Learning objectives
- Label a leaf cross-section
- Relate each tissue to its function
- Explain how stomata open and close and how transpiration is measured
1. Cross-section, top to bottom
Waxy cuticle (reduces water loss) → upper epidermis (transparent, lets light through) → palisade mesophyll (tall cells packed with chloroplasts, main photosynthetic tissue) → spongy mesophyll (air spaces for gas diffusion) → lower epidermis with stomata and guard cells → vein containing xylem and phloem.
2. How stomata work
Guard cells take in water by osmosis, become turgid, and because their inner walls are thicker they curve apart and the stoma opens. When they lose water they become flaccid and the stoma closes, conserving water.
Most stomata are on the lower surface, which is shaded and cooler — an adaptation that reduces water loss.
3. Transpiration
Transpiration is the loss of water vapour from the aerial parts of a plant, mainly through the stomata. The evaporation pulls a continuous column of water up the xylem — the transpiration stream — held together by cohesion and adhesion.
Rate increases with higher temperature, higher light intensity, faster wind speed and lower humidity. A potometer measures water uptake, which is used as an estimate of transpiration rate.
- Temperature ↑ → rate ↑
- Humidity ↑ → rate ↓
- Wind speed ↑ → rate ↑
- Light intensity ↑ → stomata open → rate ↑
Key definitions
- Transpiration
- Loss of water vapour from the aerial parts of a plant, mainly through stomata.
- Stoma
- A pore in the epidermis, bounded by two guard cells, through which gases and water vapour pass.
- Cohesion-tension theory
- Explanation of water movement up the xylem by evaporation pull and cohesion between water molecules.
Exam tips
- A potometer measures water uptake, not transpiration directly — say so, it is a mark.
- Link every structure to a function: 'many chloroplasts in the palisade layer, therefore maximum light absorption'.
Practice exam questions & model answers
Q1.Explain why the palisade mesophyll is located near the upper surface of the leaf.
It is the main photosynthetic tissue and contains the most chloroplasts, so placing it directly beneath the transparent upper epidermis maximises the light it absorbs.
Q2.Describe the mechanism by which a stoma opens.
Guard cells actively accumulate potassium ions, lowering their water potential. Water enters by osmosis, they become turgid, and because the inner wall is thicker and less elastic than the outer wall the cells curve apart, opening the pore.
Q3.State four environmental factors that increase the rate of transpiration.
High temperature, high light intensity, high wind speed and low humidity.
Continue revising
Plant Cell Structure and Function
Plant BiologyPhotosynthesis: Process, Equation and Limiting Factors
Agricultural SciencePlant Nutrition, NPK and Soil Fertility
Plant PathologyPlant Pathology Basics: Pathogens, Disease Triangle and Koch's Postulates
Put the theory to work — diagnose a real leaf photo or browse the disease library.