The following points highlight the top three types of system of plant classification. The types are: 1. Artificial Classification 2. Natural Classification 3. Phylogenetic Classification.
Plant Classification: Type # 1.
The earliest systems of classification which remained dominant from 300 B.C. up to about 1830 were artificial systems, which were based on one or a few easily observable characters of plants, such as habit (trees, shrubs, herbs, etc.) or floral characters (particularly the number of stamens and carpels).
Such types of classification using some arbitrary or at least easily observable characters, often irrespective of their affinity, is called artificial.
The sexual system of Linnaeus is a good example of artificial classification, which uses only one attribute i.e. the number of stamens for grouping plants into 24 Classes as a result of which, various unrelated taxa, which are not at all related but, similar in one respect only, have been placed under the same Class.
Plant Classification: Type # 2.
These systems of classifications are based upon overall resemblances, mostly in gross morphology, thus, utilizing as many taxonomic characters as possible, to group taxa.
Charles Darwin’s proposed theory of evolution (1859) postulates that, the present day plants have descended from those existing in the ancient past, through a series of modifications in response to changing environmental conditions, which means that all present day plants are related to each other in one way or another.
Thus, the closely related plants should naturally be grouped together. This is called natural classification. Thus, larger the number of characters shared by different taxa, the more closely related they are to each other. This is the basis of modern classification.
Plant Classification: Type # 3.
The classification systems proposed after Darwin’s theory are mostly phylogenetic i.e. they use as many taxonomic characters as possible in addition to the phylogenetic (evolutionary) interpretations. These are expressed in the form of phylogenetic trees or shrubs showing presumed evolution of the groups.
The natural systems are two-dimensional i.e. based on the data available at any time and is known as Horizontal Classification, whereas the addition of the third dimension i.e. past history or ancestral history results in phylogenetic classification also known as Vertical Classification or Evolutionary Classification.
According to Radford (1986) however there are four systems of classifications:
(a) Artificial Classifications — These systems use the habit and importance to man as taxonomic characters e.g. systems of Theophrastus, Dioscorides, etc.
(b) Mechanical Classifications — These systems use one or a few selected taxonomic characters to group taxa e.g. systems of Caesalpino, Bauhin, Ray, Tounefort and Linnaeus.
(c) Natural Classifications.
(d) Phylogenetic Classifications.