MORPHOLOGY OF FLOWERING PLANTS

Introduction:

The wide range of structures in higher plants is a source of fascination. Despite the diverse external morphologies among angiosperms, they share common features such as roots, stems, leaves, flowers, and fruits.

Standard Terms and Definitions:

To successfully classify and understand higher plants, knowledge of standard technical terms and definitions is essential. This includes understanding the variations in different plant parts, which often represent adaptations to various environmental conditions, such as adaptations for habitat specificity, protection, climbing, and storage.

Plant Parts:

1. Root System:

  • Definition: The underground part of a flowering plant.
  • Function: Anchorage, absorption of water and minerals.
  • Variations: Different root types (fibrous, taproot) based on adaptation.

2. Shoot System:

  • Definition: The portion above the ground.
  • Components:
    • Stems: Provide support and transport nutrients.
    • Leaves: Site of photosynthesis.
    • Flowers: Reproductive structures.
    • Fruits: Develop from fertilized flowers, and contain seeds.

Weed Anatomy:

Observing a weed reveals the presence of roots, stems, leaves, flowers, and fruits. Understanding the anatomy of weeds helps in appreciating the diversity in plant structures.

THE ROOT

  1. Dicotyledonous Plants:
    • Direct elongation of the radicle forms the primary root.
    • Primary root gives rise to lateral roots of various orders (secondary, tertiary, etc.).
    • Forms the taproot system.
    • Example: Mustard plant.
  2. Monocotyledonous Plants:
    • The primary root is short-lived.
    • Replaced by numerous roots originating from the base of the stem.
    • Forms the fibrous root system.
    • Example: Wheat plant.
  3. Adventitious Roots:
    • Found in plants like grass, Monstera, and banyan trees.
    • Arise from parts other than the radicle.
    • Example: Grass, Monstera, Banyan tree.
  4. Functions of Root System:
    • Absorption of water and minerals from the soil.
    • Anchorage for plant parts.
    • Storage of reserve food materials.
    • Synthesis of plant growth regulators.

Regions of the Root

  1. Root Cap:
    • Located at the apex of the root.
    • Thimble-like structure.
    • Protects the delicate root apex during soil penetration (Figure 5.3).
  2. Meristematic Activity:
    • Few millimeters above the root cap.
    • Region of active cell division.
    • Small, thin-walled cells with dense protoplasm.
    • Cells divide repeatedly.
  3. Region of Elongation:
    • Proximal to the meristematic activity region.
    • Cells undergo rapid elongation and enlargement.
    • Responsible for the lengthwise growth of the root.
  4. Region of Maturation:
    • Proximal to the region of elongation.
    • Cells gradually differentiate and mature.
    • Precedes the formation of root hairs.
  5. Root Hairs:
    • Thread-like structures originating from some epidermal cells in the region of maturation.
    • Responsible for the absorption of water and minerals from the soil.

The Stem

  • The stem is the ascending part of the axis that bears branches, leaves, flowers, and fruits.
  • It develops from the plumule of the embryo during seed germination.
  • Features:
    1. Nodes and Internodes:
      • Nodes: Regions where leaves are attached.
      • Internodes: Portions between two nodes.
    2. Buds:
      • Terminal or axillary buds are present.
    3. Color and Texture:
      • Green when young, often becoming woody and dark brown with age.
  • Functions:
    1. Branching and Support:
      • Spreads out branches bearing leaves, flowers, and fruits.
      • Provides structural support.
    2. Conduction:
      • Conducts water, minerals, and photosynthates (products of photosynthesis).
    3. Storage:
      • Some stems store food.
    4. Protection:
      • Offers protection to delicate inner tissues.
    5. Vegetative Propagation:
      • Participates in vegetative propagation, allowing the plant to reproduce asexually.

The Leaf

  • A lateral, generally flattened structure borne on the stem, developing at the node.
  • Originates from shoot apical meristems and arranged in an acropetal order.
  • Parts of a Leaf:
    1. Leaf Base:
      • Attached to the stem.
      • May have stipules (small leaf-like structures).
      • In monocots, the leaf base may expand into a sheath.
      • In some leguminous plants, the leaf base may become swollen (pulvinus).
    2. Petiole:
      • Connects the leaf base to the lamina.
      • Long, thin, flexible petioles allow leaf blades to flutter in the wind, cooling the leaf and bringing fresh air to the surface.
    3. Lamina (Leaf Blade):
      • The green expanded part with veins and veinlets.
      • Contains a midrib, a prominent vein.
      • Veins provide rigidity and act as channels for water, minerals, and food transport.
  • Functions:
    1. Photosynthesis:
      • Primary vegetative organ for photosynthesis.
    2. Support and Transport:
      • Provides structural support.
      • Veins transport water, minerals, and food materials.
    3. Adaptations:
      • The shape, margin, apex, surface, and incision of the lamina vary for environmental adaptation.

Venation in Leaves

  • Definition: The arrangement of veins and veinlets in the leaf lamina.
  • Types:
    1. Reticulate Venation:
      • Description: Veinlets form a network.
      • Example: Typically found in dicotyledonous plants.
    2. Parallel Venation:
      • Description: Veins run parallel to each other within the lamina.
      • Example: Characteristic of most monocotyledons.

Note:

  • Dicotyledonous leaves usually exhibit reticulate venation.
  • Monocotyledonous leaves generally display parallel venation.

Types of Leaves

  • Simple Leaves:
    • Characteristics:
      • Lamina is entire.
      • Incisions, if present, do not reach the midrib.
    • Axillary Bud: Present in the axil of the petiole.
  • Compound Leaves:
    • Characteristics:
      • Incisions of the lamina reach the midrib, breaking it into leaflets.
    • Axillary Bud: Present in the axil of the petiole (not in the axil of leaflets).
  • Types of Compound Leaves:
    1. Pinnately Compound Leaves:
      • Description: Multiple leaflets present on a common axis (rachis).
      • Example: Neem leaves.
    2. Palmately Compound Leaves:
      • Description: Leaflets attached at a common point (tip of petiole).
      • Example: Silk cotton leaves.

Phyllotaxy

  • Definition: Phyllotaxy refers to the pattern of arrangement of leaves on the stem or branch.
  • Types of Phyllotaxy:
    1. Alternate Phyllotaxy:
      • Description: A single leaf arises at each node alternately.
      • Examples: China rose, mustard, sunflower.
    2. Opposite Phyllotaxy:
      • Description: A pair of leaves arises at each node and lies opposite to each other.
      • Examples: Calotropis, guava.
    3. Whorled Phyllotaxy:
      • Description: More than two leaves arise at a node, forming a whorl.
      • Example: Alstonia.

Inflorescence

  • Definition: Inflorescence refers to the arrangement of flowers on the floral axis.
  • Types of Inflorescence:
    1. Racemose Inflorescence:
      • Description: The main axis continues to grow, and flowers are borne laterally in an acropetal succession.
      • Characteristics: Solitary flowers on long stalks or clustered.
      • Example: the snapdragon (Antirrhinum majus) and foxglove (Digitalis purpurea).
    2. Cymose Inflorescence:
      • Description: The main axis terminates in a flower and is limited in growth. Flowers are borne in a basipetal order.
      • Characteristics: Terminal flower with lateral flowers below.
      • Example: elderberry (Sambucus nigra)

The Flower

  • Definition: The flower is the reproductive unit in angiosperms, designed for sexual reproduction.
  • Whorls of a Typical Flower:
    1. Calyx: The outermost whorl, consisting of sepals.
    2. Corolla: The second whorl, made up of petals.
    3. Androecium: The third whorl, comprising stamens (male reproductive organs).
    4. Gynoecium: The innermost whorl, containing carpels/pistils (female reproductive organs).
  • Accessory Organs and Reproductive Organs:
    • Calyx and corolla are accessory organs.
    • Androecium and gynoecium are reproductive organs.
  • Types of Flowers:
    • Bisexual: Contains both androecium and gynoecium.
    • Unisexual: Has either stamens or carpels.
  • Symmetry:
    • Actinomorphic: Radial symmetry; can be divided into two equal halves in any radial plane (e.g., mustard, datura).
    • Zygomorphic: Bilateral symmetry; can be divided into two similar halves in one vertical plane (e.g., pea, Gilmour).
    • Asymmetric: Irregular; cannot be divided into similar halves (e.g., canna).
  • Numerical Arrangement:
    • Trimerous: Floral appendages in multiples of 3.
    • Tetramerous: Floral appendages in multiples of 4.
    • Pentamerous: Floral appendages in multiples of 5.
  • Presence of Bracts:
    • Bracteate: Flowers with bracts (reduced leaves at the base of the pedicel).
    • Ebracteate: Flowers without bracts.
  • Position of Floral Parts:
    • Hypogynous: Gynoecium at the highest position, superior ovary (e.g., mustard, china rose).
    • Perigynous: Gynoecium in the center, half-inferior ovary (e.g., plum, rose, peach).
    • Epigynous: Thalamus margin grows upward, enclosing the ovary completely, inferior ovary (e.g., guava, cucumber, ray florets of sunflower).

Parts of a Flower

  1. Calyx:
    • Definition: Outermost whorl of the flower.
    • Components: Sepals (green, leaf-like structures).
    • Functions: Protects the flower in the bud stage.
    • Types:
      • Gamosepalous: Sepals united.
      • Polysepalous: Sepals free.
  2. Corolla:
    • Composition: Petals (brightly colored for insect attraction).
    • Types:
      • Gamopetalous: Petals united.
      • Polypetalous: Petals free.
    • Variety: Tubular, bell-shaped, funnel-shaped, or wheel-shaped.
    • Aestivation: Arrangement in the floral bud (valvate, twisted, imbricate, vexillary).
  3. Androecium:
    • Composition: Stamens (male reproductive organs).
    • Structure: Stalk (filament) and anther (bilobed with pollen-sacs).
    • Variations:
      • Epipetalous: Stamens attached to petals (e.g., brinjal).
      • Epiphyllous: Stamens attached to the perianth (e.g., lily).
      • United Forms: Monadelphous (one bundle), Diadelphous (two bundles), Polyadelphous (more than two bundles).
      • Filament Length Variation (e.g., Salvia and mustard).
  4. Gynoecium:
    • Composition: Female reproductive part, one or more carpels.
    • Structure: Carpels consist of stigma, style, and ovary.
    • Ovary: Enlarged basal part; style connects to stigma.
    • Ovules: Attached to the placenta; become seeds after fertilization.
    • Types of Carpels:
      • Apocarpous: Free carpels (e.g., lotus and rose).
      • Syncarpous: Fused carpels (e.g., mustard and tomato).
    • After Fertilization: Ovary matures into a fruit.
  5. Placentation:
    • Definition: Arrangement of ovules within the ovary.
    • Types:
      • Marginal: Placenta along ventral suture (e.g., pea).
      • Axile: Placenta axial in the multilocular ovary (e.g., china rose, tomato, lemon).
      • Parietal: Ovules on the inner wall or peripheral part (e.g., mustard, Argemone).
      • Free Central: Ovules borne on the central axis without septa (e.g., Dianthus, Primrose).
      • Basal: Placenta at the base with a single attached ovule (e.g., sunflower, marigold).

THE FRUIT

  1. Definition:
    • The characteristic feature of flowering plants.
    • Mature or ripened ovary developed after fertilization.
    • Parthenocarpic fruit forms without ovary fertilization.
  2. Composition:
    • Generally consists of pericarp (wall) and seeds.
    • The pericarp may be dry or fleshy.
  3. Pericarp Structure:
    • Thick and fleshy pericarp differentiates into:
      • Outer Epicarp
      • Middle Mesocarp
      • Inner Endocarp
  4. Drupe:
    • Examples: Mango and Coconut.
    • Develop from monocarpellary superior ovaries.
    • Typically one-seeded.
    • Mango:
      • Epicarp: Thin outer layer.
      • Mesocarp: Fleshy, edible.
      • Endocarp: Stony, hard.
    • Coconut:
      • Fibrous mesocarp.

THE SEED

  1. Development:
    • Ovules after fertilization develop into seeds.
    • Seed composed of seed coat and embryo.
  2. Structure of Dicotyledonous Seed:
    • Seed Coat:
      • Outer Testa and inner Tegmen.
      • Hilum: Scar for seed attachment.
      • Micropyle: Small pore above the hilum.
    • Embryo:
      • Embryonal axis with two cotyledons.
      • Cotyledons are often fleshy with reserve food.
      • Radicle and plumule at embryo end.
    • Endosperm:
      • Present in endospermic seeds (e.g., castor).
      • Absent in non-endospermic seeds (e.g., bean, gram).
  3. Structure of Monocotyledonous Seed:
    • Endospermic Seeds:
      • Common in monocots; orchids are an exception.
      • Membranous seed coat fused with fruit wall.
      • Bulky endosperm stores food.
      • The aleurone layer separates the endosperm and embryo.
    • Non-endospermic Seeds:
      • Orchids as an example.
      • Membranous seed coat fused with fruit wall.
      • Small embryo in a groove with scutellum, plumule, and radicle.
      • Coleoptile (encloses plumule) and coleorhiza (encloses radicle).

SEMI-TECHNICAL DESCRIPTION OF A TYPICAL FLOWERING PLANT

I. Habit:

  • General appearance and growth form of the plant.

II. Vegetative Characters:

  1. Roots:
    • Type (e.g., taproot, fibrous).
    • Presence of root hairs.
  2. Stem:
    • Type (e.g., herbaceous, woody).
    • Presence of nodes and internodes.
  3. Leaves:
    • Arrangement (e.g., alternate, opposite).
    • Type (simple, compound).
    • Presence of stipules.
    • Venation pattern (reticulate, parallel).
    • Shape, margin, apex, surface, and extent of incision.

III. Floral Characters:

  1. Inflorescence:
    • Type (e.g., racemose, cymose).
    • Examples of specific inflorescences.
  2. Flower:
    • Symmetry (actinomorphic, zygomorphic).
    • Floral whorls (calyx, corolla, androecium, gynoecium).
    • Bisexual or unisexual.
    • Aestivation type (valvate, twisted, imbricate, vexillary).

IV. Floral Diagram and Floral Formula:

  • Floral diagram: Depiction of flower parts and their arrangement.
    • The position of the mother axis is indicated by a dot.
    • Whorls are drawn successively from the outermost calyx to the central gynoecium.
  • Floral formula: Symbols representing floral parts and their characteristics.
    • Symbols include Br (bracteate), K (calyx), C (corolla), P (perianth), A (androecium), G (gynoecium).
    • Additional symbols for ovary position, gender, floral symmetry, cohesion, and adhesion.
    • Fusion is indicated by brackets; and adhesion by a line above symbols.

Related posts:

  1. Phototropism
  2. Testosterone
  3. PLANT KINGDOM
  4. BREATHING AND EXCHANGE OF GASES