Angiosperm Classification


   
 
Bentham and Hooker's 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.
 
 
fig. 9.1 - Taxonomists: Bentham and Hooker
 
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 Gamopetalae
 
Flowers 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 Monochlamydae
 
The 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 Gymnospermae
This group includes the gymnosperms in which seeds are not enclosed in fruits. This class is divided into three families Gnetaceae, Confiraceae and Cycadaceae.
 
                                       fig. 9.2 - Angiosperm Classification
 
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.
 
  
     
   
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