C.A.Bessy

C. A. Bessey (1845-1915) was an American botanist, who laid the foundations of modern phylogenetic classifications. He was a student of Asa Gray and later became Professor of botany at the University of Nebraska. He was the first American to make a major contribution to plant classification, and also the first botanist to develop intentional phylogenetic classification. He based his classification on Bentham and Hooker, modified in the light of his 28 dicta and published in Ann. Mo. Bot. Gard. under the title‘The phylogenetic taxonomy of flowering plants’ (1915).

Bessey considered angiosperms to have evolved monophyletically from Cycadophyta belonging to implied bennettitalean ancestry. He was the pioneer to consider that the large-sized bisexual flowers of Magnoliaceae with spirally arranged floral parts represent the most primitive conditionin angiosperms, a theory followed by many subsequent authors.

Bessey believed in the strobiloid theory of the origin of the flower, the latter having originated from a vegetative shoot with spiral phyllomes, of which some modified to form sterile perianth, fertile stamens and carpels. Two evolutionary lines from such a flower formed strobiloideae (Ranalian line) with connation of like parts and cotyloideae (Rosalian line) with connation of unlike parts.

Ranales in dicots and Alismatales in monocots were considered to be the most primitive in each group, a fact recognized by most subsequent authors. Ranalian plants were considered to be primitive angiosperms having given rise to monocots, but unfortunately monocots were placed before dicots.

Bessey also initiated the representation of evolutionary relationships through an evolutionary diagram, a phylogram with primitive groups at the base and the most advanced at the tips of branches. His diagram, resembling a cactus plant is better known as Besseyan cactus.

John Hutchinson-- Phylogenetic

John Hutchinson (1884-1972) was a British botanist associated with the Royal Botanic Gardens, Kew, England who also served as keeper of Kew herbarium for many years. Hutchinson first proposed his classification of angiosperms in his book The Families of Flowering Plants, the first volume on Dicotyledons appearing in 1926 and the second on Monocotyledons in 1934. The classification was revised periodically, second edition in 1959 and the 3rd in 1973, one year after his demise.

Hutchinson also embarked upon an ambitious plan of revising Genera plantarum of Bentham and Hooker under the title The Genera of Flowering Plants. Unfortunately he could complete only 2 volumes of this work, published in 1964 and 1967, the project cut short by his demise.

The classification system of Hutchinson dealt only with the flowering plants, included under Phylum Angiospermae as distinct from Phylum Gymnospermae. The classification was based on 24 principles including General principles, Relating to General Habit, Relating to General Structure of Flowering plants and those Relating to Flowers and Fruits. Some of the principles are outlined below:

1. Evolution is both upwards (sympetaly,epigyny) and downwards (apetaly,unisexuality). 2. Evolution does not necessarily involve all the organs of a plant at the same time; and one organ or set of organs may be advancing while the other may be stationary or retrograding. 3. Evolution has generally been consistent and when a particular progression or retrogression has set in, it is persisted into the end of the phylum. 4. In certain groups, trees and shrubs are probably more primitive than herbs. 5. Perennials are older than biennials, and from them annuals have been derived. 6. As a rule, simple leaves precede compound leaves. 7. Bisexual flowers precede unisexual flowers, and the dioecious is probably more recent than the monoecious condition. 8. Solitary flower is more primitive than the inflorescence.

Following Bessey, Hutchinson considered flowering plants to be monophyletic, having evolved from the hypothetical cycadeoid ancestral group which he gave the name of Proangiosperms. He recognized a number of smaller groups, bound together by a combination of characters. He established Magnoliales as an order distinct from Ranales, as he considered them to have evolved on parallel lines. Hutchinson regarded Magnoliaceae as the most primitive family of the living angiosperms. He considered Dicotyledones to be more primitive and placed them before Monocotyledones, giving them a rank of Subphylum.

Whereas Hutchinson considered the woody habit to be primitive in dicots, in monocots he considered the herbaceous habit to be primitive, and the woody forms derived from the herbaceous forms. He also considered Monocotyledones also to be a monophyletic group derived from Ranales, Butomales having a link with Helleboraceae and Alismatales with Ranunculaceae.

Merits The system of Hutchinson, being based on a number of sound phylogenetic principles, and studies of a large number of plants at his disposal at Kew, shows the following improvements over earlier systems: 1. The system is more phylogenetic than that of Engler and Prantl, as it is based on phylogenetic principles, generally recognized by most authors. 2. The treatment of Magnoliales as the starting point in the evolutionary series of Dicotyledones is in agreement with prevalent views 3. Many large unnatural families have been split into smaller natural ones.Euphorbiaceae of Bentham and Hooker is split into Euphorbiaceae, Ricinaceae and Buxaceae. The family Urticaceae is similarly split into Urticaceae, Moraceae, Ulmaceae and Cannabinaceae. 4. Standards of description are very high.Useful keys are provided for the identification of families. 5. Phylograms for dicots and monocots are more superior to the Besseyan cactus. 6. The classification of Monocotyledones is sounder and generally appreciated, even keys to the identification of genera have been provided. 7. The derivation of Monocotyledones from Dicotyledones is widely agreed.

Demerits The classification of Hutchinson has largely been ignored, as it mostly did not proceed beyond family level, and gave much importance to the habit. The major drawbacks of the system are listed below: 1. The system is not useful for practical identification, arrangement in Floras and herbaria, as it does not proceed beyond the family level in the greater majority of taxa. 2. Hutchinson did not provide a full explanation for the majority of his evolutionary concepts. 3. He derives angiosperms from proangiosperms, but does not provide information about the nature of this hypothetical ancestral group. 4. The family Calycanthaceae is related to Laurales, but placed here in Rosales. 5. Hutchinson regards Magnoliaceae as the most primitive family of living Dicotyledones, but most contemporary authors consider vessel-less Winteraceae, or paleoherbs be the most primitive. 6. The monocotyledons are placed after dicotyledons, whereas the recent classifications place them between primitive angiosperms and the eudicots. 7. Family Liliaceae of Hutchinson is a large unnatural assemblage, which has been split into several smaller monophyleticfamilies like Liliaceae, Alliaceae, Asparagaceae, Asphodelaceae in the recent classification of Judd et al.(2002), APG II (2003) and Thorne (2003).

Arthur Cronquist

Arthur Cronquist (1919-1992), a leading American taxonomist, associated with the New York Botanical Garden produced a broad classification of Embryobionta along with Takhtajan and Zimmerman (1966). He produced a detailed classification of angiosperms in 1968 in his book The Evolution and Classification of Flowering Plants. The classification was further elaborated in 1981 in his book An Integrated System of Classification of Flowering Plants. The final revision was published in the second edition (1988) of The Evolution and Classification of Flowering Plants. Some realignment in Dicotyledons was published in Nordic Journal of Botany in 1983.

Cronquist gave more importance to morphology

Following Takhtajan, the angiosperms are given the name Magnoliophyta and divided into Magnoliopsida (dicots) and Liliopsida (monocots). Cronquist includes only six subclasses in dicots and recognizes five in monocots. In dicotyledons, the Ranunculidae of Takhtajan are merged with Magnoliidae and Lamiidae are not given a separate rank at subclass level, but retained in Asteridae.

Cronquist recognizes 83 orders and 386 families. Cronquist agrees with Thorne (earlier versions up to 1992) in keeping the family Winteraceae (and not Degeneriaceae as done by Takhtajan) at the beginning of dicotyledons, and included alongwith Degeneriaceae, Magnoliaceae, Annonaceae etc. in the same order Magnoliales.

The relationships of various subclasses and orders are shown with the help of a phylogram which takes the form of a bubble diagram, like other contemporary systems of classification.

Merits

1. The revision of the classification in 1981 and 1988 was presented a in comprehensive form, giving detailed information on phytochemistry, anatomy, ultrastructure and chromosomes besides morphology.

2. The text, being in English, has been readily adopted in books and floristic projects originating in the USA.

3. The system is highly phylogenetic and is based on now largely accepted phylogenetic principles. 4. The placement of Winteraceae at the beginning of dicotyledons is generally favoured by most authors including Ehrendorfer (1968), Gottsberger (1974) and Thorne (up to 1992).

5.Abolition of artificial group names such as Polypetalae, Gamopetalae, Lignosae, Herbaceae etc. has resulted in more natural grouping of taxa.Verbenaceae and Lamiaceae are thus brought under the order Lamiales.

6.Nomenclature is in accordance with the International Code of Botanical Nomenclature.

7. Placement of Magnoliidae as the most primitive group of angiosperms, dicotyledons before monocotyledons, Magnoliales at the beginning of Magnoliidae and Butomaceae at the beginning of Liliopsida, finds general agreement with other authors.

Demerits

1. In spite of being a highly phylogenetic and popular in the USA, the system is not very useful for identification and adoption in herbaria since identification keys for genera, their distribution and description are not provided. 2. Superorder, as a rank above the order is not recognized, thus showing a significant departure from the contemporary systems of Takhtajan,Thorne and Dahlgren 3. The monocotyledons are placed after dicotyledons, whereas the recent classification place them between primitive angiosperms and the eudicots. 4. Cronquist (1988) recognized Physenaceae as a family under Order Urticales, but was not sure about its exact placement.

APG Classification

There has been a considerable revival of the cladistic concepts with the utilization of molecular data and development of powerful tools of data handling. During the last decade, concept has developed into APG classification by collaborative efforts of a group of dedicated workers of ‘Angiosperm Phylogeny Group’ (K. Bremer, A. Backlund, B. Briggs, B. Bremer, M. W. Chase, M. H. G. Gustafsson, S. B. Judd, F. A. Kellogg, P. F. Stevens, M. Thulin and several others), who published a classification of 462 families of Angiosperms in 1998. These families were grouped into 40 putative monophyletic orders under a small number of informal monophyletic higher groups: monocots, commelinoids, eudicots, core eudicots, rosids, eurosids I, eurosids II, asterids, euasterids I and euasterids II.

A recent revision of APG (APG II, 2003), and continuous upgradation on Angiosperm Phylogeny website (APweb) by P. F. Stevens http://www.mobot.org/ MOBOT/research/APweb/, have resulted in considerable refinement in the APG scheme, with more and more families (and some orders) coming out of the list of unplaced taxa. The classification recognizes 45 orders of Angiosperms, of which 44 are placed in 11 informal groups, considered more or less monophyletic. One order is unassigned at the beginning of Angiosperms. A total of 457 families are recognized. The number of unplaced families has been drastically reduced in recents treatments of Stevens (2008) and Judd et al. (2008). Thorne does not belong to the Angiosperm Phylogeny Group, but has kept pace with the recent molecular developments, and is trying to balance hierarchical classification with the concept of monophyly, so sacred with the Angiosperm Phylogeny Group.

Merits

1. The system is based on the sound phylogenetic principle of constructing taxa on the basis of established monophyly.

2. The system is based on a synthesis of information from mainly morphology, anatomy, embryology, phytochemistry and more strongly on molecular studies.

3. The traditional division of angiosperms has been abandoned and various monocot taxa placed in between primitive angiosperms and eudicots, thus overcoming the problem of paraphyly in the earlier recognized two groups monocot and dicots.

4. A number of cladograms are being presented for general affinities between various groups of angiosperms based on molecular as also on information from other fields. 5. Winteraceae and Canellaceae are brought together under the same order. Their affinities are strongly supported by morphological studies and multigene analyses.

6. The number of unplaced families in various informal groups and uncertain families towards the end have been sufficiently reduced in APG II and APweb, finding ordinal places for many unplaced families of APG (1998).

Demerits

1. Classification having not proceeded below the family level, the system is not useful in practice and for adoption in herbaria and floras.

2. Although a large number of families have been assembled into more or less monophyletic orders, there still exists a large number of unplaced families, and a few unplaced genera in both APG II and APweb.

3. Although most of the orders have been assembled into informal groups, no proper names conforming to the Botanical Code have been given for these groups.

4. Although APG II places all presumed primitive families of angiosperms before monocots, APweb transfers the Magniliids to a position after monocots and commelinids.

5. Angiosperms have been given therank of a division, but there are no formal taxa between the rank of an order and division, a rather unusual phenomenon for classification systems.

6. Family Capparaceae has been merged with Brassicaceae, but the Chloroplast sequence data points to the separation of these two families as also Cleomacae. Thorne (2006) recognizes Brassicaceae, Capparaceae and Cleomaceae as distinct families