The term ‘taxonomy’ was introduced by De Candolle in the year 1813 A.D. through his “Theorie Elementarie de la Botanique”. Taxonomy can be defined as a functional science which is guided by principles that have developed with the increase in knowledge of the plants. The first scientist was Carolous Linnaeus who formulated certain principles of taxonomy in his book Critica Botanica in the year 1737 A.D. Others were Adanson (1727 A.D.-1806 A.D.); De Candolle (1778 A.D.-1841 A.D.), John Lindley (1799 A.D.-1865 A.D.) and Sir Joseph Dalton Hooker (1817 A.D.-1912 A.D.). The phylogenetic systems of Bessey (1915 A.D.) and Hutchinson (1926 A.D.) are based on modern principles.
Taxonomy as a descriptive science started mainly with the works of Tournefort (1665 A.D.-1708 A.D.), De Jussieu (1699 A.D.-1771 A.D.), Carolous Linnaeus (1707 A.D.-1778 A.D.). Most of the taxonomic works of this period were based on the observations of the similarities and dissimilarities of gross morphological characters of the plants. Plants were described and classified on the basis of these characters. With the progress in research and added knowledge, taxonomists classify the plants in such a way that give us an idea about the sequence of their evolution from simpler, earlier and more primitive types to complex, recent and more advanced types.
The system of classification proposed by Takhtajan in 1967 A.D. is mainly based on modern principles such as evolutionary sequence, genetic relationships, fossil records, morphological, cytological, physiological, anatomical, embryological, biochemistry and pollen grain characters. While developing a system of classification, the general basic principles taken into consideration are given below:
- Morphological Criteria: Ever since the beginning, morphological characters of plants were taken into consideration at the time of classification. Earlier artificial systems were based on only one or a few morphological characters. Even today, the fact cannot be denied that morphological characters have their own importance in taxonomy and all systems of classifications suggested are based on the principles of morphology. Some of the vegetative characters such as annual, biennial and perennial duration of plants, kinds of root, nature of the stem, leaf arrangement, leaf venation, presence and absence of stipules are taken into consideration. But more emphasis is laid on the floral characters because they are less affected by the change in environment. The common floral characters are presence and absence of calyx and corolla, aestivation of calyx and corolla, fused or free condition of corolla, number, arrangement and fusion of androecium and gynoecium, ovary with its placentation etc.
- Anatomical Criteria: Anatomical characters such as arrangement of vascular bundles, orientation of xylem and phloem in the bundles, nature and origin of secondary tissue and leaf trace development are used in solving taxonomic problems. These criteria are taken into consideration for the categorization in classification. It is observed that the same group of plants show close relation between the morphological characters and anatomical structures. For example, the arrangement of vascular bundle in a ring is common among the dicot and scattered vascular bundle among the monocot.
- Phylogenetic Criteria: The modern system of classification is mainly based on the phylogenetic criteria. Phylogeny may be defined as the study of the evolutionary sequences or history of organisms in different periods of the earth’s history. These systems also reflect on the genetic relationships of the plants. Engler and Prantl (1887 A.D.-1899 A.D.), system of classification can be considered as first phylogenetic system of classification. Others are Bessey, Hutchinson, Cronquist and Takhtajan.
- Palaeobotanical Criteria: Palaeobotany is the study of fossil plants. Critical examination of fossil plants help to solve taxonomic problems as well as it gives clear idea about the evolutionary relationship. Thus, palaeobotany made phylogenetic basis for classification.
- Cytotaxonomy: It is the utilization of cytological characters for the elucidation of taxonomic problems. Cytological characters such as chromosome number, shape, size and behavior during cell division are used in solving taxonomic problems.
- Chemotaxonomy: Chemotaxonomy or chemical taxonomy is defined as the application of chemical data in solving taxonomic problems. The presence and distribution of various chemical compounds in plants serve as taxonomic evidences. Although, several chemical compounds are found in plant body, only thirty three different groups of chemical compounds provide valuable taxonomic clues.
- Numerical Taxonomy: Numerical taxonomy can be defined as the grouping of taxonomic units by the application of numerical methods (data). In the classification of flowering plants, numerical taxonomy was at first introduced by Edger Anderson (1949 A.D.). The main objective of numerical taxonomy is to clarify and illustrate degrees of relationship or similarity in an objective and repeatable manner. High speed computers are used for this purpose.