Systems of Plant Classification

This is a part of online classes for the undergraduate Botany students of Government First Grade College Yelahanka Bangalore on 1 to 3 October 2020

1. Systems of Plant classification By Dr P B MALLIKHARJUNA Associate Professor GFGC YB

2. Plant classification is the placement or arrangement of plants or plant groups in an orderly sequence according to phenetic similarities, phylogenetic relationships or criteria that are more artificial . Species is the basic unit of classification Indeed more than 50 different systems of classification for plants have been proposed to date. Of these, only few are dominant and adopted is different parts of the world. However, no system is completely satisfactory in reflecting the phylogenetic relationship of the taxa concerned. One system is superior over the other to a degree. Further an ideal classification should be easy to adopt, stable and aid to memory predictive one and concise. All these classifications based on the criteria they have chosen, may fall into any one of the following systems i.e., Artificial system of classification Natural system of classification Phylogenetic system of classification

3. Artificial system of classification These are the first category and primitive type of classifications based on the Aristotelian approach. In the artificial system of classification, the plants have been classified based on the presence or absence of a single or few arbitrarily selected and easily observable characters. Owing to this, in several cases closely related plant species are placed in different groups and unrelated plants are kept together. Ex : 1) Theophrastus Classification , who has proposed an artificial classification for the plants. He has classified plants based on habit into four groups namely herbs, undershrubs , shrubs and trees .

4. Natural system of classification In natural system, all the important morphological characters of the plants are taken into account to judge their natural resemblance and relationships. The plants are thus, classified according to their natural relationships. In this system, the plants were first grouped into large higher categories and then these categories are further divided and subdivided until the basic category called the species is reached. These are very useful in the identication of plants. Ex: 1. de Condolle system of classification 2. Bentham and Hooker’s system of classification .

5. Phylogenetic system of classification These are the modern and recent system of classifications. The arrangement of plants or plant groups are mainly classified in the light of phylogenetic relatedness or the Evolutionary relationships among the plants. These classifications are based on the several characters of the plants such as morphological, anatomical, cytological, karyological , embryological, ecological and biochemical characters taken into account. Further, plant characters are weighed in the evolutionary context. For instance, arborescent habit is primitive over herbaceous habit. Ex: Engler and Prantl system of classification Cronquist classification Thakthajan classification APG (Angiosperm Phylogeny Group) classification

6. BENTHAM & HOOKER’S CLASSIFICATION (1862-1883AD) George Bentham and Joseph Dalton Hooker were British naturalists proposed the widely adopted natural system of classification for the seed plants in Genera plantarum in the Latin, published in three volumes (1862-1883 AD). It is the most widely adopted natural system of classification for the study of the flowering plants. It included names, description, illustrations, and nomenclature of seed plants (97205 species known at that time). The basis for this classification are de Jesseue and de Condolle ’ classifications. It is post-Darwinian in publication and pre-Darwinian in concept. This classification is mainly based on the several morphological arbitrarily selected characters.

7. They have classified the division- Phanerogams (flowering plants) into 3 classes containing 202 families with 7569 genera and 97205 species. They have placed the order Ranales in the beginning of the dicots which are universally considered to be the most primitive angiosperms. They believed in the theory of reduction in evolution .

8. Merits: It is the widely used natural system of classification in pedagogic deliberations There is a uniform treatment and the study judgement of taxa throughout the system. Placing the order Rannales in the beginning is justified, as several taxonomists believed that are primitive evolutionary. Placing dicots before the monocots is justified. Even though the system is not phylogenetic, the sequence of arrangement of taxa reflect the lines of evolution .

9. Demerits: Placing/interpolation of Gymnosperms in-between Dicots and Monocots is a major drawback. Division of Dicotyledons into 3 sub-- classes based on a single character i.e., P erianth has become a priori method of character weighing. The sub-class monochlamydeae of the dicots is an unnatural assemblage. Related families are kept apart such as Chenopodiaceae and Caryophyllaceae . Further, unrelated families kept together such as Juncaceae and Palmae The authors were believed in the dogma of fixity of species and immutability of species. Placing the family Orchidaceae at the beginning of monocots is not justified as it is one of the highly evolved one .

10. ENGLER & PRANTL CLASSIFICATION ( 1887-1915) Adolf Engler and Karl Prantl were the German botanists. They have proposed a phylogenetic and polyphyletic system of classification for the entire plant Kingdom i.e., from algae to angiosperms. They have published their classification in a 23 (Voluminous) magnum opus entitled “ Die Naturlichen Pflanzen familien ” (i.e., The natural plant families) between 1887 - 1915 in German language. This system is based on the foundations laid down by AW Eichler . They suggested the Amentifer theory in the interpretation of the origin of angiosperms. According to them the arborescent (woody) habit with unisexual and wind pollinated flowers produced in catkin inflorescence are primitive.

11. They have classified the plant Kingdom into 13 divisions out ofthese one to 12 th division includes the non flowering plants The 13 th division includes the flowering plants or seed plants has Embryophyta Siphanogama This division Embryophyta Siphanogama has 2 sub divisions namely Gymnospermae and Angiospermae . The subdivision Gymnospermae has 7 classes. Whereas, the sub division Angiospermae has 2 classes ( Monocotyledoneae and dicotyledoneae ) , 55 orders and 306 plant families. The class Monocotyledoneae was further divided into 11 orders starting with Pandanales ( Typhaceae ) and ending with Microspermae ( Orchidaceae ).

12. The class Dicotyledoneae further divided into 2 sub classes namely Archichlamydeae & Metachlamydeae / Sympetalae , with 44 orders including 261 families. The subclass Archichlamydeae contains the 33 orders and 201 families beginning with Verticillatae ( Casuarinaceae ) and ending with the order Umbelliflorae ( Apiaceae ). The second subclass, Metachlamydeae contains 11 orders and 60 families starting with Diapensiales ( Diapensaceae ) and ending with Companulatae (Composite).

13. Merits : Placing Gymnosperms before the Angiosperms was justified Placing M onochlamydeae and Polypetalae plant families in the Archichlamydeae is more homogeneous. Keeping the both Asteraceae and Orchidaeae families at the respective ends of dicots and monocots is justified. The taxa are extensively illustrated and also provided modern keys to the Genera and families. The work is known for its systematic thoroughness with which it is applied to the world's flora. They have taken the anatomical and embryological data along with morphological attributes in the light of evolution while classifying the plants.

14. Demerits : Placing monocots before to the dicots he is contradictory to the generally accepted line of evolution of angiosperms. However it is rectified in the later edition 11th edition (deals 1926). The system failed to be a truly phylogenetic one. This system is not a much of practical utility