Saturday, 17 January 2015

PLANT CLASSIFICATION SYSTEMS



Traditionally plant kingdom has been divided into two sub-kingdoms named as PHANEROGAMAE and CRYPTOGAMAE.

(a) CRYPTOGAMAE (phaneros-visible, gamos-marriage): All flowering plants which bear seeds are included in this group. They are also known as spermatophytes (sperma-seed, phyton- plant), since they produce seeds. These seed bearing plants are further divided into divisons gymnospermae and angiospermae. Gymnosperms have naked ovules or seeds. The angiosperms (covered seed) include all the flowering plants which produce seed and have ovules in ovary or fruit.

(b) PHANEROGAMAE (kryptos-concealed, gamos-marriage): All non flowering plants such a algae, fungi, lichen, moses and ferns are included in this sub-kingdom. The cryptogams are further classified into three divisions- THALLOPHYTA, BRYOPHTA and PTERIDOPHYTA.

A. Early attempts for classification
Aristole and other Greek philosophers divided living organisms into two groups: plants and animal,    Aristole also divided plants into 3 groups. HERBS, SHRUBS and TREES. Charaka (The father of Ayurveda), in 1st century A.D., listed 200 kinds of animals and 340 kinds of plants in his ' CHARAKA SAMHITA '.

B. Artificial System of Classification
It is based on a few morphological character of vegetative nature for grouping of organisms. e.g. early systems of classification by Aristole, Theophrastus, Pliny, Bauhin , Linnaeus etc. THEOPHRASTUS proposed the first system of artificial classification of plants on the basis of habit and classified plants into herbs, shrubs, undershrubs and trees. CAROLUS LINNAEUS (1707-1778) proposed the artificial system of classification based exclusively on nature and number of stamens and carpels. It was called as sexual system of classification. Classification by Linnaeus consisted of 24 classes, in which 23 classes were of flowering plants ( Phanerogamia ) and 24th class had flowerless plants ( Cryptogamia ). Details of this classification were published in Genera Plantarum (1737). Total 24 classes given by Linnaeus were : Monandria, Diandria  , Triandria , Tetrandria , Pentandria , Hexandria , Heptandria , Octandria , Enneandria , Decandria , Dodecandria , Iconsandria , Polyandria , Didynamia , Tetradynamia , Monadelphia ,Diadelphia , Syngensia , Gynandria , Monoecia , Polygamia and Cryptogamia.
Drawbacks:
(i) This system was based on one or a few characters, hence the diverse animals or plants were placed into limited number of groups.
(ii) Natural affinities and phylogenetic relationships were not considered.
(iii) The artificial system gave equal weight age to vegetative and sexual characters. This is not acceptable and since we know that often the vegetative characters are more easily affected by environment.

C. Natural System of Classification
Also known as HORIZONTAL system of classification or 2 D system.  Organisms in this system are classified on the basis of the natural affinities. This system uses as many taxonomic characters as possible to group organisms. This classification is mainly based on forms, relationship realizing all information available at the time of collection of plants. This also considers internal features like ultra structure, anatomy, embryology, and phytochemistry. Common natural systems were proposed by- John Ray, de Candolle, Bentham and Hooker,etc. George Bentham and J.D. Hooker gave most important natural system of classification of angiosperms and published it in three volumes of 'George Plantarum'. They described 202 families. In this system, description of plants was based on their detailed studies and dissections. This system is followed in all British Commonwealth countries including India.

D. Phylogenetic System of Classification
The term phylogeny was given by Lamarck and concept of phylogeny by E. Haeckel. Phylogeny is the evolutionary history of the organism. This system is also called '3D' or vertical system. In this system, plants are classified according to their evolutionary and genetic affinities. Organisms belonging to same texas are believed to have a common ancestor and may be represented in the form of family tree called Cladogram. A.W. Eichler modified Bentham and Hooker's system of classification by placing gymnosperms in the beginning. He is also called as the pioneer in phylogenetic system of classification. Adolph Engler and Karl A.E. Prantl, two german botanists, adopted their system in "Die naturalichen Pflanzen familien" (1887-1915).

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