Morphology of Flowering Plants

Introduction

Morphology is the study of the external form and structure of organisms. In flowering plants (angiosperms), the body is differentiated into root, stem, and leaf (vegetative organs) and flower, fruit, and seed (reproductive organs). Understanding morphology is essential for plant identification and classification.

The Root System

Roots are underground organs that anchor the plant and absorb water and minerals.

• Types of Root Systems:
• Tap root system: primary root (radicle) grows deep; lateral roots arise from it. Found in dicots (e.g., mustard, gram, banyan).
• Fibrous root system: primary root short-lived; replaced by adventitious roots from stem base. Found in monocots (e.g., wheat, maize, grass).
• Adventitious roots: arise from parts other than radicle (stem or leaf). Can be of many kinds.

• Modifications of Root:
• Storage: · Daucus · (carrot — fusiform), · Beta · (turnip-shaped), · Manihot · (fasciculated — cassava)
• Respiration (Pneumatophores): · Rhizophora · (mangrove) — vertical roots come out of water for gas exchange
• Support (Prop roots): · Ficus benghalensis · (banyan) — aerial roots from branches provide support
• Nitrogen fixation (Nodulated roots): · Pisum · (pea), · Cicer · (gram) — Rhizobium in root nodules

The Stem

The stem bears leaves, branches, flowers, and fruits. It is differentiated into nodes (where leaves arise) and internodes (regions between nodes).

• Modifications of Stem:
• Underground stems: · Rhizome · (ginger, turmeric) — horizontal, underground; · Corm · (Colocasia, Saffron) — swollen, solid, upright; · Tuber · (potato — Solanum tuberosum) — swollen tip with eyes (axillary buds); · Bulb · (onion, garlic) — very short disc-like stem with fleshy scale leaves
• Aerial stems: · Tendril · (Cucurbita — pumpkin) — slender; · Thorn · (Bougainvillea, Citrus) — protect plant; · Phylloclade · (Opuntia/cactus) — flattened, green, photosynthetic
• Sub-aerial stems: · Stolon · (strawberry, Jasmine) — runners along ground; · Offset · ( · Pistia · , · Eichhornia · ) — short internode, horizontal

The Leaf

A leaf is a lateral, flattened outgrowth from the stem at a node. Parts: petiole (stalk), leaf blade (lamina), leaf base (stipule may be present).

• Venation:
• Reticulate (net-like): veins form a network — in dicots (mango, peepal, rose)
• Parallel: veins run parallel — in monocots (wheat, maize, banana)

• Types of Leaves:
• Simple: a single, undivided lamina (e.g., mango, peepal)
• Compound: lamina divided into leaflets. · Pinnately compound · (leaflets on a common axis/rachis) e.g., · Neem · ; · Palmately compound · (leaflets arise from tip of petiole) e.g., · Silk cotton · .

• Phyllotaxy (Arrangement of leaves on stem):
• · Alternate · : one leaf per node (China rose, mustard)
• · Opposite · : two leaves per node (Calotropis, guava)
• · Whorled · : more than two leaves per node (Alstonia)

• Modifications of Leaves:
• · Tendrils · : · Pisum · (pea) — upper leaflets modified; · Lathyrus · — entire leaf into tendril
• · Spines · : · Opuntia · (cactus) — reduces transpiration
• · Phyllode · : · Acacia · — petiole becomes flat and leaf-like
• · Insectivorous modifications · : · Pitcher · ( · Nepenthes · ), · Bladder · ( · Utricularia · ), · Venus flytrap · ( · Dionaea · )

The Flower

A flower is a modified shoot for sexual reproduction. Parts (from outside to inside): Calyx (sepals) → Corolla (petals) → Androecium (stamens) → Gynoecium (carpels).

• Key Terms:
• · Bisexual · (perfect): both androecium and gynoecium ( · mustard · , · rose · )
• · Unisexual · (imperfect): only one type ( · papaya · , · watermelon · )
• · Actinomorphic · : radial symmetry (mustard, datura); · Zygomorphic · : bilateral symmetry (pea, gulmohar)
• · Epigynous · : ovary inferior (guava, cucumber, sunflower); · Hypogynous · : ovary superior (mustard, brinjal); · Perigynous · : ovary half-inferior (rose, peach)

• Placentation (arrangement of ovules in ovary):
• · Marginal · : ovules on margin of fused carpel (pea)
• · Axile · : ovules on central axis of multilocular ovary (tomato, orange)
• · Parietal · : ovules on inner wall of unilocular ovary (mustard, cucumber)
• · Free central · : ovules on central column without septa (Dianthus, Primula)
• · Basal · : single ovule at base (sunflower, marigold)

The Fruit and Seed

• Fruit develops from ovary after fertilisation; true fruit from ovary only; false fruit includes other floral parts (apple = thalamus; strawberry = thalamus).
• Seed: fertilised, mature ovule. Dicot seed ( · gram · , · pea · ): testa, tegmen, two cotyledons, embryo axis (radicle + plumule). Monocot seed ( · maize · ): scutellum (single cotyledon), aleurone layer, endosperm, coleoptile and coleorhiza.