Classification of Angiosperms (Explained With Diagram)

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Many systems of classification of angiosperms have been proposed by many taxonomists from time to time.

It can be divided into three broad categories:

i. Artificial Systems based on superficial features.

ii. Natural systems based on form relationships.

iii. Phylogenetic systems based on evolutionary and genetic relationships.

[I] Artificial systems:

These systems of classification were based on one or few morphological characters.

In some systems habit and habitat have been considered for this purpose:

(i) Theophrastus (370 – 285 BC), a Greek philosopher, in his book Historia Plantarum classified about 480 plants into four groups on the basis of their habit-herbs, undershrub’s, shrubs and trees.

(ii) Otto Brunfels (1464-1534) for the first time classified plants into Perfecti and Imperfect based on the presence or absence of flowers.

(iii) Andrea Caesalpino (1519-1603), an Italian botanist and Physician, in his book De Plant is classified about 1500 plants on the basis of habit (herbs and trees) and then subdivided them on the basis of fruits and seeds which they produced.

(iv) Joseph Pitton de Tournefort (1656-1708), a French Botanist and Physician, in his book Elements de botanique divided flowering plants into herbs and trees. He further sub-divided them on the basis of several morphological features, such as petal bearing or non-petal bearing flowers, simple or compound flowers (now referred to as polypetalous and gamopetalous), flowers regular or irregular.

(v) John Ray (1627-1705), an English naturalist, in his book Methods Plantarum Nova (1682) for the first time divided herbs, shrubs and trees into dicotyledons and monocotyledons on the basis of two or one cotyledons. Broadly he divided the plants as under:

(vi) Carolus Linnaeus (also called Carl Linnaeus) (1707-1778), a Swedish naturalist in his book Species Plantarum (1753) classified 7300 species of plants into 24 classes, mainly on the basis of number, union and length of stamens. For example, he described the classes as Monandria (1 Stamen), Diandria (with 2 stamens), Triandria (with 3 stamens) and so on Polyandria (with 20 or more stamens). This system is commonly known as sexual system of classification.

[II] Natural Systems:

In these systems the organisms are classified on the basis of their natural affinities (i.e. the basic similarities in the morphology) rather than on a single character for determining the affinities.

(i) A.L. de Jussieu (1748-1836) published a natural system of classification of plants in his book Genera Plantarum secundus ordines Naturales Disposita. He grouped all plants into 15 classes which were further divided into 100 orders (now called families).

He divided the plants into three main groups, i.e. Acotyledones (plants without cotyledons, e.g., algae, fungi, mossesetc.) Monocotyledones (plants with one cotyledon) and Dicotyledones (plants with two Cotyledons). He mainly emphasized on the number of cotyledons and their presence or absence, number of petals and their presence or absence, and position of stamens.

(ii) A.P. de Candolie (1778-1841) a French botanist published Theorie elementaire de la Boanique in which he classified about 58,000 species into 161 families. He divided plants into two major groups i.e. cellulares (non-vascular plants) and vasculares (vascular plants).

(iii) Bentham and Hooker’s Classification:

The most important and the last of the natural systems of classification of seed plants was proposed by two British taxonomists George Bentham (1800-1884), a self trained botanist, and Joseph Dalton Hooker (1817-1911), the first director of the Royal Botanical Garden, Kew (England).

They recorded precise description of most of the plants known at that time. Their monumental work which took about quarter of a century for completion was described in three volumes of Genera Plantarum, published in Latin during July 1862 and April 1883. Bentham and Hooker’s system of classification is still used and followed in several herbaria of the world. It is supposed to be the best system for the students to identify plants in the laboratory.

Salient Features of Bentham and Hooker’s system:

1. It is a classification of only the “seed plants” or phanerogams.

2. They described 97,205 species of seed plants belonging to 7,569 genera of202 families starting from Ranunculaceae up to Gramineae.

3. They classified all the seed plants into 3 groups or classes i.e. Dicotyledons (165 families), gymnosperms (3 families) and monocotyledons (34 families).

4. They included disputed orders among Ordines Anomali which they could not place satisfactorily.

5. Monocotyledons were described after the dicotyledones.

6. The dicotyledons were divided into 3 Divisions (Polypetalae, Gamopetalae and Monochlamydeae) and 14 series. Each series again divided into cohorts (modern orders) and cohorts into orders (modern families).

7. The authors did not mention anything about the origin of the angiosperms.

8. Creation of the Disciflorae, a taxon not described by the earlier taxonomists.

9. Among the Monochlamydeae, major taxa, like the series, were divided on the basis of terrestrial and aquatic habits.

10. Polypetalae carries 82 families, 2610 genera & 31,874 species. Gamopetalae carries 45 families 2619 genera & 34,556 species. Monochlamydae includes 36 families, 801 genera & 11,784 species. Similarly Monocotyledons consist 34 families, 1495 genera and 18,576 species.

Merits of Bentham and Hooker’s System:

1. 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.

2. The system is highly practical and is useful to students of systematic botany for easy identification of species.

3. The flora describes geographical distribution of species and genera.

4. The generic descriptions are complete, accurate and based on direct observations.

5. Larger genera have been divided into sub genera, each with specific number of species.

6. Dicots begin with the order Ranales which are now universally considered as to be the most primitive angiosperms.

7. Placing of monocots after the dicot is again a natural one and according to evolutionary trends.

8. The placing of series disciflorae in between thalami florae and calyciflorae is quite natural.

9. The placing of gamopetalae after polypetalae is justified since union of petalsis considered to be an advanced feature over the free condition.

Demerits of Bentham and Hooker’s System:

1. Keeping gymnosperms in between dicots and monocots is anomalous.

2. Subclass monochlamydeae is quite artificial.

3. Placing of monochlamydeae after gamopetalae does not seem to be natural.

4. Some of the closely related species are placed distantly while distant species are placed close to each other.

5. Certain families of monochlamydeae are closely related to families in polypetalae, e.g. Chenopodiaceae and Caryophyllaceae.

6. 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.

7. Liliaceae and Amaryllidaceae were kept apart merely on the basis of characters of ovary though they are very closely related.

8. There were no phylogenetic considerations

Phylogenetic Systems:

Classification based on evolutionary features is known as phylogenetic system.

Important phylogenetic systems are:

Engler and Prantl (1884-1930):

They published detailed classification in 23 volumes of “Die Naturlichen Pflanzenfamilien”. They arranged flowering plants according to increasing complexity of their floral morphology. They considered monocot’ primitive than dicots.

John Hutchinson (1884-1972):

He was author of “Families of flowering plants” (in 2 volumes) and director of Royal Botanic Garden at Kew, England. Classification proposed by him was based on 24 principles. This system is mostly followed. They placed monocots after dictos.