Taxonomy is concerned with the laws governing the classification of plants. The term taxonomy includes two Greek words taxis - arrangement and nomos- laws. Plant taxonomy is otherwise known as systematic botany. Classification, identification, description and naming the plants are the bases of plant taxonomy. The taxonomic knowledge about the plants is based on their form and structure. The knowledge gained through taxonomy is useful in the fields of medicine, agriculture, forestry, etc.
The ultimate aim of classification is to arrange plants in an orderly sequence based upon their similarities. The closely related plants are kept within a group and unrelated plants are kept far apart in separate groups.
The other aim of classification is to establish phylogenetic relationships among the different groups of plants. The plants that are closely related show more similarities than differences.
The earliest systems of classification were simple and based on one or few characters. They gave importance to vegetative characters. The later systems of classification gave more importance to floral characters because floral characters are more stable and permanent.
The different types of classification proposed by earlier taxonomists can be broadly categorized into three systems- artificial, natural and phylogenetic.
It was based on one or at most only a few superficial characters. In 1753, Carolus Linnaeus of Sweden published his book ' Species Plantarum' wherein he described 7,300 species. He divided the plants into 24 classes based on number, union, length and certain other characters of stamens. Hence, this system is also known as sexual system of classification. In those days, it was an important over other systems of classification. The importance of floral characters was felt by Linnaeus and his classification was more important than others. The main defect of this system is that totally unrelated plants are brought together in a single group and those that are closely related plants are placed in widely separated groups. For example, plants belonging to Zingiberaceae of Monocotyledons and that of Anacardiaceae of Dicotyledons had been placed in one group called Monandria, as these possess only one stamen. Another defect of this system was that no importance was given to either natural or phylogenetic relationships among different groups of plants.
In this system of classification, plants are classified based on their natural affinities. More number of characters are taken into consideration in this system. It is mainly based on all the informations that were available during the time of direct observation of plants. The most important natural system of classification of seed plants was proposed by two British botanists George Bentham and Sir Joseph Dalton Hooker. It helps to determine the relationships between various groups of plants. However, it does not attempt to bring out evolutionary relationships among different groups of plants.
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. Charles Darwin's concept of Origin of Species had given enough stimulus for the creation of phylogenetic system of classification. Adolf Engler (1844-1930) and Karl Prantl (1849-1893) of Germany published a phylogenetic system in their monograph on 'Die Naturlichen Pflanzen Familien'. In this system, floral characters such as single whorl of perianth or no perianth and unisexual flowers pollinated by wind were considered as primitive characters when compared to perianth with two whorls, bisexual flowers pollinated by insects. According to them, members of Asteraceae of dicotyledons and Orchidaceae of monocotyledons were highly advanced.