An amazing diversity and number of types of organisms are present on earth. Taxonomy is the field of biology that deals with these organisms to classify this diversity.

History of Angiosperm Classification Many systems of classification of plants have been proposed by various philosophers, herbalists and botanists based on different criteria,

• Artificial System

These systems of classification were based on one or few morphological characters. Many botanists used this system and classified angiosperms into different classes. Few of these botanists are a. Theophrastus (370 -285 B.C.) a Greek botanist, known as the father of botany considered habit as an essential character. And classified plants into four groups on the basis of their habits and separated according to flowering and non-flowering. ● herbs ● subshrubs ● shrubs ● trees b. Albertus Magnus (1200-1280 A.D.) produced a classification system that recognized for the first-time monocots and dicots. c. Carolus Linnaeus (1707–1778) was an 18th century Swedish botanist and physician. He classified plants and animals according to similarities in form and divided living things into two main kingdoms namely plant and animal kingdoms. He named the plants and animals in Latin or used latinised names in his books Species Plantarum (1753) and Systema Naturae (1758). The ​ two-kingdom classification system devised by Linnaeus is not used today. However, one of Linnaeus more appreciable systems was the system of naming organisms called binomial

nomenclature. This Swedish taxonomist known as father of taxonomy, created a sexual system of classification that divided plants into 24 classes based in large part on the number, union, and length of stamens. Secondary grouping with these classes (Order) was based on the gynoecium mostly the number of styles.

While the artificial approach allowed quick sorting and identification, its application produced 'unnatural' groupings. The next step along the path of systematizing flowering plants involved an effort, which progressed through the 1700s and first half of the 19th century to employ as many characters as needed to ensure that natural patterns of variation were reflected by the classification system.

• Natural system

Michel Adanson was the first scientist to reject all the artificial systems and support the natural systems of classification. These systems the plants were classified on the basis of their natural affinities (i.e. the basic similarities in the morphology) rather than on a character for determining the affinities. Compared to the artificial systems, it was based on the proper utilization of all facts and figures available in nature. In this system, the plants were grouped and placed into different taxa like classes, orders, families, and genera.

A large number of morphological characters were taken into consideration, in the natural systems of classification . A natural system of classification is based on overall resemblances. It was recognized that many plants that look alike share suites of morphological features (leaves, flowers, fruits) and, therefore, should be grouped together. Because, these similarities indicate their relationships. On the other hand, differences or dissimilarities of characters are used for separating the plant groups from each other. Plant groups with greater differences are considered to be unrelated or distantly related.

The main demerit of this system was that the classification was not based on evolutionary relationships. Different families had been placed in specific groups which do not show any evolutionary relationships.

a. The most important natural system of classification of seed plants was proposed by two British botanists, George Bentham (1800-1884) and Sir Joseph D. Hooker (1817-1911), in Genera Plantarum (1862-1883), recognized 97,205 species belonging to 7,569 genera of families of flowering plants.

b. August Wilhelm Eichler, (1839- 1887) was a German botanist that modified former systems to reflect a better relationship between plants. The plant kingdom was Eichler System is the first one in which the concept of Evolution was accepted and divided into non-seed (Cryptogamae) and seed plants (Phanerogamae) by him. therefore it was also the first one to be considered phylogenetic. Moreover Eichler was the first taxonomist to separate Phanerogamae in Angiosperms and Gymnosperms and the former in Monocotyledonae and Dicotyledonae.

• Phylogenetic system

These classification systems came up after Darwin’s theory of evolution was proposed and widely accepted. This system is based on evolutionary sequence as well as genetic relationships among different groups of plants. In addition to this, it employs as many taxonomic characters as possible. a. Concepts of natural selection and lineage relationships present in Origin of Species, published in 1859 by Charles Darwin had given enough stimulus for the creation of phylogenetic system of classification. b. Adolf Engler (1844-1930) and Karl Prantl (1849-1893) were two German botanists who published their work in the form of a book called Die Naturalichen Pflanzenfamilien, (The Natural Plant families) a work that appeared in 23 volumes during 1887-99. It is an evolutionary system proposed subsequent to the acceptance of Darwins Theory of evolution, based on the assumption that the flowering plants where perianth is absent are evolutionary primitive. American and European scientists were adopted Engler and Prantl’s system but not British scientists. These were following Bentham and Hooder’s system in the light of modifications produced by Bessey. It is based on Eicher’s system.

Families and orders arranged based on the complexity of floral morphology. Characters like a perianth with one whorl, unisexual flowers and pollination by wind were considered primitive as compared to perianth with two whorls, bisexual flowers and pollination by insects. They dealt with the primitive groups as well. It is in line with Adolphe-Théodore Brongniart's 1843 work. The Plant Kingdom is divided into 14 major divisions. The first 13 divisions cover algae, fungi, bryophytes and pteridophytes. The 14th division is named Embryophyta Siphonogama. It is divided into two subdivisions : Gymnospermae (Cycads and Conifers) and Angiospermae (flowering plants).

c. Charles Edwin Bessey (1845-1915), American, published The Phylogenetic Taxonomy of Flowering Plants (1915), Charles Edwin Bessey (1845-1915) made his Bessey system, with focus on the evolutionary divergence of primitive forms. The systems based on various 28 guiding rules, or “dicta”, to determine level of being, simple or advanced, of a group of plants. It is considered by many as the system most likely to form the basis of a modern, comprehensive taxonomy of the plant kingdom. Here, he considered Spermatophyta as having had polyphyletic origin, being composed by three different phyla, of which he treated only Anthophyta. Full in line with Richard Wettstein ideas. d. John Hutchinson (1884-1972), Families of Flowering Plants, Genera of Flowering Plants:-John Hutchinson proposed a radical revision of the angiosperm classification system devised by Hooker and by Engler and Prantl that had become widely accepted during the 20th century. At its simplest, his system suggested two main divisions of angiosperms, herbaceous and woody.

Modern Systems of Classification

Phylogeny, Cladistics & Cladogram

Modern taxonomy is based on many hypotheses' of the evolutionary history of organisms, known as phylogeny. As with the Scientific Method, scientists develop a hypothesis on the history of an animal and utilise modern science and technology to prove the phylogeny.

Cladistics is a classification system which is based on phylogeny. Expanding on phylogeny, cladistics is based on the assumption that each group of related species has one common ancestor and would therefore retain some ancestral characteristics. Moreover, as these related species evolve and diverge from their common ancestor, they would develop unique characteristics. Such characteristics are known as derived characteristics

The principles of phylogeny and cladistics can be expressed visually as a cladogram, a branching diagram which acts as a family (phylogenetic) tree for similar species. A cladogram can also be used to test alternative hypotheses for an animal's phylogeny. In order to determine the most likely cladogram, the derived characteristics of similar species are matched and analysed.

Cronquist (1981, 1988) used 3 major superfamilial ranks within Magnoliophyta, the flowering plants : class, subclass, and order. In his system, the dicotyledons were placed in Magnoliopsida, the monocotyledons in Liliopsida. Within Magnoliopsida, he distinguished 6 subclasses and 64 orders; in Liliopsida, 5 subclasses and 19 orders. A total of 389 families were accepted, 323 of them dicots and 66 monocots.

A.L. Takhtajan (1987) used 4 major superfamilial ranks: class, subclass, superorder, and order. Eight subclasses of dicotyledons were distinguished and 4 among the monocots. Within the dicots, Takhtajan accepted 36 superorders, 128 orders, and 429 families; within the monocots, he had 16 superorders, 36 orders, and 104 families. Recently Takhtajan (1993) has acepted 450 dicot families arranged among 145 orders, 45 superorders, and 10 subclasses. R.F. Thorne (1992) currently recognizes 437 families (351 dicots) distributed among 28 superorders (19 dicots) and 71 orders (52 dicots).

Angiosperm Phylogeny Group system

A new formal code of nomenclature is the PhyloCode (1988), were started at Harvard University in 1988. It is now development by The International Society for Phylogenetic Nomenclature. It have led to a modern system of plant taxonomy, the APG system of plant classification was published in 1998 by the Angiosperm Phylogeny Group. It was superseded in 2003 by a revision, the APG II system, and then in 2009 by a further revision, the APG III system. It is the first taxonomic work, done by a larger group of systematic botanists, worldwide. It were published in

Annals of the Missouri Botanical Garden 85: 531–553. W.S. Judd & al. (1999; 2nd edition, 2002), with the title: Plant systematics: a phylogenetic approach. The system is unusual in being based, not on total evidence, but on the cladistic analysis of the DNA sequences of three genes, two chloroplast genes and one gene coding for ribosomes.

APG I (1998) "system -An ordinal classification for the families of flowering plants", The initial 1998 paper by the APG made angiosperms the first large group of organisms to be systematically re-classified primarily on the basis of genetic characteristics.

APG II (2003)

The second paper published by the APG presents an update to the original classification of 1998. The authors say that changes have been proposed only when there is "substantial new evidence" which supports them. The proposed classification continues the tradition of seeking broad circumscriptions of taxa, for example trying to place small families containing only one genus in a larger group.

APG III (2009)

The third paper from the APG updates the system described in the 2003 paper. The broad outline of the system remains unchanged, but the number of previously unplaced families and genera is significantly reduced. This requires the recognition of both new orders and new families compared to the previous classification.APG III recognises all of the 45 orders of the previous system, as well as 14 new ones. in the eudicots, these the result of much more clearly understood relationships relative to APG II (2003).

APG IV (2016)

An update of the Angiosperm Phylogeny Group (APG) classification of the orders and families of angiosperms is presented. Several new orders are recognized. This brings the total number of orders and families recognized in the APG system to 64 and 416, respectively. Two additional informal major clades, superrosids and superasterids, that each comprise the additional orders are included in the larger clades dominated by the rosids and asterids.

Credit Notes https://www.peoi.org/Courses/Coursesen/bot/temp/bot19t38.html https://www.siyavula.com/read/science/grade-10-lifesciences/biodiversity-and-classification/09- biodiversity-and-classification-03 https://en.wikibooks.org/wiki/General_Biology/Classification_of_Living_Things/Classification_ and_Domains_of_Life

Student Activity

Learn:

● Give an account of different systems of classifications in plants