Principles Of Angiosperm Classification Essay

Hohn Hutchinson, a British botanist and formerly the office holder of museum of royal botanical garden, kew, England has given the classification of plants based on the principles followed by Bessey his classification was published in his famous book families of flowering plants in two volumes. Volume-II published in 1926 dealing with dicotyledons and volume-II published in 1934 on monocotyledons.

The system of classification was revised in British flowering plants (1948) and again the second edition of the families of flowering plants (1959) It is underlying principles are more like the besseeyan syatem than the englerian system.

This is phyogenetic system is based on the assumption that:

i) Plants with petals and sepals associated with other floral and anatomical character are primitive and more ancient than the plants without sepals.

ii) Free floral parts are more primitive than the agnate or connate parts.

iii) Spiral arrangement of floral parts sepals petals and stamens are more primitive than cyclic arrangement.

iv) Hermaphrodite condition and free stamens are primitive over the unisexual flowers and connate stamens.

v) A regular or actinomerphic flower is primitive with zygomorphic flowers.

vi) Solitary flower is more primitive than the inflorescenced flowers.

vii) Hypogyny is more primitive than epigyny and perigyny conditions.

viii) A flower with indefinite number of floral parts is primitive over few numbers of floral parts.

ix) Complete flower are primitive than incomplete flowers.

Principle’s of Hutchinon’s Classification:

Hutchinos’s classification is based on flowing principles which has been supported by others. The principles are outlined under 24 point which are as follows:

1. The evolution is both upward and downward, the former tending towards preservation and the later to their reduction and degeneration of characters.

2. Evolution does not necessarily involve call organs at one time or simultaneously.

3. Aquatic plants are derived from terrestrial and saprophytes parasites epiphytes are more recent.

4. Trees and shrubs are more primitive than herbs.

5. Perennials are more primitive than biennials and annuals.

6. Plants with vascular bundles arranged in a ring are more primitive those in which vascular bundlers are scattered.

7. Spiral phyllotaxy is primitive than whorled and opposite phyllotaxy.

8. Dioecious plants are more advanced than bisexual flowers.

9. Unisexual flower are more primitive than bisexual flowers.

10. Petalod flowers are more primitive than bisexual flowers.

11. Gamopetally is more advanced than polypetalae.

12. Zygomorphic flower are more advanced than actinomorphic flowers.

13. Hypogyny is more primitive than perigyny and epigyny.

14. Simple leaves are more primitive than compound leaves.

15. Solitary flower is more primitive than infloresceneed flowers.

16. Spirally imbricate floral parts are more primitive than whorled and valvate arrangement.

17. Apocarpy is more primitive than syncarpy.

18. Polycarpy preceedes oligocarpy.

19. Endospermic seeds with small embryo are more primitive than non endospermic seeds with large embryo.

20. Flowers with numerous stamens are more primitive than those with fewer stamens.

21. Free stamens precede the fused ones.

22. Aggregate fruits are more evolved than single fruit and capsule preceedes berry or drupe.

23. Parietal placentation is more primitive than axial and free central placentation.

24. Trees or arboreal habit are more primitive than climbers are twiners in any one family or genus.

A) Sub-phylum – I – Dicotyledonae

i) Embryo with two cotyledons

ii) Tap root system

iii) Reticulate veination of leaves

iv) Pentamerous floral parts

This sub phylum further divided into two divisions.

Division (I) : Lignosae

i) Trees and shrubs, woody plants.

ii) It includes 54 orders which begins with magnoliales and ends with verbenales.

Order – 1 – Magoliales – (Magnoniaceae)

Order – 2 – Anonales – (Annonaceae)

Order – 6 – Rosales – (Rosaeae)

Order – 7 – Leguminales – (Mimosae, Fabaceae)

Order – 30 – Cucurbitales – (Cucubitaceae)

Order – 33 – Maivales – (Malvaceae)

Order – 52 – Rubiales – (Rubiaceae)

Order – 54 – Verbenales – (Verbenaceae)

Division –II Herbaceae:

i) It includes all herbaceous plants.

ii) Plants may be annuals or binnials or perennials.

iii) This division includes 28 orders which start with ranales and ends with lamiales.

Order – 55 – Ranales – (Rannunculaceae, Nymphaceae)

Order – 59 – Rhoedales – (Papaveraceae)

Order – 60 – Cruciales (Pareitales ) - Cruciferae

Order – 72 – Umbellales – Umbelliferal (Apiaceae)

Order – 76 – Asterales – Compositae (Asteraceae)

Order – 77 – Solanales – (Solanaceae, convolvulaceae)

Order – 78 – Personales – (Acanthaceae, Scorephulariaceae)

Order – 82 – Lamiales – (Labiatae)

SUB –PHYLUM – 2- Monocoty ledonae:

i) Embryo with one cotyledon

ii) Fibrous adventitious root system

iii) Parallel veinationof leaves

iv) Closed and scattered vasular bundles

v) Trimerous flowers

This sub – phylum divided into three divisions

Division – I – Calyciferae:

i) Flowers with distinct calyx and corolla

ii) Sepals green in colour, petals colured variously.

iii) It includes 12 orders, starting with butamales and ends with zingiberales.

Order – 1 – Butamales – (Butamaceae)

Order – 8 – Commelinales – (Commelinaceae)

Order 12 – Zingiberales – (Zingiberaceae; musaceae)

Division – II – Corolliferae:

i) Both calyx and corolla are not distinct in colourantion.

ii) Sepals may be coloured other than green.

iii) Petlas and sepals present in different whorl.

It includes 14 orders begins with liliales and end with orchidales

Order – 13 – Liliales – Liliaceae

Order – 15 Aeales – Araceae

Order – 17 – Amaryllidales – Amaryllidaceae

Order – 21 – Palmales - palmales – palmae (Arecaceae)

Order – 26 – Orchidales – Orchidaceae

Division – III – Glumiflorae

i) Flower with reduced perianth

ii) Neither sepal not petal is distince and reduced to membanous lodicules.

This includes 3 orders and six familes

Order – 27 – Juncales – Juncaceae

Order – 28 – Cyperales – Cyperaceae

Order – 29 – Graminales – Graminae (Poaceae)

Merits of Hutchinson’s Classification:

i) It is most phylogenetic system of classification based on natural characteristic of plants.

ii) This system is based on evolutionary tendencies and interrelationship among angiospermic plants.

iii) Magnoliales representing arborescent plants and ranales representing herbaceaous plants which shows parallel evolution.

iv) Several big orders have been broken into small orders like rosales, paritales, malveles, leguminales etc.

v) Many families have been raised to the rank of orders, leguminosae famile raised to order leguminales.

vi) Reshuffiling or genera and families

vii) Origin of monocots from dicots and placement of first dicot and then monocot families is correct in all respect.

viii) Placing of gymnosperms before angiosperms in flowering plants.


Advertisements:

Here is a compilation of essays on the ‘Taxonomy of Angiosperms’ for class 11 and 12. Find paragraphs, long and short essays on the ‘Taxonomy of Angiosperms’ especially written for school and college students.

Essay on the Taxonomy of Angiosperms


Essay Contents:
  1. Essay on the Definition of Taxonomy
  2. Essay on Taxonomy and Systematics
  3. Essay on the Aims of Taxonomy
  4. Essay on the Principles of Taxonomy
  5. Essay on the Phases of Taxonomy
  6. Essay on Key to Identify Taxon


Essay # 1. Definition of Taxonomy:

Taxonomy (Greek, taxis = arrangement, nomous = law, rule) means the “arrangement by rules” or “lawful arrangement”. The botanists agreed on a nomenclature — arrangement of plants into convenient groups for the proper and easy handling of vast number of plants in a sui­table process following certain principles or rules.

The term Taxonomy was first introduced in plant science by A. P. de Candolle (1813), a French botanist, as the theory of plant classification. Later on and till date, it is considered as a part of plant science which includes identification, nomencla­ture and classification.

According to G. H. M. Lawrence (1955) “Taxonomy is a science which includes identification, nomenclature and classi­fication of objects and is usually restricted to objects of biological origin”. When the taxonomy is concerned with plants, it is referred to as systematic botany.

In respect to plant, both the terms are consi­dered as synonym but are not accepted by all. Both the terms are often used variously, like chemotaxonomy (based on chemical content), cytotaxonomy (based on chromosome structure and numbers), biosystematics (systematics of living organisms), etc.

Initially, the taxonomy was based on a few macro morphological informations like habit, sex organ, etc. i.e., the artificial system. Later on, the classification was developed after considering many morphological characteristics, the natural systems. During the post-Darwinian period the taxonomy was based on evolutionary relation­ships i.e., the phylogenetic systems.

However, modern taxonomy is not restricted on morphology only. It depends on other branches of botany for good information like anatomy, cytology, physiology, phytochemistry, genetics, embryology, ecology etc.

The term ‘taxon’ was first introduced by Adolf Meyer (1926), a German biologist, for the animal groups. Later, in 1948, it was Herman J. Lam who proposed the term in plant science and it was accepted in the Seventh International Congress (1950). The term “taxon” indicates a taxonomic group like a variety, species, genus or any higher group.


Essay # 2. Taxonomy and Systematics:

There is no harmonic opinion in the mea­ning and use of both the terms taxonomy and systematics. Mason (1950) considered taxonomy as a vast field in biological science including four main streams. These are systematics i.e., comparative study of the organisms, taxonomic systems, nomenclature, and documentation.

On the other hand, authors like Simpson (1961), Heywood (1967), Ross (1974) and many others have treated systematics as a field which covers the study of diversity, differentiation and the relationship that exists among the organisms. According to them, taxonomy is a part of syste­matics.

According to Solsbrig (1966), taxonomy includes nomenclature and classification but tilt massively on systematics for its concept. Later, Small (1989) reviewed the above and defined systematics as ‘The science of organisation and pattern of heritable relationships among the kinds and diversity of organisms’ and, on the other hand, taxonomy as ‘a very substantial but imprecisely separated part of systematics, that is especially concerned with the production of for­mal classifications of living things on the basis of genetic relationships’.

Finally, it can be concluded that due to loose and interchangeable use in past, a proper expla­nation at present is very difficult. The literal meaning of Taxonomy in Greek is putting in order or lawful arrangement and Systematics means putting together. Lam (1959) and Turrill (1964) boldly expressed their opinion to treat these terms synonymously and later it was followed by many others.


Essay # 3. Aims of Taxonomy:

The aim of taxonomy includes three aspects I. Identification, II. Nomenclature, and III. Classi­fication of plants. But to develop the construction of the above, the taxonomists approach in diffe­rent ways.

These are of two types:

1. Empirical approach, and

2. Interpretative approach.

1. Empirical Approach:

This type of approach is based on the thorough observation and characterisation of the organism which finally leads to the construction of a classi­fication. Due to thorough observation of many characteristics of the specimens and development of a classification, this type becomes popularly acceptable. This type of approach was developed during pre- Darwinian period.

2. Interpretative Approach:

This type of approach of classification is based on the interpretation of evolution of a taxon. This is called phylogenetic classification, which developed during post-Darwinian period (i.e., after the publication of ‘The Origin of Species by Means of Natural Selection’ (1859). This type of classification needs the data from the past history of a taxon.

The modern taxonomy made an attempt to merge the above two approaches.

The aims of modern taxonomy are:

1. To supply an appropriate method of identi­fication;

2. To contribute classification based on natural affinities of the specimens;

3. To contribute a catalogue of taxa by studying the different flora;

4. To trace the evolution after proper observa­tion and interpretation;

5. To supply an integrating and synthetic role to maintain the relationship among the diffe­rent biological fields.


Essay # 4. Principles of Taxonomy:

Taxonomy is the oldest branch of Botany and was practiced in many countries like India, Greece, Rome, China, England from long back. During early part, taxonomy was mainly aimed to develop some convenient methods of classifi­cation. One of the earliest known Indian works dealing with plants in a scientific manner is Vrikshayurveda.

Later, it was completed by Parasar before Christ. This book contained a classification based on comparative morphology of plants. The artificial system reached its climax in Carolus Linnaeus (1707-1778), the Father of modern taxonomy. Later, Michel Adanson (1727-1806) was perhaps the first person to reject all artificial systems in favour of natural system.

He proposed the idea that all characte­ristics are important in grouping of plants, which in the recent years came as Numerical Taxonomy. At that time, it was very difficult to consider all the characteristics of a plant in grouping of plants. For the above problem, importance was given on flower characteristics. In this system, a group of characteristics are con­sidered in grouping of plants.

The natural system reached its climax in George Bentham and Joseph Dalton Hooker in their book ‘Genera Plantarum’ in July 1862 and the last part in April 1883. Before the publication of their book, the phylogenetic concept came in focus after the publication of Darwin’s concept of ‘Origin of Species’ (1859).

This system is still continuing with eminent personalities like Engler (1886-1892), Hutchinson (1926-1973), Takhtajan (1969, 1980), Cronquist (1968, 1981) and many others. Thus, the principles have evolved through time.

However, Cronquist (1968) has formulated certain basic principles of Taxonomy in “Evolution and Classification of Flowering Plants”:

1. Taxa are properly established on the basis of multiple correlations of characters.

2. Taxonomic importance of a character is determined by how well it correlates with other characters. This means that the taxonomic importance of a character is determined by a posteriori rather than a priori.

3. An important feature of taxonomy is its predative value.

The most important principle of taxonomy is the multiple correlations of characters. To find out correlation of taxa with others, single character should not be considered, it should always be considered along with other characters.

The selected characters should show maxi­mum correlation with other characters. The significance of a selected characters depends on the degree of correlation. Characters with no distinct correlation with other characters is often accepted as an anomalous one and usually it is not important from the taxonomic point of view.

Thus, the regular flower (Scoparia dulcis) of Scrophulariaceae, exstipulate leaves (Corculum leptopus) of Polygonaceae, zygomorphic flower (Delphinium ajacis) of Ranunculaceae and many such exceptional characters are taken as anomalous, because they are not correlated with common characters of taxa.

The above exam­ples clearly indicate that they are not significant taxonomically, but are important from their identification. But the anomalous characters often help the taxa with other groups of plants where these same characters are normally available.


Essay # 5. Phases of Taxonomy:

The taxonomy includes identification, nomenclature and classification.

According to Davis and Heywood (1963) the classification is achieved in four (4) different consecutive phases:

1. Pioneer Phase:

This is also called explora­tory phase. In this phase, different plants of a taxon have to be collected throughout the world and identified.

2. Consolidation Phase:

In this phase, plant is studied both in field as well in the herbarium and a range of variations are evaluated. The new group or groups, if discovered, are fully described. Thus a monograph is obtained.

3. Biosystematic or Experimental Phase:

This phase deals with much more detailed know­ledge of a taxon based on the above two phases along with the geographical varia­tion, physiological characteristics, cytological characteristics, etc. The progress of this phase is very slow and it requires a team work rather than individual effort. The suc­cess in this phase is remarkable in some countries (U.K., U.S.A. etc.) but at and below the generic level.

4. Encyclopaedic or Holotaxonomic Phase:

This phase is a coordination of the above three phases.

According to Turrill (1938) the classification is of two types:

1. Alpha Classification:

The first two phases i.e., pioneer and consolidation phases are based on gross morphological characteristics and the classifications of these phases are called Alpha classification or Alpha taxo­nomy.

2. Omega Classification:

The last two phases, i.e., biosystematic and encyclopaedic phase, are based on the data collected from fields, herbarium, laboratory and library and then properly analysed with the help of compu­ter. The classifications of plants of these phases are called Omega classification or Omega taxonomy.


Essay # 6. Key to Identify Taxon:

1. Punched Card Key:

It consists of cards of appropriate size where the names of all taxa like families, genera or species for which the key have to be prepared should be printed on each. Each card has to be numbered serially. Each card should be printed with any one characteristic at any corner. All the taxa having the characteristics should be indicated by a hole in front of their name and others remain without it.

Many set of cards have to be developed, one set for each characteristics. During identification of a specimen, select the cards showing charac­teristics possessed by the specimens. The charac­teristic combination shown by the specimen will permit only one hole in the set of cards chosen. The particular sample of specimen is then assigned to that family to which the cards indi­cate the hole.

Advantages of Punched Card Key:

The punched card key is tackled by the beginners like college stu­dents, who find more interest in taxonomy. But the system is very costly for the cards, printing of names etc.

2. Dichotomous Key:

The dichotomous key consists of the following characteristics:

i. It consists of a pair of contrasting cha­racteristics i.e., cuplets and each state­ment of a couplet is called a lead.

ii. Lead should be numbered.

iii. Both the leads of a couplet begins with same word as far as possible.

iv. The characteristics used in the key should be easily observable and con­trast.

v. The quantitive characteristics are usu­ally preferred than qualitative characte­ristics.

vi. More than one contrasting characteris­tics should be selected to differentiate the closely related taxa with overlap­ping characteristics.

The dichotomous key is of two types:

a. Indented Key:

In this type the collateral leads of a couplet are arranged in yokes and leads are identified by a figure or letter. Successive yokes are arranged one above the other.

An example of dichotomous key with indented or yoked leads is given:

b. Bracketed Keys:

In this type both the leads of each couplet always remain together.

Advantage of Dichotomous Key:

This key is very suitable for the taxonomists and is far less costly. The indented key gives a visual representa­tion of the group, thereby the user can read­ily obtain a picture about the taxon. But if the key is too large then the leads of a cou­plet get widely separated from each other, on the other hand, in bracketed or parallel leads, the advantage lies that the couplets occur side by side.

Good knowledge of characteristic diffe­rences and sound knowledge of the flora is essential in the construction of a key.


Categories: 1

0 Replies to “Principles Of Angiosperm Classification Essay”

Leave a comment

L'indirizzo email non verrà pubblicato. I campi obbligatori sono contrassegnati *