Question
What is transpiration? Describe its types, the factors that affect it, and explain why it is called a “necessary evil” for plants.
(NEET + CBSE Class 11)
Solution — Step by Step
Transpiration is the loss of water vapour from aerial parts of the plant, mainly through leaves.
| Type | Through | Percentage |
|---|---|---|
| Stomatal | Stomata on leaves | ~90% |
| Cuticular | Cuticle (waxy layer) on epidermis | ~8-9% |
| Lenticular | Lenticels on stem bark | ~1-2% |
Stomatal transpiration dominates — this is why guard cells (which control stomata opening/closing) are so important.
| Factor | Effect |
|---|---|
| Temperature | Higher temp = more transpiration (faster evaporation) |
| Humidity | Higher humidity = less transpiration (smaller concentration gradient) |
| Wind | More wind = more transpiration (removes humid air near leaf) |
| Light | More light = stomata open = more transpiration |
| CO₂ concentration | Higher CO₂ = stomata close = less transpiration |
| Leaf area | More surface area = more transpiration |
The evil part: Plants lose enormous amounts of water. A maize plant loses about 200 litres of water during its growing season. Less than 5% of water absorbed by roots is used in growth — the rest is lost in transpiration.
The necessary part: Transpiration creates the transpiration pull — a suction force that pulls water up from roots through xylem to the topmost leaves. Without transpiration, water could not reach the canopy of tall trees (some over 100 m). It also:
- Cools the leaf surface (evaporative cooling)
- Helps in absorption and upward transport of minerals
- Maintains cell turgidity
Water Movement Pathway
flowchart TD
A["Soil water"] -->|"Absorption by root hairs"| B["Root xylem"]
B -->|"Transpiration pull + root pressure"| C["Stem xylem"]
C -->|"Continuous water column"| D["Leaf xylem"]
D -->|"Into mesophyll cells"| E["Evaporation from cell surfaces"]
E -->|"Diffusion through stomata"| F["Water vapour exits leaf"]
F -->|"Creates negative pressure"| G["Transpiration pull — drives the whole system"]
G -.->|"Pulls water upward"| BWhy This Works
Transpiration works because of the cohesion-tension theory. Water molecules stick to each other (cohesion) and to xylem walls (adhesion), forming an unbroken column. When water evaporates from leaves, it creates negative pressure (tension) at the top, pulling the entire water column upward.
This is strong enough to move water to the top of the tallest trees — no pump needed. The driving force is solar energy (which causes evaporation), making transpiration a solar-powered water transport system.
Common Mistake
Students think root pressure is the main force driving water up in tall trees. Root pressure can push water up only a few metres — it is significant mainly in small herbaceous plants and during low transpiration (night). In tall trees, the dominant force is transpiration pull (cohesion-tension). NEET specifically asks: “What is the main driving force for water ascent in tall trees?” — answer is transpiration pull, not root pressure.