In this article we will discuss about the Meaning and Types of Plant Classifications.
Meaning of Plant Classification:
Plant Classification is the arrangement of plants into groups and categories for a clear understanding, proper study and effective organization. According to Radford (1986) “classification is the arrangement of groups of plants with particular circumscriptions by rank and position according to artificial criteria, phenetic similarities, or phylogenetic relationships”.
The early civilization of India, Egypt and China who had a fair knowledge of plants used both as food as well as medicinal purposes, also practiced a sort of plants taxonomy.
Vrikshayurveda compiled by parasara before the beginning of the christian era is one of the earliest Indian works, which deals with plants in a scientific manner and follows a classification largely based on comparative morphology of plants, and it was considered to be more advanced than the one developed in Europe before 18th century.
Several families known as ganas are clearly distinguished in Vrikshayurveda, which are easily recognizable even today. As for example, it mentions the flowers of Samiganyam as bypogynous with a gameosepalous calyx, five petals of different sizes and the fruit a legume with the seeds on the side. This evidently indicates to the family Leguminosae.
A plant classification system has predictive value and provides an index to information storage and retrieval on that plant. Since systematics today is very much a reflection of the past, a historical review revealing the historical period and the technologies available during the historical periods needs to be summarized.
Types of Plant Classification:
A. Evolutionary Classifications:
(i) Charles Darwin (1809-1882):
His famous work includes On the Origin of Species by Means of Natural Selection (1859). Darwin was strongly influenced by observations during the voyage of the H.M.S. Beagle to the Galapagos Islands. His observation on the differences in animals (e.g. fishes) and plants on different islands prompted his mechanism for evolutionary change, i.e. natural selection.
Darwin thought then that the earth was much older than 6000 years and that evolutionary change occurs through gradual, accumulated differences. At the same time another scientist, Alfred R. Wallace, was working in the East Indies and developed a very similar theory i.e. theory of evolution.
Both theories were presented at the meetings of the London Linnaean Society in 1858, and this concept of evolutionary change was embraced by most scientists, including Haekel whose Tree of Life (1866) depicted all taxa (Protists, Plants, and Animals).
(ii) George Bentham (1800-1884) and Sir Joseph D. Hooker (1817-1911):
Genera Plantarum was published after Darwin’s Origin of Species, but it was not possible to change the system to reflect new evolutionary concepts despite the fact that both Bentham and Hooker were great champions of this theory. Bentham and Hooker’s system is post- Darwinian in chronology but pre-Darwinian in concept.
They named 200 families and 7,569 genera, with fabulously detailed, often original descriptions. Many herbaria in the world are still arranged according to this system. They recognized monocots and dicots and began the latter with the polypetalous plants.
B. Phylogenetic Classifications:
(i) Asa Gray (1810-1888):
He is not considered a major figure in systems of plant classification, but he was the first professor of botany at U.S. University. His most famous work includes A Manual of Botany of the Northern United States (1848). Gray’s Manual of Botany was published later by M. L. Fernald.
(ii) Adolph Engler (1844-1930) and K. A. E. Prantl (1849-1893):
The first phylogenetic systems came from Botanical Garden in Berlin. Engler and Prantl believed that the plant classification system should reflect the evolutionary history and their greatest work Die Naturlichen Pflanzenfamilien, adopted a system that proceeded from simple structures to complex.
Unisexual, cone-like flowers (catkins) were considered to be primitive (alder, birch, etc.). This was the first major phylogenetic classification. This system is used today in many herbaria and floras and very useful for a modern taxonomist, but the concepts of what is primitive and advanced has changed today.
(iii) Charles Edwin Bessey (1845-1915):
He was a student of Asa Gray and he developed a set of “dicta” (rules) stating which characters are primitive and which are advanced in flowering plants. Not all are considered correct today but many are. The Magnolia type flower was considered primitive, not the unisexual catkin-bearing plants. His published work is The Phylogenetic Taxonomy of Flowering Plants.
(iv) John Hutchinson (1884-1972):
He worked at Kew Gardens and published The Families of Flowering Plants and Genera of Flowering Plants. He considered woody and herbaceous to be of fundamentally different evolutionary trends. Not much of this plant classification is used today.
(v) Robert F. Thorne (1920 to present):
He works at Rancho Santa Anna Botanical Garden in Claremont, CA. His work includes A Phylogenetic Classification of the Angiosperm.
(vi) Armen Takhtajan (1910 to present):
He was initially at Lenningrad, but now spends most time at New York Botanical Garden. His work Flowering Plants: Origin and Dispersal was an influential early work. A revision of this classification appeared in 1997 as Diversity and Classification of Flowering Plants.
(vii) Rolf Dahlgren (1932-1985):
He worked at the University of Copenhagen. His work includes A Revised System of plant Classification of the Angiosperms.
(viii) Arthur Cronquist (1919-1991):
He worked most of his life at the New York Botanical Garden. Many of his ideas are Besseyan with some influence from Takhtajan. His most famous work includes An Integrated System of plant Classification of Flowering Plants (1981), which has been revised in 1988 and is much adopted today.