Angiosperms or the flowering plants exhibit wide diversity and variation with a vast range of size and structure.
The number of ways in which angiosperms can be classified depends upon the number of differentiating features & their characters.
During the history of taxonomic botany, numerous systems of classification have been devised.
The system of classification is a particular way of delimiting and arranging taxonomic groups. A framework has been devised for the purpose of scientific classification of plants which is termed as taxonomic hierarchy.
Broadly there are four system of classification of flowering plants: artificial system, natural system, phylogenetic system and modem system.
1. Artificial Systems:
It is considered the earliest system of classification. In artificial system, plants are classified on the basis of only one, or a very few, specially chosen characters. Theophrastus (370-285 B.C.), divided the plants on the basis of form and texture – Herbs, Shrubs, Under shrubs and Trees. He gave names and description to about 480 plants in his book ‘Historia plantarum’ which is the oldest botanical work in existence.
Andrea Caesalpino (1519-1603), an Italian botanist divided plants into two major groups based on woody and herbaceous habit. John Ray (1628- 1705), a British botanist divided plants into Imperfectae (non- flowering) and Perfectea (seed- plants). Carolus Linnaeus (1707- 1778), regarded as the “Father of Systematic Botany”‘, published artificial system based exclusively on floral characters.
Linnaeus sexual system based on floral characters is the most advanced of artificial classification and became very popular Artificial system of classification is relatively simple. It contains only limited information about its members and hence has little or no predictive value.
2. Natural Systems:
In natural system of classification all the important characters are taken into consideration and the plants are classified according to their related characters. The first such classification was given by A.L. de Jussieu (1748-1836). A.P. de Candolle (1778-1874), a French botanist gave a new system of classification of plants based on characteristics of vascular tissues.
However, the most important system of classification was given by Bentham and Hooker (1862- 1883), which is based on the concepts of A.P. de Candole & A.L. de Jussieu. George Bentham (1800-1894) and J. D. Hooker (1817-1911) gave a very comprehensive system of classification in their book ‘Genera Plantarum’. Although it is called natural system by their contemporaries but they are sometimes referred to as formal systems.
Bentham and Hooker’s system of classification is regarded as the most convenient and suitable system of classification because of its practical utility. Bentham and Hooker’s system of classification of angiosperms is summarized in the Table -1.1
It is important to mention that natural system of plant classification came into being before the idea of evolution was accepted in biology. The theoretical bases of classification were, therefore, unclear and vague.
3. Phylogenetic Systems:
The publication of Darwin’s origin of species changed the outlook of taxonomy. A new system of classification evolved. It classifies plants based on the course of evolutionary descent of its members. This system establishes the genetic and ancestral relationship.
The most widely used classification of phylogenetic systems is that of Adolf Engler (1844-1930) and Karl Prantl (1849-1893) and derived from that of Eichler (1883). It was designed to demonstrate a series of evolutionary advancements from simple to more complex forms. Like Bentham and Hooker, Engler and Prantl accepted the primary division of the flowering plants into Dicotyledones and Monocotyledones.
Division I. Gymnospermae – divided into 7 classes- Cycadofilicales, Cycadales, Bennettitales, Ginkgoales, Coniferales, Cordaitales, Gnetales.
Division 2. Angiospermae- Divided into 2 classes:
Classes I. Monocotyledoneae- It includes 11 orders and 45 families.
Classes II. Dicotyledoneae- divided into 2 sub- classes:
Sub- class 1. Archichlamydeae- It includes 33 orders and 201 families.
Sub-Class 2. Metachlamydeae- It includes 11 orders and 60 families.
4. Modern Systems:
Modem system of classification is based on all the available information about the plants. In practice it is a mixture of artificial, natural and phylogenetic systems of classification. A number of modem, systems have been proposed, of which the best known and most accepted are those of Takhtajan (1967) documented in ‘Flowering plants: Origin and Dispersal’. Takhtajan considered angiosperms to moniphyletic and that they arose from some very ancient groups of gymnosperms.
The Takhatajan’s system of classification is summarized as follows:
Takhtajan’s system –
I. Class Magnoliatae (Dicotyledons) – divided into 7 sub-classes:
II. Class Liliatae (Monocotyledons) – divided into 4 sub-classes:
Thus, plant classification is made possible by the discontinuous nature of variation in the plant kingdom which is a result of evolution and extinction during the long course of geological time. The basic process in classification is identification, which is a comparison of features & characteristics and deriving valid conclusions utilizing information derived from biological investigation such as key, herbaria etc. So, by classification we can organize and summarize our knowledge of the plant kingdom in a convenient manner.