo­thetical Proangiosperms having Bennettitalean characteristics.

2. He made a valuable contribution in phylogenetic classification by his care­ful and critical studies.

3. Monocots have been derived from Dicots.

4. According to him, the definitions of orders and families are mostly precise, particularly in case of subphylum Monocotyledones.

Demerits:

1. There is undue fragmentation of fami­lies.

2. Too much emphasis is laid on habit and habitat. Thus, creation of Lignosae and Herbaceae is thought to be a defect reflecting the Aristotelean view.

3. The origin of angiosperms from Bennettitalean-like ancestor is criticised by many, because the anatomical struc­tures of the early dicotyledons are not tenable with such ancestry.

Botanist # 3. Armen Takhtajan (1911):

Takhtajan was a reputed palaeobotanist of Komarov Botanical Institute of Leningrad, U.S.S.R. (now in Russia). He also made great contributions in the field of angiosperm taxo­nomy. In 1942, he proposed preliminary phylo­genetic arrangement of the orders of higher plants, based on the structural types of gynoecium and placentation.

After 12 years i.e., in 1954, the actual system of classification was published in “The Origin of Angiospermous Plants” in Russian language. It was translated in English in 1958. Later on, in 1964, he proposed a new sys­tem in Russian language. To trace the evolution of angiosperm, he was particularly inspired by Hallier’s attempt to develop a synthetic evolu­tionary classification of flowering plants based on Darwinian philosophy.

The classification was published in ‘Flowering Plants: Origin and Dispersal’ (1969) in English language. Later on, in 1980, a new revision of his system was pub­lished in “Botanical Review”.

Takhtajan (1980) included the angiospermic plants under the Division Magnoliophyta. The Magnoliophyta is divided into two classes Magnoliopsida (Dicotyledons) and Liliopsida (Monocotyledons). The class Magnoliopsida consists of 7 subclasses, 20 superorders, 71 orders and 333 families.

On the other hand, Liliopsida comprises of 3 subclasses, 8 super- orders, 21 orders and 77 families. The class Magnoliopsida starts with the order Magnoliales and ends in Asterales and the class Liliopsida begins with Alismatales and ends in Arales.

The class Magnoliopsida is considered to be monophyletic in origin, probably derived from Bennettitales-like ancestors or stocks ancestral to them. On the other hand, the Liliopsida have been considered to be originated from the stocks ancestral to Nymphaeales. He considered Magnoliopsida more primitive than the Liliopsida. The principles as adopted by Takhtajan (1980) for interpreting the evolutionary lineages in higher plants are mentioned in Table 4.2.

Merits and Demerits:

Merits:

1. The classification of Takhtajan is more phylogenetic than that of earlier sys­tems.

2. This classification is in a general agree­ment with the major contemporary systems of Cronquist, Dahlgren, Thorne, and others. Both phylogenetic and phenetic informations were adopted for delimination of orders and families.

3. Due to the abolition of several artificial groups like Polypetalae, Gamopetalae, Lignosae, Herbaceae, many natural taxa came close together, viz. Lamiaceae (earlier placed under Herbaceae) and Verbenaceae (placed under Lignosae) are brought together under the order Lamiales.

4. Nomenclature adopted in this system is in accordance with the ICBN, even at the level of division.

5. The treatment of Magnoliidae as a primitive group and the placement of Dicotyledons before Monocotyledons are in agreement with the other con­temporary systems.

6. The derivation of monocots from the extinct terrestrial hypothetical group of Magnoliidae is found to be logical.

Demerits:

1. In this system, more weightage is given to cladistic information in comparison to phenetic information.

2. This system provides classification only up to the family level, thus it is not suitable for identification and for adop­tion in Herbaria. In addition, no key has been provided for identification of taxa.

3. Takhtajan recognised angiosperms as division which actually deserve a class rank like that of the systems of Dahlgren (1983) and Throne (2003).

4. Numerous monotypic families have been created in 1997 due to the further splitting and increase in the number of families to 592 (533 in 1987), resulting into a very narrow circumscription.

5. Takhtajan incorrectly suggested that smaller families are more “natural”.

6. Although the families Winteraceae and Canellaceae showed their 99-100% relationship by multigene analyses, Takhtajan placed these two families in two separate orders.

Botanist # 4. Arthur Cronquist (1919-1992):

Arthur Cronquist was the Senior Curator of New York Botanic Garden and Adjunct Professor of Columbia University. He presented an elabo­rate interpretation of his concept of classification in “The Evolution and Classification of Flowering Plants”(1968). The further edition of his classi­fication was published in “An Integrated System of Classification of Flowering Plants” (1981).

The latest revision was published in the 2nd edition in 1988 in “The Evolution and Classification of Flowering Plants”. He discussed a wide range of characteristics important to phylogenetic system. He also provided synoptic keys designed to bring the taxa in an appropriate alignment.

He also represented his classification in charts to show the relationships of the orders within the various subclasses. His system is more or less parallel to Takhtajan’s system, but differs in details.

He considered that the Pteridosperms i.e., the seed ferns as probable ancestors of angiosperm.

The following principles were adopted by Cronquist (1981) to classify the flowering plants:

1. The earliest angiosperms were shrubs rather than trees.

2. The simple leaf is primitive than compound leaf.

3. Reticulate venation is primitive than parallel venation.

4. Paracytic stomata is primitive than the other types.

5. Slender, elongated, long tracheids with numerous scalariform pits are primitive. Further specialisation leads to shorter broad vessels with somewhat thinner walls and transverse end walls with few larger perfo­rations. Later on, the perforation becomes single and large.

6. Long and slender sieve elements with very oblique end walls where the sieve areas scattered along the longitudinal wall with groups of minute pores are primitive. Whereas, the phloem with short sieve tube elements where end walls having a single transverse sieve plate with large openings is a derived condition.

7. The area and activity of cambium and also the length of fusiform initial is more in primitive form which gradually becomes reduced in advanced one.

8. Plants with vascular bundles arranged in a ring are primitive rather than scattered vas­cular bundle as found in monocots.

9. Plants with large and terminal flowers are primitive, those may arrange in monochasia or dichasia and the other type of inflores­cences have been derived from these types.

10. Flowers with many large, free and spirally arranged petals; many linear and spirally arranged stamens and free carpels as found in Magnoliaceae are primitive, and other types got evolved through gradual reduc­tion, aggregation, elaboration and differenti­ation of floral members.

11. Plants with unisexual flowers are evolved from bisexual floral ancestors.

12. The large and indefinite number of floral members are primitive than the small and definite numbers.

13. Androecium with many stamens is primitive than the reduced numbers.

14. Linear stamens with embedded pollen sacs as found in some Magnolian genera are considered more primitive than the others.

15. Uniaperturate pollen grains are considered as primitive and the triaperturate type are derived from it.

16. Insect pollinated plants are considered as primitive from which wind pollinated plants got evolved.

17. The gynoecium comprising of many carpels arranged spirally on a more or less elonga­ted receptacle is considered as primitive. Further evolution leads to the reduction of the number of carpels which are arranged in a single whorl and then undergo further fusion.

18. Axial placentation is primitive from which other types have been evolved.

19. Anatropous ovule is primitive from which other types have been evolved.

20. Ovule with two integuments (bitegmic) is primitive and, either by fusion or abortion, unitegmic condition has been evolved.

21. Embryo-sac with 8-nuclei (Polygonum-type) is primitive from which embryo-sac with 4- nuclei (Oenothera-type) has been derived through reduction.

22. Monocotyledons have been developed from dicotyledons through abortion of one cotyledon.

23. The follicle (fruit) is considered as primitive. Further, dry and dehiscent fruit is more primitive than fleshy and indehiscent fruit.

According to him “many of the evolutionary trends bear little apparent relation to survival value and that there are some reversals”.

In 1981, he divided the Division Magnoliophyta (Angiosperms) into two classes Magnoliatae (Dicotyledons) and Liliatae (Monocotyledons). He divided Magnoliatae into 6 subclasses and 55 orders, of which magnoliales is the primitive and Asterales is the advanced taxa.

On the other hand, the class Liliatae has been divided into 4 subclasses and 18 orders, of which Alismatales is the primitive and Orchidales is the advanced taxa. The class Magnoliatae consists of 291 families and Liliatae with 61 families.

Merits and Demerits:

Merits:

1. There is general agreement of Cronquist’s system with that of other contemporary systems like Takhtajan, Dahlgren and Thorne.

2. Detailed information on anatomy, ultra- structure phytochemistry and chromo­some — besides morphology — was presented in the revision of the classi­fication in 1981 and 1988.

3. The system is highly phylogenetic.

4. Nomenclature is in accordance with the ICBN.

5. The family Asteraceae in Dicotyledons and Orchidaceae in Monocotyledons are generally regarded as advanced and are rightly placed towards the end of respective groups.

6. The relationships of different groups have been described with diagrams which provide valuable information on relative advancement and size of the various subclasses.

7. The family Winteraceae (vessel-less wood present similar to Pteridosperms) placed at the beginning of dicotyledons is favoured by many authors.

8. The subclass Magnoliidae is considered as the most primitive group of Dicotyledons. The placement of Dicotyledons before Monocotyledons finds general agreements with modern authors.

9. As the text is in English, the system has been readily adopted in different books.

Demerits:

1. Though highly phylogenetic and popu­lar in U.S.A., this system is not very use­ful for identification and adoption in Herbaria since Indented keys for genera are not provided.

2. Dahlgren (1983, 89) and Thorne (1980, 83) treated angiosperms in the rank of a class and not that of a division.

3. Superorder as a rank above order has not been recognised here, though it is present in other contemporary classifi­cations like Takhtajan, Thorne and Dahlgren.

4. The subclass Asteridae represents a loose assemblage of several diverse sympetalous families.

5. Ehrendorfer (1983) pointed out that the subclass Hamamelidae does not represent an ancient side branch of the subclass Magnoliidae, but is remnant of a transition from Magnoliidae to Dilleniidae, Rosidae, and Asteridae.

6. There is a difference in opinion with other authors regarding the systematic position of some orders like Typhales, Arales, Urticales etc.

Botanist # 5. Rolf Dahlgren (1932-1987):

Rolf Dahlgren, working at the Botanical Museum in the University of Copenhagen, Denmark, published a new method in Danish in 1974 to illustrate an angiosperm system in a text book of angiosperm taxonomy. Later on in 1975 he published “A System of Classification of Angiosperms to be Used to Demonstrate the Distribution of Characteristics” in Botanische Notiser in English.

The revised and improved version of his system gradually appeared in the subsequent years:

i. In 1980 in “Botanical Journal of the Linnean Society”.

ii. In 1981, in “Phytochemistry and angio­sperm Phytogeny” (Edited by Young and Siegier).

iii. In 1983, in “Nordiac Journal of Botany”.

In his system he included the information at different levels as much as possible. In his classi­fication he extensively used the chemical cha­racteristics. He considered the following mor­phological and chemical characteristics in his classification.

1. Morphological characteristics:

i. Chloripetalae and Sympetalae.

ii. Apocarpous, syncarpous and monocarpellate condition.

iii. Types of microsporogenesis.

iv. Bi- and trinucleate pollen grain.

v. Tenuinucellate, pseudocrassinucellate and crassinucellate.

vi. Bi- or unitegmic ovules etc.

2. Chemical characteristics:

i. Benzylisoquinoline alkaloids.

ii. Pyrrolisidine alkaloids.

iii. Tropane alkaloids.

iv. Silica bodies.

v. Ellagic acid and ellagitannins

vi. Various groups of flavonoids etc.

He did not consider angiosperms to be originated polyphyletically from different gymno­sperms, but believed that the combination of different characteristics like 8-nucleate embryo sac, secondary endosperm etc. would hardly have evolved independently from different groups of gymnosperms.

According to Dahlgren (1980), the class Magnoliopsida (Angiosperms) has been divided into two subclasses, Magnoliidae and Liliidae. The Magnoliidae includes 24 Superorders, those start with Magnoliiflorae and end in Lamiflorae; 80 orders, those start with Annonales and end in Lamiales and 346 families.

On the other hand, the subclass Liliidae includes 7 Superorders, those start with Alismatiflorae and end in Areciflorae; 26 orders, those start with Hydrocharitales and end in Pandanales and 92 families.

Merits and Demerits:

Merits:

1. Detailed information on morphology, phytochemistry and embryology was presented in the classification of Dahlgren.

2. This system is highly phylogenetic where angiosperms are ranked as a class like other recent systems.

3. The arrangement of taxa in the form of a bubble diagram gives an idea about the relationship of superorders, orders and even families.

4. The use of superorders and the suffixanae are in accordance with the other modern systems like those of Takhtajan, Thorne, etc.

Demerits:

1. This system provides classification of angiosperms only up to the family level, thus it is not suitable for identification and for adaptation in Herbaria.

2. Dahlgren classified angiosperms into dicots and monocots, which shows inconformity with the recent classifica­tion of APG II (2003) and Throne (2003).

3. Dahlgren placed monocots in-between dicots, while in modern classification monocots are placed in-between primi­tive angiosperms and the eudicots.

4. Although the families Winteraceae and Canellaceae showed their 99-100% relationship by multigene analysis, yet Dahlgren placed these two families in two separate orders.

Botanist # 6. Robert F. Thorne (1920- ):

Robert F. Thorne, an American taxonomist, associated with the Rancho Santa Ana Botanic Garden, California, U.S.A., initially published the principles of his classification in 1958 and 1963. Later, in 1968, he published “Synopsis of a putatively phylogenetic classification of the flow­ering plants” in Aliso. The subsequent revisions were published in 1974, 1976, 1981, 1983, 1992 and 2000. The electronic version of his classification was published in 1999 which was finally revised in 2003.

Thorne gave much emphasis on phytochemical approach. In addition to the above, many other different aspects were also considered by him.

These are:

1. Seed morphology.

2. Pollen morphology.

3. Comparative anatomy.

4. Embryology.

5. Cytology.

6. Host-parasite relationship.

7. Plant geography.

8. Paleobotany etc.

He believed that the Angiospermae are monophyletic.

In 1983, he divided the Class Angiospermae (Annonopsida) into two subclasses Dicotyledoneae (Annonidae) and Monocotyledoneae (Liliidae). The Dicotyledoneae is further divided into 19 superorder’s which start with Annoniflorae and ends in Asteriflorae; 41 orders, which start with Annonales and ends in Asterales; and 297 families.

On the other hand, Monocotyledoneae is further divided into 9 superorders, which start with Liliiflorae and end in Commeliniflorae; 12 orders, which start with Liliales and ends in Zingiberales.

Thus, he preferred the name Annonopsida for angiosperms, Annonidae for dicots, replacing Magnoliflorae by Annoniflorae and Magnoliales by Annonales. He, however, abandoned these nomenclatures and adopted the conventional names Magnoliopsida, Magnoliidae and Magnoliales since 1992.

Robert F. Thorne (1992) revised his classi­fication (“Classification and Geography of Flowering Plants”) and published it in Botanical Review. He followed the arrangements of different taxa in descending order such as subclasses (-idae), superorders (-anae), orders (-ales), sub­orders (-inae), families (-aceae), subfamilies (-oideae), and tribes (-ineae). He treated the flow­ering plants as Class with an initial bifurcation into two Subclasses, Magnoliidae (Dicots) and Liliidae (Monocots).

The subclass Magnoliidae has been divided into 19 superorders, 52 orders and the subclass Liliidae has been divided into 9 super- orders, 24 orders. The Magnoliidae starts with the order Magnoliales and ends in Lamiales, whereas the Liliidae starts with Triuridales and ends in Restionales.

Merits and Demerits:

Merits:

1. Detailed information on molecular systematics and chemotaxonomy was presen­ted in the classification of Throne.

2. This system is highly phylogenetic where angiosperms are ranked as a class like those of other recent systems.

3. The traditional groups, dicots and mono­cots have been abolished and angio­sperms are divided into 10 subclasses which are in conformity with the recent phylogenetic thinking.

4. Several closely related taxa are placed nearer to one another, viz. the orders Malvales, Urticales, Rhamnales and Euphorbiales have been included under the superorder Malviflorae.

Demerits:

1. This system has no practical utility for identification and for adoption in Herbaria, because identification keys for genera are not provided.

2. The systematic position of the five genera — namely Emblingia, Guametela, Haptanthus, Heteranthia and Pteleocarpa — ha not been mentioned.

3. The placement of Asteridae before Lamiidae is not justified.

4. Segregation of Grewiaceae from Tiliaceae is highly questionable, as in the recent APG classification all the members of Tiliaceae, Bombacaceae and Sterculiaceae are placed under Malvaceae.