Demonstration of Transpiration: Bell Jar Experiment Method

The bell jar experiment proves that plants lose water vapour from their leaves — transpiration. A watered potted plant is covered with a transparent bell jar, and within hours tiny droplets condense on the jar's inner wall: vapour the leaves released.

It matters because transpiration drives the plant's water transport — leaf evaporation pulls fresh water and dissolved minerals up from the roots. Seeing invisible vapour become visible droplets makes an abstract process concrete.

Key Concepts

• Transpiration: loss of water vapour from plant aerial parts, mainly through leaf pores called stomata.
• Stomata: tiny adjustable pores (mostly on leaf undersides) through which vapour escapes and carbon dioxide enters.
• Condensation: vapour cooling on the jar's inner surface and turning back into liquid droplets.
• Control experiment: an identical setup without the plant (or with the soil sealed in polythene), run to rule out other water sources.
• Transpiration pull: the suction created by leaf evaporation that draws the water column up the xylem.

How It Works

Water absorbed by roots travels up the stem, evaporates from leaf cells, and diffuses out through open stomata. Inside the sealed jar the vapour saturates the trapped air and condenses as droplets on the cooler glass. The procedure:

1. Take a healthy, well-watered potted plant and seal the pot's soil surface with a polythene sheet (so soil evaporation cannot contribute).
2. Cover the whole plant with a clean, dry bell jar, sealing its rim to the bench with vaseline or a rubber sheet.
3. Set up an identical control: a pot of soil without a plant, sealed the same way under a second bell jar.
4. Leave both in bright warm light for 3–4 hours.
5. Observe: the plant jar mists with droplets; the control stays nearly dry.

The only difference between the jars is the plant, so the droplets must come from its leaves — direct proof of transpiration.

Worked Example

Question: To measure transpiration quantitatively, a student seals a potted plant in a polythene bag (pot and soil wrapped, only shoot exposed). The whole setup weighs 2.450 kg at 9 am and 2.418 kg at 9 am the next day. How much water did the plant transpire in 24 hours?

1. Initial mass = 2.450 kg = 2450 g.
2. Final mass = 2.418 kg = 2418 g.
3. Mass lost = 2450 − 2418 = 32 g.
4. Since the soil is sealed, this loss can only be water vapour from the shoot.
5. Answer: 32 g of water transpired in 24 hours (about 1.33 g per hour) — a small plant can move tens of grams of water daily.

Common Misconceptions

• "The droplets come from the wet soil." Wrong — that is exactly why the soil is sealed with polythene and a plant-free control jar is run alongside.
• "Transpiration is plants 'sweating' waste." Wrong — it is not waste removal; it cools the leaf and, crucially, powers the upward pull of water and minerals.
• "Only leaves transpire." Mostly true but incomplete — green stems and flowers also lose some vapour, just far less than leaves.

Real-World Applications

• Irrigation scheduling: farmers estimate crop water needs from transpiration rates to water efficiently.
• Greenhouse control: managing humidity prevents fungal disease while keeping transpiration (and nutrient uptake) healthy.
• Forestry: transpiration from forests recycles rainfall — the Amazon makes much of its own rain this way.

The bell jar proves transpiration simply: a plant under glass mists the jar while a plant-free control stays dry, so the water must come from the leaves. That invisible vapour stream hauls water and minerals up from the roots — the quiet pump behind almost everything a plant does.