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.

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