Bentham and Hooker's system of classification, merits and demerits,

  Bentham & Hooker’s system of classification 

•  The system of classification of seed plants was proposed by Bentham and Hooker.
•  It’s the most well-developed natural classification system, it was published in 3 volume work Genera plantarum (1862-83).
• They described 97,205 species of seed plants belonging to 7,569 genera of 202 families starting from Ranunculaceae up to Gramineae.
• The delimitation of genera was based on natural affinities and was pre-Darwinian in concept.
• The system divided all the seed plants into three classes: -

– Dicotyledons (165 families) 

– Gymnosperms (3 families) and 

– Monocotyledons (34 families).

Class 1: Dicotyledons: -

•  Seed with 2 cotyledons.
• Flowers with pentamerous or tetramerous.
• Reticulate venation.

Subclass 1: Polypetalae: -

•  Polypetalae: sepals and petals distinct, petals free (14 series, 25 orders and 165 families).

– Thalami florae: flowers hypogynous, stamens many, disc absent.

• 6 Orders: Ranales, Parietales, Polygalineae, Caryophyllineae, Guttiferales and Malvales.

– Disciflorae: flowers hypogynous, disc present below the ovary.

•  4 Orders: Geraniales, Olacales, Celastrales and Sapindales.

– Calyciflorae: flowers perigynous or epigynous.

• 5 Orders: Rosales, Myrtales, Passiflorales, Ficoidales and Umbellales.

Subclass 2: Gamopetalae: -

•  Gamopetalae: Sepals and petals distinct, petals fused. 

– Inferae: ovary inferior.

• 3 orders: Rubiales, Asterales and Campanales.

– Heteromerae: Ovary superior, stamens in one or two whorls, carpels more than two.

•  3 orders: Ericales, Primulales and Ebenales.

– Bicarpellatae: Ovary superior, stamens in one whorls, carpels 2.

•  4 orders: Gentianales, Polemoniales, Personales and Lamiales.

Subclass 3: Monochlamydeae: -

• Monochlamydeae: flowers apetalous, perianth lacking or if present not differentiated into sepals and petals.

– Curvembryeae: embryo coiled, ovule usually one.

– Multiovulate aquaticae: aquatic plants, ovules many. 

– Multiovulate terrestress: terrestrial plants, ovules many. 

– Microembryeae: embryo minute.

– Daphnales : carpel one, ovule one.

– Achlamydosporae: ovary inferior, unilocular, ovules 1-3. 

– Unisexuales: flowers unisexual.

– Ordines anomali: relationship uncertain.

Class 2: Gymnospermae: -

•  Ovules naked have three families.

Class 3: Monocotyledons: -

• Flowers trimerous, venation parallel.

– Microspermae : ovary inferior, seeds minute. 

– Epigynae: ovary inferior, seeds large. 

– Coronarieae : ovary superior, carpels united, perianth colored. 

– Calycinae: ovary superior, carpels united, perianth green.

– Nudiflorae: ovary superior, perianth absent.

– Apocarpae: ovary superior, carpels more than 1, free.

– Glumaceae : ovary superior, perianth reduced, flowers enclosed in glumes.

  Merits of Bentham and Hooker’s System  

• Each plant has been described either from the actual specimen or preserved herbarium sheets so that the descriptions are detailed as well as quite accurate.
•  The system is highly practical and is useful to students of systematic botany for easy identification of species.
• The flora describes geographical distribution of species and genera.
• The generic descriptions are complete, accurate and based on direct observations.
•  Larger genera have been divided into sub genera, each with specific number of species.
•  Dicots begin with the order Ranales which are now universally considered as to be the most primitive angiosperms.
•  Placing of monocots after the dicot is again a natural one and according to evolutionary trends.
• The placing of series disciflorae in between thalami florae and calyciflorae is quite natural.
• The placing of gamopetalae after polypetalae is justified since union of petals is considered to be an advanced feature over the free condition.

  Demerits of Bentham and Hooker’s System  

•  Keeping gymnosperms in between dicots and monocots is anomalous.

Subclass monochlamydeae is quite artificial.

•  Placing of monochlamydeae after gamopetalae does not seem to be natural.
•  Some of the closely related species are placed distantly while distant species are placed close to each other.
• Certain families of monochlamydeae are closely related to families in polypetalae, e.g. Chenopodiaceae and Caryophyllaceae.
•  Advanced families, such as Orchiadaceae have been considered primitive in this system by placing them in the beginning. Placing of Orchidaceae in the beginning of monocotyledons is unnatural as it is one of the most advanced families of monocots.
• Similarly, Compositae (Asteraceae) has been placed near the beginning of gamopetalae which is quite unnatural.
• Liliaceae and Amaryllidaceae were kept apart merely on the basis of characters of ovary though they are very closely related.
• There were no phylogenetic considerations