Unit 1: Morphology

Modified Roots, Stems, and Leaves

Plants modify their basic parts (root, stem, leaf) to perform special functions like storage, support, protection, and propagation.

Modified Roots

• For Storage: Fleshy tap roots (e.g., Conical in Carrot, Fusiform in Radish, Napiform in Turnip).
• For Support:
  • Prop Roots: Grow down from branches to the ground (e.g., Banyan).
  • Stilt Roots: Grow obliquely from the basal nodes of the stem (e.g., Sugarcane, Maize).
• For Respiration: Pneumatophores (Respiratory roots) grow vertically upwards in waterlogged, saline soil (e.g., Mangroves like Rhizophora).

Modified Stems

• Underground (Storage/Perennation):
  • Rhizome: Fleshy, horizontal stem (e.g., Ginger, Turmeric).
  • Tuber: Swollen end of an underground branch, has "eyes" (buds) (e.g., Potato).
  • Bulb: Condensed stem with fleshy scale leaves (e.g., Onion, Garlic).
  • Corm: Solid, vertical, condensed stem (e.g., Colocasia).
• Aerial Modifications:
  • Tendrils: Coiled structures for climbing (e.g., Grapes).
  • Thorns: Hard, woody, pointed structures for protection (e.g., Bougainvillea, Citrus).
  • Phylloclade: Flattened, green stem that performs photosynthesis (leaves are reduced to spines) (e.g., Opuntia).

Modified Leaves

• Tendrils: For climbing (e.g., Pea).
• Spines: For protection and reducing water loss (e.g., Cactus).
• Storage Leaves: Fleshy leaves of Onion/Garlic.
• Phyllode: The petiole becomes flattened and green, performing photosynthesis (e.g., Acacia).
• Insectivorous Leaves: (e.g., Pitcher plant, Venus flytrap).

Phyllotaxy

Definition: Phyllotaxy is the pattern of arrangement of leaves on the stem or a branch.

• Alternate: A single leaf arises at each node (e.g., Hibiscus, Mustard).
• Opposite: A pair of leaves arises at each node, opposite to each other (e.g., Calotropis, Guava).
• Whorled (Verticillate): More than two leaves arise at a node and form a whorl (e.g., Nerium, Alstonia).

Inflorescence

Definition: The arrangement of flowers on the floral axis (peduncle).

Inflorescence Types

1. Racemose: The main axis continues to grow, and flowers are borne laterally in an acropetal succession (oldest at the bottom, youngest at the top).
   • Raceme: Pedicellate flowers (e.g., Mustard).
   • Spike: Sessile flowers (e.g., Achyranthes).
   • Umbel: Pedicellate flowers arise from a common point (e.g., Coriander).
   • Capitulum (Head): Sessile flowers (florets) arranged on a flat receptacle (e.g., Sunflower, Asteraceae family).
2. Cymose: The main axis terminates in a flower, limiting its growth. Flowers are borne in a basipetal succession (oldest at the top, youngest at the bottom).

Special Types of Inflorescence

• Cyathium: A single female flower (pistil) is surrounded by many male flowers (single stamens), all enclosed in a cup-shaped involucre (e.g., Euphorbia).
• Verticillaster: A type of cymose inflorescence seen in the Lamiaceae (Tulsi) family, where clusters of flowers form false whorls at the nodes.
• Hypanthodium: A fleshy, flask-shaped receptacle with a small opening, bearing flowers on its inner wall (e.g., Ficus/Fig).

Evolution of Inflorescence

The evolutionary trend is generally from solitary, terminal flowers to complex inflorescences. This change aids in pollination by making the flowers more conspicuous to pollinators and allows for efficient resource allocation.

Flower Structure

Aestivation

Definition: The mode of arrangement of sepals or petals in a floral bud with respect to other members of the same whorl.

• Valvate: Margins of petals/sepals touch each other without overlapping (e.g., Calotropis).
• Twisted (Contorted): One margin overlaps the next, in a regular pattern (e.g., Hibiscus, Cotton).
• Imbricate: Margins overlap, but not in a regular pattern (e.g., Cassia).
• Vexillary (Papilionaceous): Found in the Pea family. The large "standard" petal overlaps two "wing" petals, which in turn overlap two fused "keel" petals.

Placentation

Definition: The arrangement of ovules within the ovary.

• Marginal: Ovules are on the fused margin of a single carpel (e.g., Pea).
• Axile: Ovules are on the central axis in a multilocular (chambered) ovary (e.g., Tomato, Hibiscus).
• Parietal: Ovules are on the inner wall of a unilocular ovary (e.g., Mustard, Cucumber).
• Basal: A single ovule is attached at the base of the ovary (e.g., Sunflower, Marigold).
• Free-central: Ovules are on a central axis, but there are no septa (walls) (e.g., Dianthus).

Flower as a Modified Shoot

This theory states that a flower is a modified, condensed shoot specialized for reproduction.

• Thalamus: A condensed stem with highly shortened internodes.
• Floral Organs (Sepals, Petals, Stamens, Carpels): These are considered modified leaves that serve reproductive or protective functions.

Fruits and Seeds

Types of Fruits

A fruit is a mature or ripened ovary.

1. Simple Fruits: Develop from a single carpel or syncarpous ovary of a single flower.
   • Fleshy: (e.g., Drupe - Mango; Berry - Tomato; Pome - Apple).
   • Dry (Dehiscent): (e.g., Legume - Pea; Capsule - Cotton).
   • Dry (Indehiscent): (e.g., Achene - Sunflower; Caryopsis - Wheat).
2. Aggregate Fruits: Develop from a multicarpellary, apocarpous ovary of a single flower (e.g., Strawberry, Raspberry).
3. Multiple (Composite) Fruits: Develop from a complete inflorescence (e.g., Pineapple, Jackfruit, Fig).

Structure of Fruits and Seeds

• Fruit Structure: The fruit wall (pericarp) is often differentiated into an outer epicarp, middle mesocarp, and inner endocarp.
• Seed Structure: A seed consists of a seed coat (testa, tegmen) and an embryo. The embryo has an embryonal axis (radicle, plumule) and one (monocot) or two (dicot) cotyledons. Some seeds have endosperm (food storage), while others (non-endospermic) store food in the cotyledons.

Floral Formula and Floral Diagram

Floral Formula

A shorthand summary of a flower's structure using symbols. It is essential for family descriptions.

Common Symbols:

• Br: Bracteate
• Ebr: Ebracteate
• ⊕ (Actinomorphic): Radially symmetrical.
• % (Zygomorphic): Bilaterally symmetrical.
• ⚥: Bisexual (Hermaphrodite)
• K: Calyx (sepals). e.g., K(5) = 5 fused sepals.
• C: Corolla (petals). e.g., C5 = 5 free petals.
• P: Perianth (when calyx and corolla are not distinct).
• A: Androecium (stamens). e.g., A(9)+1 = 10 stamens, 9 fused and 1 free.
• G: Gynoecium (carpels). e.g., G(2) = 2 fused carpels; G(2) (with a line below G) = Superior ovary.

Floral Diagram

A diagram showing the ground plan of a flower. It represents the number of parts, their arrangement, how they are related (fused or free), and their position relative to the mother axis (the stem from which the flower arises, shown as a dot).

Role of Morphology in Taxonomy

Morphology (the study of external form and structure) is the classical and most fundamental basis for plant taxonomy.

• Accessibility: Morphological characters are easily observable in the field or from herbarium specimens, without needing complex equipment.
• Stability: Floral characters (like placentation, aestivation, number of parts) are generally more stable and conserved (less likely to change) than vegetative characters (like leaf size), making them more reliable for classification.
• Key Formation: Most taxonomic keys are based almost entirely on morphological characters, allowing for practical identification.
• Historical Basis: Major classification systems (like Bentham & Hooker) were built using detailed morphological comparisons.