Plant Morphology

Introduction

Morphology deals with the study of external forms and features of different plant organs. Angiosperms are characterised by the presence of roots, stems, leaves, flowers, and fruits. The underground part is the root system, while the portion above ground is the shoot system.

5.1 The Root

Roots provide anchorage, absorb water/minerals, store food, and synthesise plant growth regulators.

Types of Root Systems

• Tap Root System: Direct elongation of the radicle forms the primary root. Found in Dicotyledonous plants (e.g., Mustard).
• Fibrous Root System: Primary root is short-lived and replaced by many roots originating from the base of the stem. Found in Monocotyledonous plants (e.g., Wheat).
• Adventitious Roots: Roots arise from parts other than the radicle (e.g., Grass, Monstera, Banyan tree).

5.1.1 Regions of the Root

• Root Cap: Thimble-like structure protecting the tender apex.
• Region of Meristematic Activity: Small, thin-walled cells with dense protoplasm that divide repeatedly.
• Region of Elongation: Cells undergo rapid elongation/enlargement (responsible for growth in length).
• Region of Maturation: Cells differentiate and mature. Epidermal cells here form root hairs for absorption.

5.2 The Stem

The stem develops from the plumule of the embryo. It bears nodes (where leaves are born) and internodes (portions between nodes). Functions include spreading branches, conduction of water/minerals/photosynthates, storage, support, and vegetative propagation.

5.3 The Leaf

A lateral, flattened structure for photosynthesis. Consists of leaf base, petiole, and lamina. The leaf base may bear stipules or become swollen (pulvinus in legumes).

• Venation: Arrangement of veins/veinlets.
  • Reticulate: Network formed (Dicots).
  • Parallel: Veins run parallel (Monocots).
• Types:
  • Simple: Lamina is entire or incisions do not touch the midrib.
  • Compound: Incisions reach the midrib, breaking leaf into leaflets. (Pinnately compound e.g., Neem; Palmately compound e.g., Silk Cotton).
• Phyllotaxy: Pattern of leaf arrangement.
  • Alternate: Single leaf at each node (China rose, Mustard).
  • Opposite: Pair of leaves at each node (Calotropis, Guava).
  • Whorled: More than two leaves at a node (Alstonia).

5.4 The Inflorescence

The arrangement of flowers on the floral axis.

• Racemose: Main axis continues to grow; flowers borne laterally in acropetal succession.
• Cymose: Main axis terminates in a flower (limited growth); flowers borne in basipetal order.

5.5 The Flower

The reproductive unit possessing four whorls: Calyx, Corolla, Androecium, and Gynoecium. Swollen end of stalk is thalamus.

General Characteristics

• Symmetry:
  • Actinomorphic (Radial): Divided into equal halves in any radial plane (Mustard, Datura).
  • Zygomorphic (Bilateral): Two similar halves only in one vertical plane (Pea, Gulmohur).
  • Asymmetric: Cannot be divided into two similar halves (Canna).
• Position on Thalamus:
  • Hypogynous: Ovary is Superior (Mustard, China rose).
  • Perigynous: Ovary is Half-Inferior (Plum, Rose).
  • Epigynous: Ovary is Inferior (Guava, Cucumber).

5.5.1 Parts of a Flower

• Calyx (Sepals): Outermost, protective. Gamosepalous (united) or Polysepalous (free).
• Corolla (Petals): Brightly coloured. Aestivation types:
  – Valvate: Margins touch without overlapping (Calotropis).
  – Twisted: One margin overlaps the next (China rose).
  – Imbricate: Overlap irregularly (Cassia).
  – Vexillary: Standard overlaps wings, which overlap keel (Pea/Bean).
• Androecium (Stamens): Male organ. Can be Epipetalous (on petals) or Epiphyllous (on perianth). Fusion: Monoadelphous (China rose), Diadelphous (Pea), Polyadelphous (Citrus).
• Gynoecium (Carpels): Female organ. Apocarpous (free carpels e.g., Lotus) or Syncarpous (fused e.g., Tomato).
  – Placentation: Marginal (Pea), Axile (Lemon), Parietal (Mustard), Free central (Dianthus), Basal (Sunflower).

5.6 The Fruit & 5.7 The Seed

The Fruit

Mature ripened ovary. If formed without fertilisation, it is parthenocarpic. The wall is called pericarp (Epicarp, Mesocarp, Endocarp).

• Drupe: Stony endocarp (e.g., Mango, Coconut). In Coconut, mesocarp is fibrous.

The Seed

• Dicotyledonous: Seed coat (Testa/Tegmen), Hilum (scar), Micropyle (pore). Embryo consists of embryonal axis, radicle, plumule, and two cotyledons.
• Monocotyledonous: Generally endospermic (except Orchids).
  – Outer covering of endosperm is aleurone layer.
  – Single large cotyledon called scutellum.
  – Plumule covered by coleoptile; Radicle covered by coleorhiza.

5.8 & 5.9 Description of Families

Floral Formula Symbols

Br (Bracteate), K (Calyx), C (Corolla), P (Perianth), A (Androecium), G (Gynoecium). G (Superior ovary), Ḡ (Inferior ovary). ⊕ (Actinomorphic), % (Zygomorphic).