Principles of Classification: While developing a system of classification of organisms, certain basic principles are observed. Some of these are as follows:
Morphology: Morphology forms the primary basis for classifying organisms into various taxonomic groups or taxa. In earlier artificial systems, only a few morphological characters were taken into consideration. The plant classification system proposed by Linnaeus was based mainly on the characteristics of stamens and carpels. The similarities in the morphological characters are used for grouping the plants together. Plant groups with greater differences are considered to be unrelated or distantly related. All flowering plants with ovules enclosed in an ovary cavity are grouped together as Angiosperms whereas, the angiosperms are further classified into two classes: Dicotyledons and Monocotyledons, on the basis of differences of the characters of root system, leaf venation, flower symmetry and number of cotyledons in the embryo.
Phylogenetic considerations: In recent systems of classification of plants, a greater emphasis is given on the phylogenetic arrangement of plant groups, an arrangement which is based on the evolutionary sequence of the plant groups.
Chemical taxonomy or chemotaxonomy: Chemotaxonomy is the application of phytochemical data to the problems of systematic botany. Nearly 33 different groups of chemical compounds in plants have been found to be of taxonomic significance.
Numerical taxonomy: Application of numerical methods and data in the classification of taxonomic units is called numerical taxonomy. Edgar Anderson in 1949 was the first to make use of numerical taxonomy in the classification of flowering plants. It involves quantitative estimation of taxonomic characters from all parts of the plant from all stages of its life cycle. The numerical data thus collected for various plant groups is tabulated systematically using computers.
Conclusion: Classification is essential for the proper study and easy reference of life forms. Systematics deals with identification, nomenclature and taxonomic classification of organisms. In the systematic classification of organisms, various taxa are arranged in the descending order of their taxonomic categories as per the taxonomic hierarchy. Modern taxonomy makes use of the data from all branches of botany, genetics, cytology, ecology, chemotaxonomy, and numerical taxonomy in order to develop a phylogenetic system of classification of plants.