Angiosperms

Comparision of Dicotyledons and Monocotyledons

Character Dicotyledons Monocotyledons

Morphology Tap roots Adventitious roots

Recticulate venation Parallel venation

Tetra-or pentamerous flowers Trimerous flowers

Anatomy Vascular bundles of stem Vascular bundles of stem are scattered in

arranged in a ring the ground tissue

2-6 vascular bundles in root Many vascular bundles in root

In stem, each vascular bundle is open In stem, each vascular bundle is closed

Secondary growth occurs Secondary growth does not occur

Classification of Angiosperms

After Linnaeus work on classification of plants, the taxonomists realized the necessity of information on natural history of vegetation and the affinities at various levels of hierarchy. The studies were aimed at evolving natural systems of classification of plants.

One of the most well known natural systems of classification of angiosperms was proposed by two British systematists, George Bentham (1800–1884) and Joseph Dalton Hooker (1817–1911). They recorded precise description of most of the plants known at that time and classified them according to their system of classification, which also included Gymnosperms. This monumental work which took about quarter of a century for compilation, was published in three volumes of Genera Plantarum (1883).

Outline of Bentham and Hooker’s System of Classification

The phanerogams, based on their morphological characters, such as leaf arrangement and venation pattern; number of members in floral whorls, like calyx, corolla, androecium and gynoecium number of cotyledons in the seed and seeds with or without cover, where divided into 3 classes–Dicotyledons (165 families); Gymnosperms (3 families ) and Monocotyledons (34 families). Total of 202 families were classified.

Class I : Dicotyledons (Gk. Di = two, Kotyledon = seed leaf)

Radicle of seed forms persistent primary root.

Perennial stems, exhibiting secondary growth, possess concentric arrangement of tissues.

The vascular bundles of stem are arranged in the form of a ring (eustele) and open i.e., cambium is present between the xylem and phloem.

The leaves exhibit reticulate (net-like) venation and show varied arrangement like alternate, spiral or whorled.

The flowers are tetramerous or pentamerous having four or five members in the various floral whorls, respectively.

The embryo of seeds of dicotyledons are with two cotyledons as the name indicates.

Bentham and Hooker further divided dicotyledons into three sub-classes, namely Polypetalae, Gamopetalae and Monochalamydae on the basis of the number and nature of floral leaves. The three sub-classes have been further divided into series considering mainly the position of ovary with respect to the other floral parts. Series have been further categorized into ‘cohorts’ (equivalent to orders in other systems) and ‘orders’ (equivalent to modern families).

A brief account of the main characters used by them to distinguish these sub-classes and series of dicotyledons is as follows.

(a) Sub-class Polypetalae (Gk. Poly = free, petalae = petals)

The perinath is in two whorls, i.e., the sepals and petals are distinct and form two separate whorls.

The corolla has 4-5 petals in a whorl which are free from one another.

Carples often numerous, free or united.

It has three series–Thalamiflorae, Disciflorae and Calyciflorae.

Series (i) Thalamiflorae

Flowers are hypogynous and ovary is superior. No disc is present below ovary.

Series (ii) Disciflorae

Flowers are hypogynous with superior ovary.

Series (iii) Calyciflorae

Flowers are perigynous or epigynous and the ovary is mostly inferior.

(b) Sub-class Gamopetalae (Gk. Gamos = fused, petalae = petals)

—The perianth is in two whorls, i.e., sepals and petals are distinct.

—The corolla comprises 4-5 petals which are either partially or completely united to one another (gamopetalous).

Series (i) Inferae

Flower shows epigynous condition and ovary is inferior.

The stamens are equal to the number of petals.

Series (ii) Heteromerae

Flower is hypogynous and ovary is superior with more than two carpels being united (syncarpous).

Stamens are either equal or twice the number of petals and free from corolla

Series (iii) Bicarpellatae

Flower is hypogynous with superior ovary

Carpels usually two or more and united.

(c) Sub-class Monochlamydeae (Gk. Monos = one, chlamydeae = Seriate or whorl)

—Flowers are incomplete

—Flowers usually possess only one whorl of perianth which is sepaloid. Sepals and petals are not distinct.

Series (i) Curvembryeae

Flowers are unisexual

Stamens are generally equal to the number of tepals.

Embryo is curved around the endosperm.

Ovule is usually one.

Series (ii) Multiovulate aquaticae

Submerged aquatic plants with syncarpous ovary having many ovules

Series (iii) Multiovulate terrestres

Terrestrial plants with syncarpous ovary having many ovules.

Series (iv) Microembryeae

Embryo minute, endospermic seeds, free or united carpels, one ovule per ovary.

Series (v) Daphanales

Ovary usually with one carpel, single ovule.

Series (vi) Achlamydosporae

Ovary usually inferior, unilocular with 1–3 ovules.

Series (vii) Unisexuales

Flowers unisexual, ovules 1–2, in each ovary.

Series (viii) Ordines anomali

Families of uncertain relationship but close to series unsexuales.

Class II : Gymnospermae

This class has been placed in between the dicotyledons and monocotyledons.

Perianth absent, ovules naked, cotyledons two or more, etc.

It includes 3 families–Cycadaceae, Coniferae and Gnetaceae.

Class III : Monocotyledons (Gk. Monos = single, Kotyledon = seed leaf)

Radicle of seed forms short – lived primary root, which is replaced by adventitious root.

Root is with pith and several vascular bundles (6–20).

Stem contains scattered vascular bundles present in uniform ground tissue. Vascular boundless are closed.

Secondary growth is absent.

The leaves are simple with parallel venation.

The flowers are trimerous having 3 members in each floral whorl.

The seeds possess only one colyledon.

On the basis of nature of perianth and condition of the ovary, the monocotyledons have been divided into seven series. These are :

Series (i) Microspermae

Epigynous flowers with inferior unilocular ovary having three parietal placentae, rarely trilocular with axile placentation

The seeds are small and exalbuminous (non-endospermic).

Series (ii) Epigynae

Epigynous flower with inferior ovary.

Series (iii) Coronariae

Hypogynous flower with superior ovary.

Series (iv) Calycineae

Hypogynous flower with superior ovary

Series (v) Nudiflorae

Hypogynous flower with superior ovary.

Perianth is reduced to hairs and scales or absent

Series (vi) Apocarpae

Hypogynous flower with superior ovary

Perianth shows either two whorls or sometimes it may be totally absent

Carpels free (apocarpous)

Series (vii) Glumaceae

Flowers are sessile and arranged in spikelets.

Perianth is either absent or scaly and highly reduced.

Ovary is unilocular and has a single ovule.

Merits of Bentham and Hooker’s System of Classification

Bentham and Hooker’s system of classification of flowering plants is based on the actual examination of specimens and preserved herbarium sheets.

Description is quite accurate not only for the individual species but also for the families and the genera described in the system.

The descriptions of plant species are easy to follow and are of practical utility for identification of species up to the family level.

Although their system represents a natural an not a phylogenetic approach, some aspects of the system does show affinity with modern concept of evolution. For example, the order Ranales is placed in the beginning of the arrangement, and this has now been established amongst the most primitive orders on the basis of recent taxonomic findings.

The placement of monocots after the dicots (from which they have been derived) also appears to be in accordance with the evolutionary trends.

It was the first natural system of classification of flowering plants.

Drawbacks

Placing of Gymospermae in between dicots and monocots has not been acceptable and satisfactory.

The system also does not take into account several important floral characters and neglects any evolutionary considerations of genus, family and order.

Monochlamydeae as a sub-class has been found to be artificial. It has plants related to Polypetalae.

The placement of families like Asteraceae in the beginning of Gamopetalae and Orchidaceae in Microspermae do not justify the recent evolutionary approach.

Interestingly, Darwin’s study was available to Bentham and Hooker but somehow they did not consider evolutionary trends in their classification to rectify these lacunae.