Exhaustive Notes on the Morphology and Structural Organisation of Flowering Plants

THE HISTORY AND SIGNIFICANCE OF BIOLOGICAL DESCRIPTION

• The description of diverse life forms on Earth was initially conducted solely through observation, using the naked eye or magnifying tools like lenses and microscopes.
• This descriptive phase of biology primarily focused on gross structural features, encompassing both external and internal characteristics, as well as observable and perceivable living phenomena.
• Naturalists focused on description before the establishment of experimental biology and physiology. Consequently, biology was regarded as natural history for a long period.
• Detailed descriptions were essential for the later emergence of reductionist biology. While early students might have found the detail tedious, these descriptions allowed scientists to eventually focus on living processes rather than just life forms and structure.
• Structural descriptions facilitate the framing of research questions within the fields of physiology and evolutionary biology.
• Structural organization is often categorized into morphological (external) and anatomical (internal) features, typically presented separately for plant and animal kingdoms.

BIOGRAPHICAL PROFILE: KATHERINE ESAU ($1898$ – $1997$)

• Katherine Esau was born in Ukraine in $1898$. She pursued agricultural studies in Russia and Germany before earning her doctorate in the United States in $1931$.
• Her early research demonstrated that the curly top virus is transported through plants via the food-conducting phloem tissue.
• In $1954$, she published Plant Anatomy, a work that adopted a dynamic and developmental approach to plant structure. This text had a significant global impact and sparked a revival in the discipline.
• In $1960$, she published The Anatomy of Seed Plants, a comprehensive work often described as the ‘Webster’s of plant biology.’
• In $1957$, Esau was elected to the National Academy of Sciences, becoming only the sixth woman to receive that distinction.
• She was awarded the National Medal of Science by President George Bush in $1989$.
• Peter Raven, director of Anatomy and Morphology at the Missouri Botanical Garden, noted upon her death in $1997$ that she had absolutely dominated the field of plant biology even at the age of $99$.

MORPHOLOGY OF FLOWERING PLANTS: GENERAL OVERVIEW

• Angiosperms exhibit a vast diversity in external structure (morphology) but share common basic organs: roots, stems, leaves, flowers, and fruits.
• Understanding higher plants requires knowledge of standard technical terms and definitions for successful classification.
• Plant variations serve as adaptations to specific environments, providing functions such as protection, climbing, storage, and survival in various habitats.
• A typical flowering plant consists of two main parts:
  • The root system: The underground portion of the plant.
  • The shoot system: The portion located above the ground.

THE ROOT SYSTEM

• In most dicotyledonous plants, the radicle elongates directly to form the primary root, which grows into the soil.
• The primary root produces lateral roots of different orders, categorized as secondary and tertiary roots.
• Types of Root Systems:
  • Tap Root System: Comprises the primary root and its lateral branches. Example: mustard plant.
  • Fibrous Root System: In monocotyledonous plants, the primary root is short-lived and replaced by numerous roots originating from the base of the stem. Example: wheat plant.
  • Adventitious Roots: Roots that develop from parts of the plant other than the radicle. Examples: grass, Monstera, and the banyan tree.
• Main Functions of the Root System:
  • Absorption of water and minerals from the soil.
  • Provision of proper anchorage to the plant.
  • Storage of reserve food materials.
  • Synthesis of plant growth regulators.

REGIONS OF THE ROOT

• Root Cap: A thimble-like structure at the root apex that protects the tender tip as it grows through the soil.
• Region of Meristematic Activity: Located a few millimeters above the root cap. Cells here are very small, thin-walled, contain dense protoplasm, and divide repeatedly.
• Region of Elongation: Located proximal to the meristematic region. Cells undergo rapid enlargement and elongation, increasing the root's length.
• Region of Maturation: Located proximal to the elongation zone. Cells here differentiate and mature.
• Root Hairs: Fine, delicate, thread-like structures formed by epidermal cells in the maturation region. These are responsible for absorbing water and minerals from the soil.

THE STEM

• The stem is the ascending axis of the plant that bears branches, leaves, flowers, and fruits.
• It develops from the plumule of the embryo during seed germination.
• Key Features:
  • Nodes: The regions where leaves are attached.
  • Internodes: The portions of the stem between two successive nodes.
  • Buds: May be terminal (at the tip) or axillary (in the leaf axil).
  • Stems are generally green when young, often becoming woody and dark brown as they mature.
• Functions of the Stem:
  • Spreading branches to display leaves, flowers, and fruits.
  • Conduction of water, minerals, and photosynthates.
  • Specialized functions including food storage, support, protection, and vegetative propagation.

THE LEAF

• The leaf is a lateral, typically flattened structure that develops at the node and bears a bud in its axil; this axillary bud eventually develops into a branch.
• Leaves originate from shoot apical meristems and follow an acropetal arrangement.
• Primary function: Photosynthesis.
• Structure of a typical leaf:
  • Leaf Base: Attaches the leaf to the stem. It may bear two small lateral structures called stipules. In monocots, the leaf base often forms a sheath around the stem. In some legumes, the leaf base is swollen, known as a pulvinus.
  • Petiole: Holds the lamina to light and allows it to flutter in the wind to cool the leaf and refresh the air at its surface.
  • Lamina (Leaf Blade): The green expanded part containing veins and veinlets. The central prominent vein is the midrib.
• Venation:
  • Reticulate: Veinlets form a network; characteristic of dicotyledonous plants.
  • Parallel: Veins run parallel to each other; characteristic of monocotyledonous plants.
• Types of Leaves:
  • Simple Leaf: Lamina is entire or, if incised, incisions do not reach the midrib.
  • Compound Leaf: Incisions reach the midrib, dividing the lamina into leaflets. Buds are found in the axil of the petiole but not in the axils of individual leaflets.
  • Pinnately Compound: Leaflets are arranged on a common axis called the rachis (e.g., neem).
  • Palmately Compound: Leaflets are attached at a single point at the tip of the petiole (e.g., silk cotton).
• Phyllotaxy: The arrangement of leaves on the stem:
  • Alternate: A single leaf at each node in an alternate pattern (e.g., china rose, mustard, sunflower).
  • Opposite: A pair of leaves at each node (e.g., Calotropis, guava).
  • Whorled: More than two leaves at a node forming a circle (e.g., Alstonia).

THE INFLORESCENCE

• A flower is a modified shoot where the apical meristem transforms into a floral meristem. Internodes do not elongate, and the axis becomes condensed.
• Floral appendages are produced laterally at nodes instead of leaves. A solitary flower occurs when a shoot tip transforms into a single flower.
• Inflorescence is the arrangement of flowers on the floral axis.
• Types of Inflorescence:
  • Racemose: The main axis continues to grow, and flowers are borne laterally in acropetal succession.
  • Cymose: The main axis terminates in a flower, limiting growth. Flowers are borne in basipetal order.

THE FLOWER

• The reproductive unit of angiosperms, facilitating sexual reproduction.
• Structure: Four whorls on a swollen stalk end called the thalamus or receptacle:
  • Accessory Organs: Calyx and Corolla.
  • Reproductive Organs: Androecium and Gynoecium.
  • Perianth: Term used when calyx and corolla are indistinct (e.g., lily).
• Sexuality:
  • Bisexual: Contains both androecium and gynoecium.
  • Unisexual: Contains only stamens or only carpels.
• Symmetry:
  • Actinomorphic (Radial): Can be divided into equal halves in any radial plane (e.g., mustard, datura, chilli).
  • Zygomorphic (Bilateral): Can be divided into similar halves only in one specific vertical plane (e.g., pea, gulmohur, bean, Cassia).
  • Asymmetric (Irregular): Cannot be divided into similar halves by any plane (e.g., canna).
• Numerical Classification: Flowers can be trimerous, tetramerous, or pentamerous (appendages in multiples of $3$, $4$, or $5$).
• Bracteate: Flowers with bracts (reduced leaves) at the base of the pedicel. Ebracteate flowers lack bracts.
• Position of Floral Parts on Thalamus:
  • Hypogynous: Gynoecium is at the highest position; ovary is superior (e.g., mustard, china rose, brinjal).
  • Perigynous: Gynoecium is central, and other parts are on the rim of the thalamus at the same level; ovary is half inferior (e.g., plum, rose, peach).
  • Epigynous: Thalamus margin grows upward to enclose and fuse with the ovary; other parts arise above the ovary. Ovary is inferior (e.g., guava, cucumber, ray florets of sunflower).

PARTS OF A FLOWER

• Calyx: The outermost whorl made of sepals. Gamosepalous (united sepals) or polysepalous (free sepals).
• Corolla: Composed of petals, which attract insects for pollination. Gamopetalous (united) or polypetalous (free). Shapes include tubular, bell-shaped, funnel-shaped, or wheel-shaped.
• Aestivation (arrangement of sepals/petals in bud):
  • Valvate: Margins touch without overlapping (e.g., Calotropis).
  • Twisted: One margin overlaps the next (e.g., china rose, lady’s finger, cotton).
  • Imbricate: Margins overlap but not in a specific direction (e.g., Cassia, gulmohur).
  • Vexillary (Papilionaceous): Five petals; one large standard overlaps two wings, which overlap two keels (e.g., pea, bean).
• Androecium: Composed of stamens (filament and anther). Anthers are usually bilobed with two pollen-sacs each. Staminode refers to a sterile stamen.
  • Attachments: Epipetalous (attached to petals, e.g., brinjal); Epiphyllous (attached to perianth, e.g., lily).
  • Cohesion: Monoadelphous (one bundle, e.g., china rose); Diadelphous (two bundles, e.g., pea); Polyadelphous (multiple bundles, e.g., citrus).
  • Variation in length: Examples include Salvia and mustard.
• Gynoecium: Female part made of carpels (stigma, style, ovary). Ovules are attached to a placenta in the ovary.
  • Apocarpous: Carpels are free (e.g., lotus, rose).
  • Syncarpous: Carpels are fused (e.g., mustard, tomato).
• Placentation (ovule arrangement within ovary):
  • Marginal: Placenta forms a ridge along the ventral suture (e.g., pea).
  • Axile: Placenta is axial in a multilocular ovary (e.g., china rose, tomato, lemon).
  • Parietal: Ovules develop on the inner wall; ovary becomes two-chambered due to a false septum (e.g., mustard, Argemone).
  • Free Central: Ovules on central axis, septa absent (e.g., Dianthus, Primrose).
  • Basal: Single ovule at the base of the ovary (e.g., sunflower, marigold).

THE FRUIT AND SEED

• Fruit: A ripened ovary developed after fertilization. Parthenocarpic fruits develop without fertilization.
• Pericarp: The fruit wall, which can be dry or fleshy (epicarp, mesocarp, endocarp).
• Drupe: A one-seeded fruit from a monocarpellary superior ovary. In mango, the mesocarp is fleshy; in coconut, it is fibrous.
• Seed: Developed from ovules. Consists of a seed coat and an embryo.
• Dicotyledonous Seed: Seed coat has an outer testa and inner tegmen. Hilum is the attachment scar; micropyle is the pore above it. Embryo has two cotyledons, a radicle, and a plumule. Endospermous seeds (e.g., castor) vs. non-endospermous (e.g., bean, gram, pea).
• Monocotyledonous Seed: Generally endospermic (except orchids). In cereals like maize, the seed coat is fused with the fruit wall. The endosperm is separated from the embryo by a proteinous aleurone layer. Cotyledon is called the scutellum. The plumule is enclosed in a coleoptile and the radicle in a coleorhiza.

SEMI-TECHNICAL DESCRIPTION AND FLORAL FORMULA

• Plants are described in a standard sequence: habit, vegetative features (roots, stem, leaves), and floral features (inflorescence, flower parts).
• Floral Formula Symbols:
  • $Br$: Bracteate
  • $K$: Calyx
  • $C$: Corolla
  • $P$: Perianth
  • $A$: Androecium
  • $G$: Gynoecium; $\\underline{G}$: Superior Ovary; $\\overline{G}$: Inferior Ovary
  • $\\text{♂}$: Male; $\\text{♀}$: Female; $\\text{☇}$: Bisexual
  • $\\oplus$: Actinomorphic; $\\text{ℴ}$: Zygomorphic
  • Brackets $()$ denote fusion; a line above symbols denotes adhesion.
• Floral Diagram: Shows the number of parts, their arrangement, and relations. The mother axis is represented by a dot at the top.

FAMILY SOLANACEAE (POTATO FAMILY)

• Distribution: Tropics, subtropics, and temperate zones.
• Vegetative Characters: Mostly herbs or shrubs; stem is herbaceous or rarely woody, underground in potato ($Solanum\,tuberosum$); leaves are alternate, simple, and exstipulate with reticulate venation.
• Floral Characters: Inflorescence is solitary, axillary, or cymose. Flowers are bisexual and actinomorphic. Calyx has $5$ united, persistent sepals with valvate aestivation. Corolla has $5$ united petals with valvate aestivation. Androecium has $5$ epipetalous stamens. Gynoecium is bicarpellary and syncarpous with a superior, bilocular ovary and swollen placenta.
• Economic Importance:
  • Food: Tomato, brinjal, potato.
  • Spice: Chilli.
  • Medicine: Belladonna, ashwagandha.
  • Fumigatory: Tobacco.
  • Ornamental: Petunia.

QUESTIONS & DISCUSSION

• How is a pinnately compound leaf different from a palmately compound leaf?
• Explain with suitable examples the different types of phyllotaxy.
• Define the following terms: (a) aestivation (b) placentation (c) actinomorphic (d) zygomorphic (e) superior ovary (f) perigynous flower (g) epipetalous stamen.
• Differentiate between (a) Racemose and cymose inflorescence (b) Apocarpous and syncarpous ovary.
• Draw the labelled diagram of the following: (i) gram seed (ii) V.S. of maize seed.
• Take one flower of the family Solanaceae and write its semi-technical description. Also draw their floral diagram.
• Describe the various types of placentations found in flowering plants.
• What is a flower? Describe the parts of a typical angiosperm flower.
• Define the term inflorescence. Explain the basis for the different types inflorescence in flowering plants.
• Describe the arrangement of floral members in relation to their insertion on thalamus.