Botanical Plants Classification
The division Gymnospermae was placed in between dicotyledons and monocotyledons and is further classified into three orders.
This system has some refinements over that of de Condolle’s namely erection of a new series Disciflorae in their Polypetalae and its incorporation between the Thalamiflorae and Calciflorae, and revision of apetalous taxa. The gymnosperms were treated as the third taxon, collateral with and placed between dicotyledons and monocotyledons. Various genera grouped under “orders” and different orders under “cohort” were based on overall similarities and differences. Certain orders which could not be satisfactorily placed by them in any cohort, were treated anomalous orders (Ordines anomali). This clearly indicates that Bentham & Hooker attempted at a natural classification of seed plants taking into consideration all the data available to them.
Merits and Demerits of Bentham & Hooker’s System of Classification
- Though this system is not very natural yet it is very easily workable, and is important from the point of view of its applications.
- This system has been worked out as a result of very careful comparative examination of all known genera of phanerogams.
- This classification makes the basis for the arrangement of plants in Kew herbarium and other important herbaria of common wealth countries.
- In this system, greater emphasis has been given on the contrast between free and united petals e.g., the class dicotyledons is divided into three sub-classes, Polypetalae, Gamopetalaeand
- A special feature of this system is the addition of Disciflorae and a curious arrangement of dividing certain groups on the basis of aquatic and terrestrial characters.
- In the class monocotyledons, the stress is being given to the relative position of the ovary and perianth characteristics.
- This establishes no phylogenetic relationship in different taxa of plants.
- The greatest disadvantage is the retention in the group Monochlamydeae a number of orders which show affinities with those in which a biseriate perianth is a rule. The Monochlamydeae is considered aside from
- The position of gymnosperms between dicots and monocots is only for convenience rather than an indicating of affinities.
- This system is based mainly on single and mostly artificial characters, with the result that closely related families are widely separated from each other.
- The position of Orchidaceae and Scitamineae at the beginning of monocots is not satisfactory.
- In monocots, much stress is being given to the relative position of the ovary and perianth characters in determining the affinities that seems unjustified by the comparative study of the orders, e.g., the families Iridaceae and Amaryllidaceae exhibit greater affinities to Liliaceae than to Scitamineae and Bromeliaceae with which they are allied in this system because of common character of epigynae.
- The origin of angiosperms was not established.
Englar & Prantll’s System of Classification
Englar along with his associate, Prantll expanded his system and published their work in 23-volume Die Naturilichen Pflazenfamilien between 1887-1915.
This system includes original description of a large number of plants with keys for their identification. Engler believed that simple naked, unisexual flowers were primitive. Such flowers are usually found in Amentifers (Catkin inflorescence plants). According to him these flowers have probably derived from gymnospermous ancestors with unisexual strobilus bearing either micro or megasporophylls. This concept is known as Englerian Concept, regarding the origin of angiospermous flower. The origin of bisexual flower was considered from a cluster of male and female flowers within same inflorescence.
The Englerian school of thought also suggest that the unisexual flowers were wind-pollinated like cones of gymnosperms and that the different groups of angiosperms have been derived from various groups of gymnosperms and pteriophytes suggesting a Polyphyletic Origin. Also in his system, the precedence has been given to monocots over dicots.
Engler and Prantll classified the plant kingdom into 14 Divisions. The Divisions are:
- Embryophyta Siphonogama: This division includes seed plants.
The Embryophyta Siphonogama was further classified into two Sub-Divisions:
- Angiospermae:Angiosperms are classified into three classes, the Monocotyledonsand the Dicotyledons, together consisting of 303 families.
The Monocotyledons have been considered more primitive than the dicotyledons and therefore, have been placed before them. They are classified into 11 orders and 45 families starting with the unisexual and apetalous Pandanales followed by the aquatic Helobiae, Triuridales, Glumiflorae, Princcps, etc, and ending in highly evolved Microspermae.
The Dicotyledons are classified into two sub-classes:
(i) Archichlamydeae (Polypetlae)
(ii) Sympetalae (Gamopetlae)
Which were further divided into 44 orders and 258 families.
Archichlamydeae include 33 orders of perianthless as well as polypetalous (lowers, while the Sympetalae includes 11 orders of gamopetalous flowers. The orders Verticellatae, Fagales, Salicales, etc., with perianthless unisexual flowers are placed in the beginning of the dicotyledons, followed by the polypetalous orders such as Centrospermac, Ranales, etc. And ending in the sympetalous orders such as Contortae, Tubiflorae, etc. The Compositae is regarded as highly evolved family. The order Centrospermac, Ranales, etc and ending in the sympetalous orders such as Contortae, Tubiflorae, etc. The Composiate is regarded as highly evolved family. The order Centrospermae is regarded as holding an intermediate position connecting the monocotyledons on one hand and the dicotyledons on the other.
A broad outline of Engler and Prantll’s modified system is as follow:
Plant Kingdom was divided into 14 Divisions and these divisions are:
- Embryophyta Siphonogama
Embryophyta Siphonogama was divided into following two Sub-Divisions.
This Sub-division of Gymnospermae includes 7 classes.
This Sub-division of Gymnospermae includes 2 classes.
- Monocotyledonae has 14 orders 53 families.
- Helobiae (9)
- Triuridales (1)
- Liliflorae (17)
- Juncales (2)
- Bromeliales (1)
- Commelinales (8)
- Graminales (1)
- Principes (1)
- Synanthae (1)
- Spathiflorae (2)
- Pandanales (3)
- Cyperales (1)
- Scitamineae (5)
- Microspermae (1)
- Dicotyledonae has 48 orders 291 families. Further classified into two Sub-Classes.
Archichilamydeae (Flowers without perianth, polypetalous) has 37 orders 227 families.
- Casuarinales (1)
- Juglandales (2)
- Balanopales (1)
- Leitnerales (2)
- Salicales (1)
- Fagales (2)
- Urticales (5)
- Tubiflorae (26)
- Plantanginales (1)
- Balanophorales (1)
- Medusandrales (1)
- Polygonales (1)
- Centrospermae (12+1)
- Cactales (1)
- Magnoliales (22)
- Ranunculales (7)
- Piperales (4)
- Aristolohciales (3)
- Guttiferales (16)
- Sarraceniales (3)
- Papaverales (6)
- Batales (1)
- Rosales (19)
- Hydrostachyales (1)
- Podostemales (1)
- Geraniales (9)
- Rutales (12)
- Sapindales (10)
- Juliniales (1)
- Celastrales (3)
- Rhamnales (3)
- Malvales (7)
Merits & Demerits of Engler and Prantll’s System of Classification:
- The merits of Engler and Prantll’s system lies in the broad treatment of the whole plant kingdom and it excellent arrangement of the orders and families in a phylogenetic way.
- The gymnosperms are treated separately in this system.
- This system has resulted from the systematic thoroughness with which Englar and his colleagues applied the system to the flora of the world and thus this system has been accepted in every part of the world.
- The large artificial group of Bentahm and Hooker’s system, the Monochlamydeae has been completely abolished, and its families have been distributed among the related forms with free petals in the large series of Englar’s system,
- The treatment of orchids to be more highly evolved than grasses is satisfactory.
- The Sympetalae of this system corresponds the Gamopetalae of Bentham and Hooker’s system.
- This system obscure, the phylogeny of angiosperms, which according to Engler had a polyphyletic origin from an unknown and hypothetical taxon of extinct gymnosperms.
- The acceptance of the derivation of dichlamydous flowers from, monochlamydous flowers is objectionable.
- In this system, the monocots have been considered to be more primitive than dicots, which does not correspond to the present day knowledge.
- Derivation of bisexual flowers from unisexual flowers and parietal placentation from axile placentation are unsatisfactory.
- A primitive order like Helobiae has been placed between two more advanced orders the – Pandanales and Glumiflorae, which is not satisfactory.
- The position of Amentiferae and Centrospermae at the beginning of dicots before Ranales is not satisfactory.
Armen Takhtajan System of Classification:
Takhtajan, a leading Russia plant taxonomist is an international authority on phytogeography, origin and phylogeny of flowering plants. He developed a preliminary Phylogenetic diagram of the orders of higher plants presented in 1942 in a proper on structural types of gynoecium and placentation.
The system is based on Phylogenetic principles was first published in 1954 in Russia language and translated in English in 1958 in the book Origin of Angiospermous Plants. Later he traced the evolution of angiosperms and elaborate his system in De Evolutinder Angiospermen (1959) and proposed a new system in Systema et Phylogenia Magnoliophytorum (1966). Subsequently major revision of his classification were made in 1980, 1987, 1997.
He classified angiosperms upto family level. He believed in the monophyletic origin of angiosperms. They were evolved from the seed ferns Lyginopteridophyta. He divided division Magnoliophyta into two classes. Magnoliopsida (dicots) and Liliopsida (monocots). He considered that Magnoliopsida are primitive and Liliopsida have been derived from Magnoliales under Magnoliopsida. The two classes are divided into two subclasses, superorders, orders and families. He accounted 13,000 genera (10,000 in Magnoliopsida and 3,000 in Liliopsida) and 2,50,000 species (190,000 in Magnoliopsida and 60,000 in Liliopsida).
The outline of classification proposed by Takhtajan is as under.
Outline of system of Angiosperms Classification proposed by Takhtajan
Division: Magnoliophyta (Angiosperms)
Class 1. Magnoliopsida
Sub Class 1. Magnoliidae
Sub Class 2. Nymphaeidae
Sub Class 3. Nelumbonidae
Sub Class 4. Rannunculidae
Sub Class 5. Caryophllidae
Sub Class 6. Hamamelididae
Sub Class 7. Dilleniidae
Sub Class 8. Rosidae
Sub Class 9. Cornidae
Sub Class 10. Asteridae
Sub Class 11. Lamiidae
Class 2. Liliopsida (Monocotyledons)
Sub Class 1. Lamiidae
Sub Class 2. Commelinidae
Sub Class 3. Arecidae
Sub Class 4. Alismatidae
Sub Class 5. Triurididae
Sub Class 6. Aridae
Takhtajan considered that angiosperms arose under environmental stress like drought etc.
- Growth Habit:
The evolution of angiosperms began with small, relative weakly branched woody forms. Large trees regarded to be originated from primitive small woody angiosperms. Woody plants are primitive than herbaceous and deciduous woody plants.
Palmate venation is considered to be evolved from pinnate venation. Parallel venation is advanced than the reticulate venation. Alternate leaf arrangements is more primitive.
Stomata with subsidiary cells are advance then lacking subsidiary cells.
- Nodal Structure:
The unilacunar nodal structure has been derived from tri-pentalacunar types.
- Woody Anatomy:
The evolution of vessels is from vessels with scalariform perforations to vessels with simple perforations.
Cymose inflorescence is primitive, the simple cymose give compound cymose and then evolved racemose.
- Floral Structure:
There is a gradual transition from spiral to cyclic arrangement and to the fixation of number of floral parts.
The stamens of earliest angiosperms were leaf-like pinnate microphyllus with marginal situated microsprangia.
- Pollen Grains:
The main trend of evolution of dicot pollen is from monocolpate to tricolpate and from tricolpate to triporate.
Apocarpous with large number of ovules is more primitive than syncarpous condition.
Unitegmic ovules arose from bitegmic ovule. Thus unitegmic is primitive than bitegmic ovule. Mesogamic and chalazogamic have evolved from progamic condition.
Anemophily arose from entomophily.
The tetrasporic embryo sac is evolved from primitive monosporic.
Primitive seeds were 5-10mm long, with abundant endosperms, minute undifferentiated embryo, seed coat with multiple integument is the basic primitive characters.
Many seeded follicles, developed from a multicarpellate, apocarpous gynoecia, is most primitive fruit type.
Merits & Demerits of Takhtajan’s System of Classification:
The Important merits of Takhtajan’s system of classification are as follow:
- It reflects the evolutionary relation better than the older systems.
- It is based on 67 phyletic principles.
- He considered the angiosperms the monophyletic and monocots to have originate from dicots.
- Angiosperms are treated as one division Magnoliophyta.
- The sub class Magnoliidae is considered as the most basic group from other sub class.
- Families are small homogenous units, comprising closely related genera.
- Sub class is not based on single character but on consideration of all known informations.
Despite, that Takhtajan tried to remove deficiencies of his system in 1997, some of the defects are still left, which include:
- The main drawback is derivation of monocot from the sock ancestral to Nymphaeales.
- The narrowing defined taxa have resulted in the unwarranted splitting of related groups.
- The system is although the Phylogenetic but still not helpful for classification up to family level.
- Monocot are placed after dicots recent systems of classification place them between primitive angiosperms and Eudicots.