Bentham and Hooker classification

Bentham and Hooker classification

George Bentham and Joseph Dalton Hooker - Two English taxonomists who were closely associated with the Royal Botanical Garden at Kew, England have given a detailed classification of plant kingdom, particularly the angiosperms.

They gave an outstanding system of classification of phanerogams in their Genera Plantarum which was published in three volumes between the years 1862 to 1883. It is a natural system of classification. However, it does not show the evolutionary relationship between different groups of plants, in the strict sense. Nevertheless, it is the most popular system of classification particularly for angiosperms. The popularity comes from the face that very clear key characters have been listed for each of the families. These key characters enable the students of taxonomy to easily identify and assign any angiosperm plant to its family.

Bentham and Hooker have grouped advanced, seed bearing plants into a major division called Phanerogamia. This division has been divided into three classes namely:

1. Dicotyledonae         2. Gymnospermae and            3. Monocotyledoneae

Class Dicotyledonae

This group includes angiosperms in which the seed bears two cotyledons and leaves exhibit reticulate venation. It is divided into three subclasses - Polypetalae, Gamopetalae and Metachlamydae.

Sub-class Polypetalae

The flowers contain distinct non-essential whorls calyx and corolla. In the corolla petals are free. This sub-class includes 8 series Thalamiflorae, Disciflorae and Calyciflorae.

• Series Thalamiflorae: Many stamens in the androecium. Flower is hypogynous.

• Series Disciflorae: Hypogynous flowers with a cushion-like disc around or below the ovary.

• Series Calyciflorae: Flowers epigynous or perigynous. Thalamus is in the form of a cup.

Sub-class GamopetalaeFlowers with distinct calyx and corolla. In the corolla petals are fused. This sub-class includes 3 series.

• Series Inferae: Flowers with inferior ovary.

• Series Heteromerae: Flowers with superior ovary. Number of carpels - more than two.

• Series Bicarpellatae: Flowers with superior ovary. Number of carpels - two.

Sub-class MonochlamydaeThe flowers are with only one non-essential whorl (perianth) or absence of non-essential whorls. It includes 8 series.

• Curvembryae: Usually single ovule, embryo coiled around the endosperm.

• Multiovulate Aquaticae: Aquatic plants with syncarpous ovary and many ovules.

• Multiovulate Terrestris: Terrestrial plants with syncarpous ovary and many ovules.

• Microembryae: Only one ovule, small, tiny embryo endospermic seed.

• Daphnales: Only one carpel and single ovule.

• Achlamydosporae: Ovary inferior, 1 to 3 ovules - unilocular.

• Unisexuales: Flower unisexual, perianth usually absent.

• Ordines Anomali: (Anomolous families) Plants with uncertain systematic position but closer to unisexuales

Class GymnospermaeThis group includes the gymnosperms in which seeds are not enclosed in fruits. This class is divided into three families Gnetaceae, Confiraceae and Cycadaceae.

                               

Class Monocotyledonae

This group includes angiosperms in which the seed bears only one cotyledon. The leaves exhibit parallel venation. It is divided into the following seven series.

• Microspermae: Ovary is inferior, seeds are minute and non-endospermic.

• Epigynae: Ovary inferior, seeds are large and endospermic.

• Coronarieae: Ovary superior, perianth petalloid.

• Calycinae: Ovary superior, perianth sepalloid.

• Nudiflorae: Perianth reduced or absent. Seeds are endospermic.

• Apocarpae: Carpels more than one, free, seeds are endospermic.

• Glumaceae: Perianth reduced or absent, scaly bracts present.

Each of the series mentioned under dicotyledonae and monocotyledonae have been further divided into orders and families. Bentham and Hooker classified the angiosperms into 202 families. They were able to provide distinct diagnostic key characters to each of these families. This is the reason for the popularity of Bentham and Hooker's classification particularly amongst the taxonomists.

Engler and Prantl regarded Angiosperms as having arisen from a hypothetical extinct group of gymnosperms called Protoangiosperms related to unknown taxa of gymnosperms. They considered gymnosperms to be primitive and hence placed them before angiosperms. Similarly, monocots were initially considered more primitive and hence placed before dicots. However, in the last edition of the “Syllabus” (Melchior, 1964), the dicots were placed before monocots. They distinguished between primitive and advanced conditions.Engler believed in the Amentiferous origin of the angiosperm flower. According to this concept, the unisexual flowers without perianth (achlamydeous) or with one whorl of perianth (monochlamydeous; and borne in catkin inflorescence with wind pollination) were primitive whereas those with two whorls of perianth (dichlamydeous - distinguished into sepals and petals) were advanced. The bisexual flowers were believed to have evolved from a cluster of male and female flowers (held together in the same inflorescence and appearing like a single flower (called a   pseudoanthium), while the surrounding perianth evolved later. Therefore the families with naked (Achlamydeous = without perianth), unisexual and wind pollinated flowers were placed at the beginning, while those with dichlamydeous (perianth in two whorls – calyx and corolla), bisexual and insect pollinated flowers were placed at a later (= advanced) position in the classification. Thus the different families were arranged in an ascending series with simple groups at the beginning, followed by families with increasing complexity and elaboration of flowers. Engler’s system thus did not recognize the significance of reduction in evolution, and equated simplicity with primitiveness. However, it is now known that in many cases simplicity may be the result of evolutionary reduction. This has led to the suggestion that the classification was not phylogenetic in a true sense, and may be regarded as a step towards phylogeny.Engler and Prantl considered the angiosperms to be polyphyletic in origin, and derived from seed ferns and gymnosperms through Amentiferae.The classification covered the entire plant kingdom from algae to angiosperms which has been divided into 13 divisions. The first 11 divisions are Thallophytes, 12^th division is Embryophyta Asiphonogama (plants with embryos but no pollen tubes; Bryophytes and Pteridophytes) and the 13^th division is Embryophyta Siphonogama (plants with embryos and pollen tubes) which includes seed plants. This is divided into 2 subdivisions: 1. Gymnospermae, 2. Angiospermae.The subdivision Angiospermae is further divided into 2 classes:Class 1. MonocotyledoneaeClass 2. DicotyledoneaeClass 1. Monocotyledoneae include 11 orders starting with order Pandanales with naked unisexual flowers and ending with order Microspermae with family Orchidaceae. Orchidaceae is considered as the most advanced family and evolved over grasses.Class 2. Dicotyledoneae is divided into 2 subclasses: Subclass 1. Archichlamydeae and Sublass 2. Sympetalae or Metachlamydeae.  Archichlamydeae include 33 orders starting with order Verticillatae and ending with order Umbelliflorae. It included achlamydeous or monochlamydeous families at the beginning and dichlamydeous and polypetalous families in the end. Hence, Archichlamydeae comprised of the Poylpetalae and Monochlamydeae of Bentham and Hooker’s classification. Sympetalae included Gamopetalous flowers with 11 orders starting with order Diapensiales and ending with order Campanulatae.

• Merits of Engler and Prantl System of Classification
• The classification incorporated evolution and was the beginning of phylogenetic classifications.
• The system classified all the plant families from the algae to angiosperms.
• Gymnosperms were considered primitive and placed before angiosperms.
• The taxa were fully illustrated, described along with identification keys upto family level in Syllabus der Pflanzenfamilien, up to genus level in Die naturlichen Pflanzenfamilien and for many families along with their species in Das Pflanzenreich.
• The description of families also contained information on embryology, anatomy and geographical distribution.
• The classification was very elaborate, extensive and is still followed in many herbaria and floras of the world.
• The polypetalous condition was considered primitive than gamopetalous condition which is also the present phylogenetic view.
• The Polypetalae and Monochlamydeae of Bentham and Hooker’s classification were merged into a single subclass Archichlaymdeae which resulted in proper placement of many families. For example family Illecebraceae is combined with family Caryophyllaceae. Families Amaranthaceae, Chenopodiaceae and Caryophyllaceae are included in the same order Centrospermae.
• Families Compositae and Orchidaceae in dicots and monocots respectively are considered advanced are placed at the end of each group.

• Demerits of Engler and Prantl System of Classification

• The classification though not intentionally was more natural and less phylogenetic.

• The classification equated simplicity of structure with primitiveness and did not recognize the simplicity due to evolutionary reduction. Thus the classification was not phylogenetic in true sense.

• Monocotyledons were placed before dicots but this placement was reversed in the 1964 edition of Syllabus der Pflanzenfamilien.

• The naked flowers of Amentiferous families were considered primitive, but evidences from floral anatomy, wood anatomy and palynology suggest these are not primitive but simple due to the disappearance of perianth and evolutionary reduction.

• Unisexual flowers were treated as primitive, but evidence from floral anatomy has shown that the unisexual condition is derived from bisexual condition by suppression of either androecium or gynoecium.

• Monochlamydeous flowers (one whorl of perianth) were treated primitive over dichlamydeous flowers (distinct calyx and corolla) which is not accepted by modern taxonomists.

• Angiosperms were considered to be polyphyletic in origin, but modern taxonomists regard angiosperm to be monophyletic in origin.

• Ranales are considered primitive by modern taxonomists, but they have been placed after Centrospermae at a position not accepted by modern taxonomists.

• Araceae in Monocotyledons are derived from Liliaceae, but Araceae have been placed before Liliaceae, a position not accepted by modern taxonomists.

• Helobieae consisting of primitive groups have been placed between two advanced orders Pandanales and Glumiflorae.