Classification is the system or arrangement of plants into groups and groups into different levels of categories in a sequential manner on the basis of their origin, relationship and structural similarities. Different systems of plant classifications are given below.
(i) Artificial System:
Only one or a few easily observable characters are taken arbitrarily for classification of plants. This is because artificial system which is the earliest system of classifying plants and very little was known about plants by that time.
Carlous Linnaeus (1707-78) adopted an artificial system in his book. Species Plantarum. He classified the plant kingdom into 24 classes.
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The first 23 classes dealing with the flowering plants and the last class with non-flowering plants. The flowering plants were classified on the basis of number, length and fusion of stamens, such as Monandria, Diandria, Didyanamia, Monadelphia, Syngenesia, etc. The classes were divided into several orders on the basis of pistils present in the flower, e.g. Monogyna, Digyna, etc. Linnaean system was immediately accepted because of its simpleness but was then replaced by natural system of classification due to the demerits like criteria used were arbitrary and based on random morphological characters, many closely related plants were placed in different groups and vice-versa.
(ii) Natural System:
This system is based on natural relationship between the plants and permanently retained sexual and structural characters. According to similarities and differences, the plants are placed in bigger groups like division which are subsequently divided and subdivided into smaller groups like subdivisions, classes, families , genera and ultimately species.
Gregor Bentham and Sir J.D. Hooker proposed the most important natural system of classification in their monumental work Genera Plantarum (1862-1883).
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Phanerogams (seed plants) were classified into classes: Dicotyledones, Gymnospermae and Monocotyledones. Dicotyledones were divided into sub-classes: polypetalae (petals free), gametopetalae (petals united) and monochlamydeae (sepals and petals not differentiated). These sub-classes and monocotyledons were divided into different series. The series were further divided into cohorts (=orders) and cohorts into orders (families). Though this system has tremendous practical utility and is better than an artificial system, it is based on a single character and is not at all phylogenetic.
(iii) Phylogenetic System:
This system is based on morphological characters, natural affinities and evolutionary and genetic relationships of the plants. Some of the important so-called phylogenetic systems are Engler and Prantl’ surrounded system, Hutchinson’s system and Takhtajan system.