Put a Plastic Bag over a Branch: A Bag of Water Drops Reveals Transpiration

The question this experiment answers is beautifully simple: where does the water a plant drinks actually go? Three plastic bags build a clean set of controls, and an oil-sealed measuring cylinder turns "how much water escaped" into a number you can read off a scale — one set of materials that covers both the qualitative and the quantitative side.

There is no open flame and no chemical involved; this is one of the lowest-risk experiments on the site. The result does take a few hours to appear, so it works best as an observation task you set up ahead of time.

Safety first (read this before you start)

• Cut branches with scissors or pruning shears. If students do the cutting, demonstrate the grip first: angle the cut downward and keep fingers away from the blade.
• Take cuttings only from plants on your own balcony or in your own yard. Greenery at school or around the neighborhood is not yours to trim — get the property manager's permission first. That, too, is a lesson in civic awareness.
• Petroleum jelly is a safe topical product, but never put it in your mouth or rub your eyes; wash your hands after applying it.
• Plastic bags are a suffocation hazard — keep them out of reach when younger siblings are around, and sort them into the proper waste stream when the experiment ends.
• Handle glass measuring cylinders gently. Lay down newspaper when pouring the cooking-oil seal; oil on the floor is very slippery.

Materials and equipment

• 1–2 healthy potted plants (pothos, rose, or gardenia all work; the more intact leaves the better);
• 3 clear plastic bags (food-storage bags are fine, with no holes) and 3 twist ties or rubber bands;
• 2 measuring cylinders or narrow-necked bottles, a little cooking oil (for the oil seal), and clean water;
• 2 freshly cut leafy branches (one keeping all of its leaves, one with every leaf removed);
• a small tub of petroleum jelly and cotton swabs (for the stomata-blocking extension);
• a marker, a ruler, a notebook — and a kitchen scale if you have one.

Step-by-step

Step 1: three bags of controls (qualitative)

1. Pick two similar spots on the potted plant: one leafy branch and one branch that is stem only, with its leaves removed (strip those leaves half a day in advance so the wounds can dry).
2. Cover each with a clear plastic bag and tie the mouth tightly around the base of the branch — do not seal the pot and soil inside.
3. Blow some air into the third bag and tie it shut as a blank control, hung in the same environment.
4. Put the whole pot somewhere with gentle sunlight and moving air and leave it for 3–5 hours (warm, sunny weather speeds things up).

Step 2: oil-sealed cylinders (quantitative)

1. Add equal amounts of water to the two cylinders, then drip in a layer of cooking oil until the surface is completely covered. The oil seal blocks direct evaporation from the water surface, so any drop in the reading can only come from the branch.
2. Stand the leafy branch in one cylinder and the leafless branch in the other, matching leaf count and thickness as closely as you can; note the starting level and the time.
3. Keep both on the same windowsill and read the levels every half day for 2–3 days. If you have a kitchen scale, weigh each whole setup as well for a second, independent record.

Step 3 (extension): which side of the leaf holds the stomata

1. On one branch, choose three leaves of similar size: coat one all over its upper surface with petroleum jelly, coat one all over its lower surface, and leave one uncoated.
2. Watch for two or three days and compare the order in which the leaves wilt. Whichever surface the jelly smothers is the surface whose stomata are sealed shut.

What you'll see

• Within a few hours the leafy branch's bag fogs over, then gathers real droplets — unmistakable in sunlight;
• the leafless branch's bag shows only the faintest mist, and the empty bag stays dry throughout;
• in the oil-sealed cylinders, the level on the leafy side falls noticeably day after day while the leafless side barely moves; more leaves and sunnier weather widen the gap;
• in the petroleum-jelly extension, the leaf with its lower surface coated holds out longest, while the upper-coated and uncoated leaves wilt sooner — evidence that most plants keep their stomata concentrated in the lower epidermis.

Why the water leaves through the leaves

The droplets on the bag are water that left the leaf as water vapor and condensed back to liquid on the cooler plastic. That liquid–gas–liquid route is transpiration: the loss of water, as vapor, from the living surfaces of a plant — mainly its leaves — into the atmosphere.

The vapor's exit is the stoma, a tiny pore ringed by a pair of guard cells that can open and close it. The same pore is also the intake for the carbon dioxide that photosynthesis needs — the raw material used in the green-leaf starch experiment comes in through this very door. In most land plants the stomata cluster on the lower epidermis, which is exactly what the petroleum-jelly extension sets out to confirm.

The number worth pausing over: of all the water the roots absorb, only about one to five percent stays in the plant to power photosynthesis and build cells. Over ninety percent leaves by transpiration. It looks wasteful, but it is a survival strategy:

• Transpirational pull: water constantly escaping from the leaves drags the water column in the stem and roots upward, like a drink rising in a straw, carrying water and minerals from root to crown. A tree tens of meters tall has no pump — this pull does most of the lifting;
• Cooling: vaporizing water carries away a great deal of heat, just as sweating cools us. Midsummer shade under a tree feels cooler than shade under an eave partly because the whole canopy is "sweating" together;
• Feeding the water cycle: the air above a forest is moister and rains more; transpiration is a major source of atmospheric water.

Now reread everyday life with this principle: gardeners cut away most branches and leaves when transplanting a sapling, choose an overcast evening to do it, and rig shade netting over newly planted trees — all for the same reason. Until the roots re-establish, keep transpiration losses as low as possible so the leaves cannot "pump the roots dry".

Troubleshooting common failures

• Hardly any droplets even in the leafy bag: temperature too low, light too weak, or not enough time. Move the pot somewhere warm, bright and airy and wait another half day; indoors in winter, allow a full day.
• Droplets in the empty bag too: the bag was not dry to begin with, or damp soil or moss got sealed inside. Restart with a dry bag and tie the mouth around the branch only.
• Both cylinder readings fall together: the oil seal is not covering the water completely, so the surface is evaporating on its own. Top up the oil to full coverage, or switch to bottles with narrower necks.
• Plenty of vapor in the leafless branch's bag: the leaves were stripped just now and the wounds are still leaking moisture. Strip the leaves, let the branch rest half a day, then bag it.
• All three petroleum-jelly leaves wilt together: the coat is too thin to seal the stomata, or the three leaves differ too much in age. Coat to a glossy thickness and pick leaves of similar age.

When students can explain why the droplets can only have come from the leaves, what the oil seal is guarding against, and why losing ninety percent of the water is not really a waste, the cause-and-effect chain of transpiration has clicked into place.

Notes for teachers and parents

• Timing and groups: the result takes hours, so assign the setup one afternoon and discuss it the next day; at home, run it as a "bag before dinner, check before bed" observation task. Groups of three: one bags, one records, one photographs.
• Say it before anyone touches anything: get permission before cutting any branch, and pass scissors tip-down.

The step most often skipped. The blank-bag control and the cylinder oil seal are the two shortcuts people take. Skip them and the phenomenon still shows up — but the proof collapses: both the droplets and the falling water level gain alternative explanations. In discussion, deliberately ask "couldn't the droplets have been in the bag all along?" and let students discover the value of the controls for themselves.

How to tell they really get it. They can draw the route "roots absorb — stem transports — leaves release" with the stomata marked in, and use it to explain two everyday facts: why transplanted trees are stripped of leaves, and why it is cool under a big tree. That's the bar.