Systems of Classification – Artificial, Natural and Phylogenetic

Types Of Classification

In the study of botany, the classification of plants is a fundamental task aimed at organizing the immense diversity of plant species into manageable categories. Due to the vast number of plant species and their complex relationships, it is essential to classify them into hierarchical groups based on similarities and differences. This classification process is generally categorized into three main types: artificial, natural, and phylogenetic. Each of these systems has distinct methodologies and purposes.

1. Artificial Classification

• Overview: Artificial classification systems are designed primarily for convenience. They utilize easily observable or arbitrary characteristics to group plants, often without regard for their evolutionary relationships. This type of classification simplifies the study and identification of plants but may not accurately reflect their natural relationships.
• Key Characteristics:
  • Arbitrary Criteria: Uses specific, easily identifiable traits, such as the number of stamens, to classify plants.
  • Historical Example: Linnaeus’s classification system is a notable example, where he categorized plants based on the number of stamens and pistils. This system facilitated plant identification and study but did not account for evolutionary relationships.
  • Functional Emphasis: Provides a practical framework for organizing plant species for study and reference, though it may not reflect true botanical relationships.

2. Natural Classification

• Overview: Natural classification systems aim to reflect the relationships among plants as they occur in nature. This approach groups plants based on shared evolutionary traits and genetic similarities, presenting a more accurate representation of plant relationships.
• Key Characteristics:
  • Reflective of Natural Relationships: Plants are classified based on their natural relationships and shared ancestry.
  • Example: The classification system developed by Bentham and Hooker exemplifies natural classification. This system arranges plants into hierarchical groups that mirror their natural relationships and evolutionary history.
  • Functional Emphasis: Aims to group plants in a way that represents their true biological and evolutionary connections.

3. Phylogenetic Classification

• Overview: Phylogenetic classification systems classify plants according to their evolutionary history and developmental lineage. This system seeks to reflect the evolutionary relationships and common ancestors of different plant groups.
• Key Characteristics:
  • Evolutionary Focus: Plants are classified based on their evolutionary development and genealogical relationships.
  • Challenges: Due to incomplete fossil records and the complexity of plant evolution, no phylogenetic system is perfect. However, phylogenetic systems are often based on natural classification principles.
  • Examples: Prominent examples include the systems proposed by Engler and Prantl, Hutchinson, and Takhtajan. These systems use evolutionary data to categorize plants, aiming to reflect their historical and genetic lineage.
  • Functional Emphasis: Provides insights into the evolutionary processes that have shaped plant diversity and relationships over time.

A. Artificial System Of Classification

The artificial system of classification, primarily associated with Carolus Linnaeus, represents a historical approach to categorizing plants based on distinct, observable characteristics. Linnaeus, a Swedish botanist born in 1707, developed this system during his extensive exploration and collection of plant specimens. His methodology, outlined in seminal works such as Systema Naturae (1735) and Species Plantarum (1753), was instrumental in laying the groundwork for modern taxonomy, despite its inherent limitations.

Basis of Classification

Linnaeus’s classification system is referred to as “artificial” or “sexual” because it relies predominantly on the reproductive structures of plants. This method categorizes plants according to the number, arrangement, and characteristics of stamens and carpels. Linnaeus’s approach is also termed “numerical classification” due to its reliance on the number of stamens. The key elements of Linnaeus’s system include:

1. Number of Stamens: Plants are classified based on the count of stamens they possess, ranging from one to more than twenty. For instance:
   • Monandria: One stamen (e.g., Canna, Salicornia)
   • Diandria: Two stamens (e.g., Olea, Veronica)
   • Triandria: Three stamens (e.g., various grasses)
2. Size of Stamen: The size of the stamens is another criterion used for classification, though it is less commonly emphasized in modern systems.
3. Cohesion of Filaments: This involves the fusion of filaments into bundles, with classifications such as:
   • Monadelphia: Stamens forming one bundle (e.g., Hibiscus)
   • Diadelphia: Stamens forming two bundles (e.g., Pisum)
4. Cohesion of Anthers: Classification based on the fusion of anthers, leading to categories like:
   • Syngenesia: Anthers fused but filaments free (e.g., many composites like Sunflower)
5. Distribution of Sex: Plants are classified according to the distribution of male and female flowers:
   • Monoecia: Both male and female flowers on the same plant (e.g., Cucurbita)
   • Dioecia: Male and female flowers on different plants (e.g., Papaya)
6. Plant Without Flowers: Certain plants are classified separately when they lack visible flowers, such as:
   • Cryptogamia: Plants like algae, fungi, and mosses

Outline of the System

Linnaeus’s system was detailed and extensive, dividing the plant kingdom into 24 classes based on the aforementioned characteristics. Each class contained genera and species, diagnosed in Linnaeus’s key works:

• Class I: Monandria (One stamen)
• Class II: Diandria (Two stamens)
• Class III: Triandria (Three stamens)
• Class IV: Tetrandria (Four stamens)
• Class V: Pentandria (Five stamens)
• Class VI: Hexandria (Six stamens)
• Class VII: Heptandria (Seven stamens)
• Class VIII: Octandria (Eight stamens)
• Class IX: Enneandria (Nine stamens)
• Class X: Decandria (Ten stamens)
• Class XI: Dodecandria (Eleven to Nineteen stamens)
• Class XII: Icosandria (Twenty or more stamens attached to calyx)
• Class XIII: Polyandria (Twenty or more stamens attached to receptacle)
• Class XIV: Didynamia (Two short and two long stamens)
• Class XV: Tetradynamia (Two short and four long stamens)
• Class XVI: Monadelphia (Stamens in one bundle)
• Class XVII: Diadelphia (Stamens in two bundles)
• Class XVIII: Polyadelphia (Stamens in many bundles)
• Class XIX: Syngenesia (Anthers fused)
• Class XX: Gynandria (Stamens adnate to the gynoecium)
• Class XXI: Monoecia (Male and female flowers on the same plant)
• Class XXII: Dioecia (Male and female flowers on different plants)
• Class XXIII: Polygamia (Male, female, and bisexual flowers on the same plant)
• Class XXIV: Cryptogamia (Concealed flowers)

Limitations and Legacy

Although Linnaeus’s system was groundbreaking, it was inherently artificial. It categorized plants based on a single characteristic—the number of stamens—ignoring broader evolutionary relationships. Linnaeus himself recognized the limitations of this system, noting that it was a practical tool for identification rather than a reflection of natural relationships among plants. He later advocated for a more holistic approach that would consider multiple characteristics.

Despite these limitations, Linnaeus’s contributions were foundational. His introduction of binomial nomenclature, which assigns each species a two-part Latin name, remains a cornerstone of botanical classification. His work paved the way for future taxonomic systems that integrate more comprehensive evolutionary concepts, as exemplified by Charles Darwin’s theory of evolution introduced a century later.

B. Natural System Of Classification

Following the contributions of Carolus Linnaeus, botanists began to seek a classification system that more accurately reflected the natural relationships among plants. This shift in focus led to the development of natural systems of classification, which aimed to group plants based on their evolutionary relationships and shared characteristics rather than on a single trait. One of the most influential natural classification systems was developed by George Bentham and Joseph Dalton Hooker.

1. Bentham and Hooker’s System

George Bentham (1800-1884) and Joseph Dalton Hooker (1817-1911) made significant advancements in plant classification with their work, “Genera Plantarum” (1862-1883). This monumental effort encompassed the names, descriptions, and classifications of approximately 97,205 species of seed plants known at the time. Their system marked a substantial departure from Linnaeus’s artificial classification by organizing plants based on natural affinities and overall similarities.

Classification Outline and Basis

Bentham and Hooker’s system was based on the principle of form-relationships, reflecting the natural relationships between plant genera. The major components of their classification include:

• Phanerogams (Seed Plants):
  • Classes:
    • Dicotyledons: Plants with two cotyledons in the seed, characterized by reticulate venation in leaves.
    • Gymnosperms: Plants with naked seeds, positioned between Dicots and Monocots.
    • Monocotyledons: Plants with one cotyledon in the seed and parallel venation in leaves.
• Dicotyledons Subdivisions:
  • Polypetalae: Plants with free petals.
  • Gamopetalae: Plants with fused petals.
  • Monochlamydeae: Plants lacking distinct petals but possessing a perianth.
  • Polypetalae Series:
    • Thalamiflorae: Series characterized by flowers with ovaries positioned in relation to the thalamus.
    • Disciflorae: Flowers with a well-developed disc and superior ovary.
    • Calyciflorae: Flowers with perigynous or epigynous conditions.
  • Gamopetalae Series:
    • Inferae: Bicarpellary, syncarpous flowers with an inferior ovary.
    • Heteromerae: Flowers with more than two carpels.
    • Bicarpellatae: Bicarpellary, syncarpous flowers with a superior ovary.
  • Monochlamydeae Series:
    • Divided into seven series, including Curvembryae and Ordines Anomali.
  • Class Gymnospermae:
    • Includes families like Cycadaceae, Coniferae, and Gnetaceae.
  • Class Monocotyledons:
    • Divided into seven series with Microspermae and Glumaceae as significant examples.

Advantages of Bentham and Hooker’s System

The natural classification system proposed by Bentham and Hooker has several notable advantages:

• Detailed Descriptions: Provides thorough descriptions of genera based on living specimens, which enhances the accuracy of taxonomic judgments.
• Practical Utility: Facilitates plant identification up to the family level, making it practical for various applications.
• Geographic Distribution: Includes information on the geographical distribution of genera.
• Updated Information: Regular updates through resources like Index Kewensis maintain the system’s relevance.
• Evolutionary Alignment: While not strictly phylogenetic, the system’s placement of Ranales and the position of Monocots align with modern evolutionary understanding.

Limitations of Bentham and Hooker’s System

Despite its contributions, Bentham and Hooker’s system has limitations:

• Gymnosperms Placement: The positioning of Gymnosperms between Dicots and Monocots lacks logical consistency.
• Polypetalae and Gamopetalae Confusion: The classification of families with mixed petal conditions creates inconsistencies.
• Monochlamydeae Classification: The separate classification of Monochlamydeae does not reflect the evolutionary relationships with closely related families.
• Orchidaceae Placement: Including Orchidaceae in the starting series of Monocots does not align with recent evolutionary trends.

C. Phylogenetic systems of classification

Phylogenetic systems of classification focus on organizing plants according to their evolutionary relationships. Given the complexities of plant evolution and the limitations of fossil records, these systems utilize available data to infer evolutionary connections. Key examples of phylogenetic classification systems include those proposed by Adolf Engler and Karl A.E. Prantl, Hutchinson, and Takhtajan. Each of these systems presents a unique approach to understanding plant relationships based on evolutionary principles.

1. Engler and Prantl System of Classification

Adolf Engler and Karl A.E. Prantl’s system of plant classification, first detailed in Engler’s “Syllabus” (1892) and subsequently expanded in their comprehensive work, “Die Naturlichen Pflanzenfamilien” (1887-1915), represents a significant development in botanical taxonomy. This system, later revised by Hans Melchior in 1964, is grounded in phylogenetic principles, aiming to reflect evolutionary relationships among plants.

Outline and Basis of Classification

1. Plant Kingdom Division:
   • Engler and Prantl’s system initially divided the plant kingdom into thirteen divisions, a number later expanded to sixteen by Melchior.
   • These divisions encompass a broad spectrum of plant groups including Bacteria, Algae, Fungi, Bryophytes, and Pteridophytes.
2. Subdivision Embryophyta Siphonogamia:
   • This subdivision is further divided based on the type of ovules:
     • Gymnospermae (Naked Ovules): Includes seven orders, starting with Cycadofilicales, regarded as primitive, and ending with Gnetales, which represent a more advanced group of Gymnosperms.
     • Angiospermae (Enclosed Ovules): Divided into two major groups:
       • Monocotyledonae: Characterized by leaf venation and flower structure, with 11 orders and 45 families initially, later expanded to 14 orders and 53 families. The sequence starts with Pandanales and ends with Microspermae.
       • Dicotyledonae: Further split into:
         • Archichlamydeae: Includes 33 orders and 206 families (expanded to 37 orders and 227 families in the revision). This group is defined by the condition of the perianth and petal arrangement, with families ranging from Verticillatae to Umbelliflorae.
         • Sympetalae (Metachlamydeae): Comprises 11 orders and 52 families (expanded to 64 families). The sequence starts with Ericales and concludes with Campanulales, with Asteraceae as the last family, noted for its high evolutionary status among Dicots.
3. Evolutionary Tendencies:
   • Although Engler and Prantl did not consider their system as fully phylogenetic, it largely reflects evolutionary tendencies. For instance, Gymnosperms are placed before Angiosperms, and highly evolved families like Asteraceae and Orchidaceae are positioned accordingly.

Advantages of Engler and Prantl System

1. Evolutionary Alignment:
   • The system accurately reflects the evolutionary sequence of plant groups, such as the placement of Gymnosperms before Angiosperms and the high evolutionary status of Asteraceae and Orchidaceae.
2. Comprehensive Scope:
   • It provides a broad framework for plant classification, covering all major groups from bacteria to angiosperms, and includes modern identification keys for various plant groups.
3. Inclusion of Anatomical Data:
   • For the first time, anatomical characteristics were considered in the classification, adding depth to the system’s approach.

Limitations of Engler and Prantl System

1. Primitive Flower Concept:
   • The classification’s emphasis on primitive flower types, such as unisexual and naked flowers, conflicts with contemporary evolutionary understanding.
2. Primitive Monocots:
   • The system erroneously categorizes Monocots as more primitive than Dicots, a view not aligned with modern evolutionary concepts.
3. Large Grouping Issues:
   • The amalgamation of Apetalous and Polypetalous families into Archichlamydeae creates an unwieldy classification, with an extensive number of orders and families.
4. Practical Utility:
   • The system’s complexity limits its practical application, reducing its effectiveness for practical taxonomy.

2. Hutchinson’s System of Classification

John Hutchinson, a distinguished British botanist, developed a comprehensive system of plant classification based on 24 phyletic dicta, or principles, which he outlined in his seminal work, The Families of Flowering Plants (1926-1934). This system, refined in later editions, categorizes plants into two primary volumes: one for dicotyledons and another for monocotyledons. The most recent revisions appeared in 1959 and 1971, reflecting ongoing developments in botanical science.

Outline and Basis of Classification

Hutchinson’s system is built upon 24 key principles regarding plant evolution and morphology. These principles can be summarized as follows:

1. Evolutionary Trends: Evolution tends to proceed upwards towards more complex forms and downwards towards simplification. For instance, the evolution of flowers often moves from complex, multi-part structures to simpler, reduced forms.
2. Organ Evolution: Not all plant organs evolve simultaneously. Different parts may advance or regress at different rates.
3. Consistency in Evolution: Once a specific evolutionary trend starts, it generally persists throughout the phylum. For example, a trend towards zygomorphy in flowers or the reduction in stamens.
4. Primitive vs. Advanced Forms: Trees and shrubs can be more primitive compared to herbaceous plants within the same family, while climbing plants may have evolved from these more primitive forms.
5. Habit Evolution: Perennials are considered more primitive than biennials, and annuals are derived from these more stable forms. Aquatic plants are derived from terrestrial ancestors.
6. Vascular Bundle Arrangement: Dicotyledons with collateral vascular bundles are considered more primitive compared to monocotyledons with scattered vascular bundles.
7. Leaf Arrangement: Spiral leaf arrangements are more primitive compared to opposite or whorled arrangements.
8. Leaf Structure: Simple leaves are generally more primitive than compound leaves.
9. Flower Structure: Unisexual flowers are considered more advanced than bisexual ones, and dioecious plants are more recent compared to monoecious ones.
10. Inflorescence Types: Solitary flowers are more primitive compared to those in inflorescences, with the highest forms being umbel and capitulum.
11. Floral Parts: Spirally imbricate floral parts are primitive compared to whorled and valvate arrangements.
12. Flower Complexity: Many-parted (polymerous) flowers are considered more primitive compared to few-parted (oligomerous) flowers.
13. Petal Presence: Apetalous flowers derive from petaliferous ones through reduction.
14. Petal Fusion: Polypetaly (free petals) is more primitive than gamopetaly (fused petals).
15. Symmetry: Actinomorphy (radial symmetry) is more primitive than zygomorphy (bilateral symmetry).
16. Gynoecium Structure: Hypogyny (superior ovary) is the most primitive, with perigyny and epigyny (inferior ovaries) being derived forms.
17. Carpel Fusion: Apocarpy (free carpels) is more primitive than syncarpy (fused carpels), though carpels may become free again in some cases.
18. Carpel Number: Polycarpy (many carpels) precedes oligocarpy (few carpels).
19. Seed Structure: Endospermic seeds with small embryos are older than non-endospermic seeds with large embryos.
20. Stamen Number: Indefinite stamens are more primitive compared to a few stamens.
21. Stamen Fusion: Separate stamens are more primitive than connate ones.
22. Fruit Type: Aggregate fruits are considered more advanced compared to single fruits, with capsules preceding berries or drupes.

Classification Framework of Hutchinson’s system

Based on these principles, Hutchinson’s classification system divides plants as follows:

1. Dicotyledons:
   • Lignosae (Woody): Includes 54 Orders and 251 Families, starting with Magnoliales and ending with Verbenales.
   • Herbaceae (Herbaceous): Includes 28 Orders and 100 Families, beginning with Ranales and concluding with Lamiales.
2. Monocotyledons:
   • Calyciferae: Starting with Butomales and ending with Zingiberales.
   • Corolliferae: Starting with Liliales and ending with Orchidales.
   • Glumiflorae: Starting with Juncales and ending with Graminales.

Advantages of Hutchinson’s system

Hutchinson’s system is noted for several strengths:

• High Standard of Description: Detailed descriptions and valuable features are provided for plant families.
• Practical Value: The classification system is practical and widely used.
• Phylogenetic Approach: Incorporates evolutionary relationships, particularly in monocots and dicots.
• Inclusion of Geographic Distribution: Considers the geographic distribution of genera.

Limitations of Hutchinson’s system

However, there are criticisms of Hutchinson’s system:

• Monophyletic Emphasis: Heavy reliance on the monophyletic origin of angiosperms from a hypothetical group, Proangiosperms.
• Family Separation: Some closely related families are separated due to the emphasis on herbaceous versus woody plants.
• Complexity: The system can be complex, with some classifications not aligning with modern botanical understanding.

3. Takhtajan’s System of Classification

Takhtajan’s system of plant classification represents a significant advancement in the phylogenetic classification of Angiosperms. Proposed by the distinguished Russian palaeobotanist, Armen Takhtajan, this system has undergone several revisions since its inception. The initial version was introduced in 1942, focusing primarily on the structural types of gynoecium and placentation. Takhtajan subsequently refined this system in 1954, with further updates published in 1958 and 1964. The most comprehensive revision came in 1980.

Key Features of Takhtajan’s System

1. Monophyletic Origin
   • Takhtajan’s classification is grounded in the concept that Angiosperms (Magnoliophyta) have a monophyletic origin. This implies that all flowering plants descend from a single ancestral group, which Takhtajan referred to as Proangiosperms.
2. Hierarchical Structure
   • The system divides Angiosperms into two primary classes: Magnoliopsida (Dicots) and Liliopsida (Monocots). This division reflects Takhtajan’s view that Dicots are more primitive compared to Monocots.

Outline of Takhtajan’s Classification System (1980)

Magnoliophyta (Angiosperms)

• Total: 419 Families
• Divided into:
  1. Magnoliopsida (Dicots)
     • Further divided into 7 subclasses:
       • Magnolidae
         • Super Order: Magnolianae
         • Order: Magnoliales
         • Family: Magnoliaceae
       • Ranunculidae
         • Order: Ranunculales
         • Family: Ranunculaceae
       • Hamamelididae
         • Order: Trochodendrales
       • Caryophyllidae
         • Order: Caryophyllales
         • Family: Caryophyllaceae
       • Dilleniidae
         • Order: Dilleniiales
         • Family: Dilleniaceae
       • Rosidae
         • Order: Rosales
         • Family: Rosaceae
       • Asteridae
         • Order: Asterales
         • Family: Asteraceae (Ending Family)
  2. Liliopsida (Monocots)
     • Further divided into 3 subclasses:
       • Alismatidae
         • Order: Alismatales
         • Family: Butomaceae
       • Lilidae
         • Order: Triuridales
         • Family: Triuridaceae
         • Order: Orchidales
         • Family: Orchidaceae
         • Order: Poales
         • Family: Poaceae
       • Arecidae
         • Order: Arales
         • Family: Arecaceae (Ending Family)

Characteristics of Takhtajan’s System

• Comprehensive Classification: Takhtajan’s system includes detailed categorizations across a wide range of taxonomic ranks, from subclasses to families. This detailed approach enables a thorough representation of Angiosperm diversity.
• Phylogenetic Basis: The system is strongly rooted in phylogenetic principles, using evolutionary relationships to structure the classification.
• Emphasis on Multiple Data Types: Takhtajan integrated a variety of biological data into his classification system, including morphological, cytological, palynological, anatomical, embryological, cytogenetic, biochemical, and palaeobotanical information. This multifaceted approach provides a robust framework for understanding plant evolution.

Comparison with Other Systems

• Comparison with Bentham and Hooker: Takhtajan’s system shares similarities with Bentham and Hooker’s classification in terms of recognizing major taxa such as classes and subclasses. However, Takhtajan introduced modifications at various levels, aiming to address some of the limitations of previous systems.
• Criticisms and Merits: While Takhtajan’s system has been praised for its comprehensive use of available data and its phylogenetic orientation, it has also faced criticism for its complexity and the artificial nature of some subdivisions. The system’s reliance on specific characters for categorization has been noted as a potential limitation.

Merits of Takhtajan’s Classification

1. Phylogenetic Orientation
   • Detailed Evolutionary Relationships: Takhtajan’s system is notably more phylogenetic compared to earlier classifications. It emphasizes evolutionary relationships and lineage diversification, reflecting a deeper understanding of plant evolution.
   • Use of Phylogenetic Data: The system integrates comprehensive phylogenetic data, helping to construct a more accurate evolutionary framework for Angiosperms.
2. Compatibility with Contemporary Systems
   • Alignment with Major Systems: Takhtajan’s classification generally aligns with other significant contemporary systems such as those proposed by Cronquist, Dahlgren, and Thorne. This agreement ensures that Takhtajan’s system is relevant and applicable within the broader context of plant taxonomy.
   • Consistency with Modern Views: The placement of major taxa, like the consideration of Magnolidae as primitive and the positioning of Dicots before Monocots, is consistent with views held by other contemporary taxonomists.
3. Adherence to Nomenclatural Standards
   • ICBN Compliance: Takhtajan’s nomenclature for various plant groups adheres to the International Code of Botanical Nomenclature (ICBN). This adherence ensures that the naming conventions used are standardized and universally recognized, facilitating clearer communication within the scientific community.

Demerits of Takhtajan’s Classification

1. Limited Scope of Classification
   • Family-Level Classification: The system provides a detailed classification primarily up to the family level. This limitation means that further subdivisions within families are not as thoroughly addressed, potentially missing finer taxonomic distinctions.
2. Debate on Primitive Families
   • Controversial Primitive Families: Takhtajan identified Degeneriaceae as the most primitive angiosperm family. However, this view is contested by some modern taxonomists who consider Winteraceae to be more primitive. Such discrepancies highlight the ongoing debates and evolving perspectives in plant taxonomy.
3. Emphasis on Cladistics
   • Cladistic vs. Phenetic Data: Takhtajan’s system places greater emphasis on cladistic information rather than phenetic data. While cladistics focuses on evolutionary relationships based on shared derived characteristics, phenetics considers overall similarities and differences among organisms. This emphasis can limit the system’s ability to incorporate comprehensive phenetic information, potentially affecting its applicability in certain contexts.

References

• https://egyankosh.ac.in/bitstream/123456789/16417/1/Unit-2.pdf
• https://www.bhu.ac.in/Content/Syllabus/Syllabus_300620200412053945.pdf
• https://www.shivajicollege.ac.in/sPanel/uploads/econtent/9ab8759378b1544a8fe9098e5da09528.pdf
• https://bieap.apcfss.in/Uploads/Materials/ilovepdf_merged.pdf
• https://gdcbhojpur.com/files/Benthom.pdf
• https://www.upcollege.ac.in/Upload/econtent/1302.pdf
• https://mis.alagappauniversity.ac.in/siteAdmin/dde-admin/uploads/1/PG_M.Sc._Botony_346%2012_Plant%20Taxonomy_MSc%20Botany.pdf
• https://bncollegebgp.ac.in/wp-content/uploads/2020/04/Classification-of-Angiosperm.pdf
• https://gcwgandhinagar.com/econtent/document/1587192818Document%20from%20Renu%20Sharma%20(1).pdf
• https://gcwgandhinagar.com/econtent/document/1587192785Document%20from%20Renu%20Sharma%20(2).pdf
• https://dspmuranchi.ac.in/pdf/Blog/classification-Part1.pdf
• https://www.ladykeanecollege.edu.in/files/userfiles/file/Dr_%20S_%20Nongbri%20III%20Sem%20ppt.pdf