Short Answer Questions — Chapter 12: Plant Anatomy and Plant Physiology
Plant tissues are grouped into two main categories:
1. Meristematic Tissue (actively dividing)
- Apical meristem: At root tips and shoot tips → growth in length
- Lateral meristem: Along the sides (cambium) → growth in width
- Intercalary meristem: At nodes of grasses → elongation
2. Permanent Tissue (non-dividing)
(a) Simple permanent tissue:
- Parenchyma: Most common; storage, packing, photosynthesis
- Collenchyma: Mechanical support in young stems; elongated with thickened corners
- Sclerenchyma: Mechanical support; thick, lignified walls; dead cells; fibres and sclereids
(b) Complex permanent tissue:
- Xylem: Water and mineral conduction (upward); vessel elements, tracheids
- Phloem: Food conduction; sieve tube elements, companion cells
Photosynthesis is the process by which green plants synthesise glucose from CO₂ and H₂O using sunlight energy, in the presence of chlorophyll.
Overall equation:
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
(Carbon dioxide + Water → Glucose + Oxygen)
Conditions required: Sunlight, chlorophyll (in chloroplasts), CO₂, and water.
Two stages:
Transpiration is the loss of water in the form of water vapour from aerial parts of plants (mainly through stomata, but also through cuticle and lenticels).
Types:
- Stomatal transpiration (~90% of total) — through stomata
- Cuticular transpiration (~10%) — through the waxy cuticle
- Lenticular transpiration (very small) — through lenticels in bark
Advantages of transpiration: 1. Ascent of sap: Creates transpiration pull (negative pressure) that draws water upward from roots to leaves through xylem. 2. Cooling: Evaporation of water from leaves cools the plant surface (similar to sweating in animals). 3. Mineral absorption: Drives absorption of minerals from soil along with water. 4. Maintains turgidity: Keeps cells turgid, maintaining plant shape.
Disadvantages: 1. Plants may lose more water than they can absorb during drought → wilting 2. During hot dry days, stomata close to prevent water loss → photosynthesis decreases (CO₂ can't enter)
| Feature | Xylem | Phloem |
|---|---|---|
| Function | Conducts water and minerals upward (roots → leaves) | Conducts food (sucrose) from leaves to all parts |
| Direction | Mostly upward (unidirectional) | Upward and downward (bidirectional) |
| Cells | Tracheids, vessels (dead cells), fibres, xylem parenchyma | Sieve tubes, companion cells (living), phloem parenchyma, phloem fibres |
| Cell state | Mostly dead cells at maturity | Living cells |
| Force | Transpiration pull, root pressure | Osmotic pressure (Münch's pressure-flow) |
| Location | Centre of vascular bundle | Periphery of vascular bundle |
Leaf is the primary organ of photosynthesis.
External structure:
- Lamina (blade): Flat, expanded surface to capture maximum light
- Petiole: Stalk attaching leaf to stem
- Veins: Network of xylem and phloem
- Midrib: Main central vein
Internal structure (cross-section): 1. Upper epidermis: Single cell layer, waxy cuticle, no chloroplasts; prevents water loss 2. Palisade mesophyll: Elongated cells with many chloroplasts; primary site of photosynthesis 3. Spongy mesophyll: Loosely packed cells with air spaces; CO₂ exchange 4. Vascular bundles (veins): Xylem (upper) and phloem (lower) 5. Lower epidermis: Contains stomata with guard cells
Functions of leaf: 1. Photosynthesis — food synthesis 2. Transpiration — water loss 3. Gas exchange — CO₂ in, O₂ out (photosynthesis); O₂ in, CO₂ out (respiration) 4. Storage — some leaves store food (e.g., onion)
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