Transpiration

Chapter in a nutshell: Transpiration is the evaporative loss of water as water vapour from the aerial parts (mainly leaves) of a plant — only ~2% of absorbed water is used by the plant, while ~98% is lost this way. Though seemingly wasteful, it cools the plant, creates the suction force that drives the ascent of sap, and distributes water and minerals; this chapter covers its kinds, mechanism, factors, significance, adaptations, and the related processes of guttation and bleeding.

1. What is Transpiration?

Transpiration is the loss of water in the form of water vapour from the leaves and other aerial parts of the plant — i.e. the evaporative loss of water from the aerial parts (leaves and stem).

• Plants continuously absorb water through their roots; it is conducted upward through the stem (xylem) to all aerial parts, including leaves.
• Only about 2% of this water is used in photosynthesis and other activities; the rest (~98%) is lost as vapour during transpiration.
• It is a vital and partly physical process, controlled by both internal and external factors.

Transpiration is NOT excretion — it removes no metabolic waste and is not the deliberate elimination of "excess water," so it is wrong to call it an excretory process.

2. Demonstration of Transpiration

3. Measurement of Transpiration

1. Weighing method: A potted plant (soil/pot covered to stop other evaporation) is weighed before and after; the loss in weight is water transpired. A graduated tube with a leafy shoot (no roots) and oil on the water can also measure loss by volume (1 cc = 1 g).

2. Potometer method: Poton = drink, meter = measure. A potometer measures the rate of water uptake by a cut shoot, assumed almost equal to the water transpired. Ganong's measures uptake rate; Darwin's demonstrates suction force; Garreau's demonstrates unequal transpiration from the two surfaces of a dorsiventral leaf. In Ganong's potometer, a twig is cut obliquely under water and fixed in a water-filled apparatus with no air spaces; an air bubble in the graduated capillary moves as suction pulls water from the beaker, and the distance moved gives the volume taken up.

Limitations of the potometer: (i) introducing the air bubble is not easy; (ii) the twig may not stay alive long; (iii) outside temperature shifts the bubble's position; (iv) it measures water uptake, not water actually transpired.

4. Kinds of Transpiration

• Cuticle is a waxy layer on both leaf surfaces. The thicker the cuticle, the lesser the evaporation — desert plants have thick cuticles.
• Lenticels are openings on older stems allowing gas diffusion; lenticels never close — they remain open all the time.

5. Mechanism of Stomatal Transpiration

Stomata are minute openings in the leaf epidermis (1,000–10,000 per cm²). Each stoma is bordered by two bean-shaped guard cells containing chloroplasts and a nucleus.

Pathway of water vapour out of the leaf:

1. Water absorbed by roots rises up the stem and reaches leaf tissues via the veins (xylem vessels).
2. Spongy mesophyll cells expose their surfaces to the intercellular spaces and give out a thin water film.
3. This film evaporates, saturating the intercellular air with vapour.
4. Vapour diffuses (high → low concentration) through the spaces to the sub-stomatal space, then escapes through the stomata.

Sequence (exam favourite): xylem → mesophyll cells → intercellular space → sub-stomatal space → stoma.

Most water travels along cell walls by imbibition; only a small part enters cells by osmosis. This evaporation pulls more water up — the transpiration pull can draw water ~50 m in tall trees. More transpiration occurs from the undersurface of a dicot leaf, as more stomata are on the lower surface.

Stomatal regulation: Stomata open in the day (CO₂ for photosynthesis), close at night, depending on guard-cell turgidity — turgid cells open the pore, flaccid ones close it. Mechanisms: starch → sugar interconversion and active K⁺ transport (K⁺ enters guard cells in light → water enters by osmosis → turgid → stoma opens).

Midday wilting: In plants like balsam, leaves wilt at midday despite ample soil water — transpiration exceeds water absorption. In the cooler evening, turgidity returns and leaves stand erect.

6. Factors Affecting the Rate of Transpiration

A. External (environmental) factors

B. Internal factors

• Water content of leaves: low water → leaves wilt → stomata close → transpiration reduced (a natural water-conserving response, indirectly due to stomatal closure).
• Leaf area / structure: larger area → more transpiration; thick cuticle, sunken or fewer stomata → less.

Maximum transpiration occurs in hot, dry, windy weather. It is least in cool, humid, still air.

7. Adaptations to Reduce Excessive Transpiration

Plants of dry climates (xerophytes) have permanent adaptations to curtail water loss:

8. Significance of Transpiration

1. Cooling effect: evaporation lowers leaf-surface temperature — protects leaves on hot days (intense heat destroys enzymes).
2. Suction force (ascent of sap): evaporation concentrates cell sap, raising osmotic pressure; this draws water from cells below in sequence and ultimately from the soil — the transpiration pull / transpiration stream.
3. Distribution of water and minerals: transpiration from leaf tips draws water and dissolved minerals towards all branches and twigs.
4. Affects climate / brings rain: vast forests release huge quantities of vapour, raising atmospheric moisture and helping bring rain.

Scale of water loss: a sunflower ~½ litre/day; a maize plant ~2 litres/day; a large apple tree ~30 litres/day. Forests contribute to rain — transpiration is the secret.

9. Guttation and Bleeding (Direct Loss of Water)

Some plants lose water directly as liquid, not vapour, in two ways:

• Guttation (gutta = a drop): loss of water as droplets from the margins/tips of leaves through special pores called hydathodes. Occurs in warm, humid conditions, usually at night/early morning, when humidity hampers transpiration but roots keep absorbing water — building root pressure that forces out water. Common in banana, nasturtium, colocasia, balsam, strawberry. (Guttation droplets are NOT dew.)
• Bleeding: oozing of cell sap from injured/cut surfaces, assisted by root pressure; e.g. milky latex from cut Calotropis / Euphorbia.

Worked / Structured Examples

Q1. Trace the path of a water molecule from soil to atmosphere. A: Soil water → root hair (osmosis/imbibition) → root cortex → root xylem → stem xylem (rises by cohesion + root pressure + capillarity + transpiration pull) → leaf veins → mesophyll cell walls → evaporates into intercellular spaces → sub-stomatal space → out through stoma as vapour.

Q2. Four China-Rose leaves (room, 24 hrs): A — vaseline on upper surface, B — on lower, C — both, D — uncoated. Which limps most / least, and why? A: Most limp = A (upper coated, but most stomata are on the lower surface, so it still transpires heavily). Least limp = C (both surfaces blocked → transpiration almost stopped).

Q3. A leafy shoot is in a test-tube of water with oil on the surface; the tube loses weight. Why oil? Why would a single flower not serve as well? A: Oil prevents direct evaporation from the water surface, so weight loss is due only to the shoot's transpiration. A single flower has few/no leaves, hence minimal stomata and little measurable loss.

Q4. In Ganong's potometer, what happens to the air bubble if kept (i) in dark, (ii) in sunlight, (iii) in front of a fan? A: (i) Dark — stomata close, transpiration ↓, bubble moves slowly. (ii) Sunlight — stomata open, transpiration ↑, faster. (iii) Fan/wind — vapour swept away, fastest.

Q5. Cobalt chloride paper is clipped to both surfaces of a leaf for 30 minutes. What change occurs? A: Paper on the lower surface turns pink much faster (more stomata → more transpiration); the paper on the upper surface stays blue or turns pink very slowly. Proves more transpiration from the lower surface of a dicot leaf.

Key Terms — Quick Glossary

Common Mistakes to Avoid

• Don't say transpiration is loss "from the whole plant" — it is from the aerial parts (leaves and stem), not roots.
• Don't call transpiration an excretory process — it removes no metabolic waste; only pure water changes to vapour.
• Don't confuse guttation (droplets, via hydathodes, at night, humid) with transpiration (vapour, via stomata, in day); droplets at leaf margins are not dew.
• Don't state a potometer measures water transpired — it measures water uptake.
• Don't say more transpiration occurs from the upper surface of a dicot leaf — it is the lower (under) surface.
• Don't mix indicator colours: cobalt chloride paper is blue when dry, pink when moist.

Likely Exam Questions (with crisp answers)

1. Define transpiration. — The loss of water in the form of water vapour from the leaves and aerial parts of a plant.
2. Name the three kinds of transpiration. — Stomatal, cuticular, and lenticular.
3. Which kind is maximum and which is minimum? — Maximum: stomatal; minimum: lenticular.
4. Why does more transpiration occur from the lower surface of a dicot leaf? — Because more stomata are present on the lower (under) surface.
5. What is a potometer? Name a type. — A device measuring the rate of water uptake by a cut shoot; e.g., Ganong's potometer.
6. State two limitations of the potometer. — (i) Introducing the air bubble is difficult; (ii) it measures uptake, not actual transpiration.
7. Give two characteristics of cobalt chloride paper that suit transpiration experiments. — It is blue when dry and turns pink when moist — a clear visible moisture indicator.
8. List four advantages of transpiration. — Cooling, suction force (ascent of sap), distribution of water/minerals, and raising atmospheric moisture (brings rain).
9. State three external factors affecting transpiration. — Sunlight intensity, temperature, wind velocity (also humidity, CO₂, atmospheric pressure).
10. Under what conditions is transpiration fastest? — Hot, dry, and windy weather.
11. Why do some leaves wilt at midday despite well-watered soil? — Midday transpiration exceeds root absorption → cells lose turgidity → wilting; recovers in the cool evening.
12. List three adaptations to reduce transpiration. — Sunken stomata, thick cuticle, reduced/spiny leaves (also fewer stomata, rolled leaves).
13. Differentiate transpiration and guttation. — Transpiration: vapour, via stomata/lenticels/cuticle, in daytime, regulated, cools plant. Guttation: droplets (with minerals), via hydathodes, at night, in humid conditions, unregulated, no cooling.
14. Differentiate guttation and bleeding. — Guttation: droplets from uninjured leaf margins (hydathodes). Bleeding: sap oozes from injured/cut surfaces. Both use root pressure.
15. How does transpiration help bring rain? — Forests release enormous vapour, raising atmospheric moisture and favouring cloud formation and rainfall.
16. Why must the leaf stay attached in a transpiration experiment? — A detached leaf cannot draw water from roots and stops transpiring; attached, it continues normal uptake and loss.