Angiosperms | Botany Optional for UPSC PDF Download

Systematics

Angiosperms, commonly known as flowering plants, represent a remarkable and diverse group within the Plantae kingdom, comprising approximately 300,000 species. These botanical wonders have claimed their dominance, currently accounting for a staggering 80% of all known green plants. The defining feature of angiosperms is their unique reproductive system, wherein the ovule, or egg, is fertile and develops into a seed within an enclosed ovary. This ovary is typically nestled within a flower, a crucial part of these plants that houses the male and female reproductive organs or both. The subsequent fruit formation from mature floral organs is a distinguishing characteristic of angiosperms, setting them apart from gymnosperms, such as conifers and cycads, where seeds are exposed on the surfaces of reproductive structures like cones.

General Characteristics

• Angiosperms boast a well-defined and thoroughly characterized classification. Notably, there is no ambiguity in classifying plant species as either angiosperm or non-angiosperm. Even within the fossil record, no forms suggest a connection to any other plant group, although some fossils represent specific plant components without clear categorization.
• Angiosperms are primarily seed plants, a feature that distinguishes them from all other plants except gymnosperms, with conifers and cycads as the most familiar representatives. The key distinction lies in the ovules; angiosperms enclose their ovules within an ovary, while gymnosperms have exposed ovules during pollination, never enclosed within an ovary. Additionally, angiosperms employ a specialized structure called the stigma to collect pollen, which is typically positioned above the ovary on a slender structure known as the style. In contrast, gymnosperm pollen grains are received through an aperture atop the ovule, known as the micropyle.
• The female gametophyte of angiosperms, referred to as the embryo sac, is minuscule and contains a limited number of nuclei, typically around eight. Unlike gymnosperms, angiosperms lack partitioned cytoplasm by cell walls, and the nuclei have direct or indirect associations with it. In sexual reproduction, one of the embryo sac's nuclei becomes the egg, uniting with one of the sperm nuclei from the pollen tube. Two additional embryo sac nuclei merge with the second sperm nucleus from the pollen tube, forming a triple-fusion nucleus that eventually develops into the endosperm, a multicellular food-storage tissue found in seeds.

Angiosperm Phylogeny Group

• The Angiosperm Phylogeny Group (APG) is an informal, multinational consortium of systematic botanists who collaborate to establish a taxonomic consensus for flowering plants (angiosperms). This consensus reflects the latest findings on plant relationships revealed through phylogenetic research. 
• The APG has released four successive versions of a classification system, published in 1998, 2003, 2009, and 2016. This collaborative effort was prompted by perceived deficiencies in prior angiosperm classifications, which did not adhere to monophyletic groupings, i.e., groups encompassing all descendants of a common ancestor.
• The influence of APG publications is steadily growing, leading numerous major herbaria, including the Royal Botanic Gardens, Kew, to reorganize their collections to align with the most recent APG classification. Additionally, the influential World Checklist of Selected Plant Families from Kew is undergoing updates according to the APG III system, highlighting the acceptance and adoption of this evolving classification.

Angiosperm Classification and the APG

• Before the era of genetic evidence, the classification of angiosperms relied primarily on morphology, especially flower characteristics, and biochemical analysis. However, the availability of precise genetic data and the application of phylogenetic methods in the 1980s transformed the field. While some aspects of prior classifications were confirmed or clarified, others were fundamentally revised. 
• The catalyst for this transformation was a groundbreaking molecular study in 1993 involving 5,000 flowering plants and a photosynthesis gene (rbcL). This study yielded unexpected insights into plant relationships, challenging previously held notions. This scientific breakthrough necessitated extensive collaboration among scientists, leading to the birth of the Angiosperm Phylogeny Group (APG) for a more universally accepted and stable standard for angiosperm classification.
• As of 2016, three versions of the APG classification system have been released: APG II (2003), APG III (2009), and APG IV (2016), each superseding the previous version. These classifications are subject to evolution as new research emerges, reflecting the dynamic nature of science.

Principles of the APG System

The APG’s classification concepts were established in the original publication of 1998 and have remained intact in subsequent editions.
These are, in brief, the following:

• The Linnean order and family structure should be preserved. “The family is fundamental to the systematics of flowering plants.” An ordinal classification of families is offered as a “universally useful reference tool.” Orders are regarded as particularly valuable in the classroom and in the study of family interactions
• Monophyletic groups should exist (i.e. consist of all descendants of a common ancestor). The primary reason for rejecting present systems is that they lack this quality; they are not phylogenetic
• The boundaries of organisations such as orders and families are defined broadly. Thus, it is argued that a small number of larger orders will be more beneficial. Families and orders having a single genus and family are avoided wherever possible without breaching the overriding requirement of monophyly
• The term clades is more loosely employed above or parallel to the level of orders and families. (Some clades were later given formal names in a study related to the APG system’s 2009 revision.) The authors state that while naming all clades in a phylogenetic tree is “impossible and undesirable,” systematists must agree on names for select clades, particularly orders and families, to promote communication and discussion.

Conclusion

The Angiosperm Phylogeny Group (APG) serves as an international platform for systematic botanists to collaborate and establish a consensus on the taxonomy of flowering plants (angiosperms). Their work has revolutionized our understanding of plant relationships, incorporating genetic evidence and evolving with the latest scientific findings. As the APG classification system gains prominence, it influences the organization of herbaria collections and botanical research worldwide, ensuring a more accurate and up-to-date understanding of the mesmerizing world of angiosperms.

Anatomy

The study of flowering plants' internal structure, commonly known as plant anatomy, delves deep into the intricate world of plant organs through the observation of cross-sections. Flowering plants, also known as angiosperms, encompass a diverse array of tissues, each with its unique characteristics. Understanding these tissues is fundamental to comprehending the complexities of plant growth and function. 

Types of Tissues

At the core of plant anatomy are tissues, which are groups of cells with a shared origin and function. These tissues can be broadly categorized into two main types:

Meristematic Tissues

Meristematic tissues consist of cells with the remarkable ability to divide, contributing significantly to a plant's growth. They are further divided into three subgroups:

• Apical Meristem: Found at the tips of roots and shoots, apical meristems are primarily responsible for increasing the plant's length.
• Intercalary Meristem: These short-lived meristems occur between mature tissues and are capable of forming branches and flowers. They play a vital role in early plant development.
• Lateral Meristem: Also known as secondary meristem, lateral meristem is found in mature regions of roots and shoots. It contributes to secondary growth and is responsible for the formation of structures like fascicular vascular cambium.

Permanent Tissues

• These are derived from meristematic tissue and are composed of cells that have lost the ability to divide.
• They are divided into simple and complex tissues, which are further divided.
• The simple tissues constitute the parenchyma, which is thin-walled cells with cellulose in the cell wall and performs photosynthesis, storage and secretion. The collenchyma are formed of closely packed isodiametric cells, providing mechanical support and the sclerenchyma which are made up of dead cells also meant for mechanical support, having two types of cells, fibres, and sclereids.
• The complex tissues consist of the xylem, conducting minerals and water to other parts of the plant and the phloem, which transports food material to various parts of the plant.

Simple Tissues

Simple tissues include:

Complex Tissues

Complex tissues, such as xylem and phloem, are made up of multiple cell types that function as a unit:

• Xylem: This tissue conducts water and mineral salts from the roots to other parts of the plant. It provides mechanical support and consists of tracheids, vessels, xylem parenchyma, and xylem fibers.
• Phloem: Responsible for transporting food materials from the leaves to all parts of the plant, phloem comprises sieve tube elements, companion cells, phloem parenchyma, and phloem fibers.

The Tissue System

The tissue system in plants can be classified into three main categories:

• Epidermal Tissue System: This system includes structures like root hairs, epidermis, cuticle, epidermal hairs, stomata, and trichomes.
• Ground Tissue System: Comprising sclerenchyma, parenchyma, and collenchyma, the ground tissue system forms the bulk of the plant's structure.
• Vascular Tissue System: This system is composed of xylem and phloem. Vascular bundles within this system are responsible for conducting water, minerals, and food materials throughout the plant.

Embryology

Embryology, a field of biology, concentrates on the examination of micro and megasporogenesis, gametophyte growth, fertilization, and the development of endosperm, embryos, and seed coatings in plants. It serves as a pivotal tool in resolving taxonomic dilemmas across different classification levels within the plant realm. The importance of embryology in taxonomic research was initially highlighted by German embryologist Schnarf in 1931. According to experts like Jones and Luchsinger, embryological traits have proven invaluable in distinguishing connections within plant families, genera, and species, though their usefulness wanes when applied to higher taxonomic tiers such as orders, subclasses, or classes.
Prominent authorities such as Maheshwari, Bhojwani, Bhatnagar, and Radford have pinpointed several critical embryological characteristics with special relevance in taxonomic considerations.
These encompass:

• Existence and Nature of Anther Tapetum: The type of tissue surrounding the anther contributes to taxonomy.
• Quantity and Arrangement of Anther Loculi: The configuration of chambers within the anther holds taxonomic significance.
• Type of Anther andothecium: The structure and type of the anther are substantial factors.
• Quadripartition of Microspore Mother Cell: The division pattern of microspore mother cells is a crucial embryological feature.
• Developed Pollen Grains: The characteristics of mature pollen grains can assist in classification.
• Evolution, Structure, Position, Vasculation, and Orientation of Ovule: These factors in ovule development provide valuable taxonomic data.
• Origin of Sporogenous Tissue in Ovule: Understanding the source of sporogenous tissue within the ovule is essential for taxonomy.
• Megasporogenesis and Growth of Embryo Sac: The formation of the embryo sac is a taxonomically significant process.
• Presence of Aril: The presence or absence of an aril is a distinguishing trait.
• Form of Embryo Sac: The shape and structure of the embryo sac can aid in classification.
• Fertilization: The process of fertilization is crucial in taxonomy.
• Variety of Embryo: The characteristics of the embryo are essential for classification.
• Type of Embryogeny: The method of embryo development contributes to taxonomic insights.
• Formation of Endosperm: Understanding endosperm development is vital for classification.
• Type of Haustorium Formation: The presence and type of haustorium are relevant taxonomic features.
• Seed Coat: Characteristics of the seed coat are important in classification.
• Cotyledons: The number and characteristics of cotyledons provide taxonomic insights.

A Few Examples of Role of Embryology in Taxonomy

• Distinguishing Dicots and Monocots: Angiosperms, the flowering plants, are categorized into dicotyledons (dicots) and monocotyledons (monocots), primarily based on a key embryological feature – the number of cotyledons, or seed leaves.
• The Unique Characteristics of Caryophyllales: Caryophyllales, a specific order within angiosperms, possesses distinctive embryological traits. These include trinucleate pollen, bitegmic crassinucellate ovules that exhibit campylotropous or amphitropous orientations, seeds with peripheral embryos, and perisperm with minimal or no endosperm. These characteristics are exclusive to Caryophyllales, also known as centrospermae according to Cronquist's 1968 classification.
• Helobiae: This monocotyledonous order, sometimes treated as a subclass in more recent classification systems, is recognized by the presence of the helobial type of endosperm.
• Orchidales: Members of the Orchidales order are characterized by a distinctive embryological trait – the presence of undifferentiated embryos and very limited or no endosperm.
• Podostemaceae: This family is identified by the formation of pseudoembryo sacs, created through the disintegration of nucellar cells beneath the embryo sac.
• Onagraceae: The Onagraceae family is distinguished by the presence of the Onagrad type of embryo sac.
• Cyperaceae: In flowering plants belonging to the Cyperaceae family, four functional microspores develop from each microspore mother cell.
• Lemnaceae: Phylogenetic research suggests that Lemnaceae likely originated from either the Helobiales or the Araceae.
• Crassulaceae: Embryological investigations of Crassulaceae propose that it should be classified within the order Rosales, in close proximity to the Saxifragaceae family.
• Parnassia: While Parnassia is generally considered a member of the Saxifragaceae family, its embryological characteristics significantly differ from those of other genera within Saxifragaceae.

Palynology

Palynology is the scientific discipline devoted to the examination of plant pollen and spores and its applications. The term "palynology" originates from the Greek word "palynein," which means "to scatter." Over the past three decades, researchers like Erdtman, Wagenitz, Stix, Raj, Chanda, and Nair have recognized the significance of pollen characteristics in taxonomy.

Pollen Characteristics and Taxonomy

Pollen characteristics play a vital role in solving various taxonomic puzzles and interpreting the origin and evolution of different plant groups.
The following aspects of pollen have been of particular interest to taxonomists:

• Pollen Types: Pollen grains within angiosperms are categorized into two basic types: monosulcate and tricolpate.
  • Monosulcate pollen grains are found in primitive dicotyledons, certain monocotyledons, pteridosperms, and cycads. These grains are boat-shaped and feature a single long germinal furrow and aperture.
  • Tricolpate pollen grains are characteristic of advanced dicotyledons. They are spherical in shape and possess three germinal apertures.
• Number of Nuclei in Pollen: The number of nuclei in pollen at the time of dispersal has taxonomic significance. Angiosperm pollen can be binucleate or trinucleate based on the division of the generative nucleus, with the binucleate condition considered more primitive.
• Pollen Grain Shape: The shape and symmetry of a pollen grain, along with the architecture of its wall, exine stratification, sculpture, and aperture characteristics, are fundamental attributes useful in taxonomy.
• Stenopalynous and Eurypalynous Taxa: Taxa with consistent and characteristic pollen types are termed stenopalynous or unipalynous. In contrast, eurypalynous taxa exhibit variation in pollen types, including size, aperture, exine stratification, and other features.
• NPC System: The classification of pollen is based on the Number-Position-Character (NPC) analysis. This system groups taxa with the same NPC formula together and separates those with differing NPC patterns. It provides a three-dimensional classification and aids in creating diagnostic keys below the family level.

Examples of Palynology in Taxonomy

Palynology has proven valuable in taxonomy through various applications:

• Palynological analysis of 16 Indian Cyperus species enabled the creation of a key for differentiation based on pollen characteristics.
• Distinct pollen morphoforms based on apertureal features.
• Recognition of angiosperm pollen grains characterized by a massive exine and thin intine.
• Identification of different species within a genus using exine patterns.
• Determination of phylogenetic relationships through pollen characters.
• Observation of pollen grains arranged in tetrads in several dicot and monocot families.