Stomata, turgor and wilting sim
Dim or brighten the lamp over a potted plant on a balance and water it from a beaker, then wait an hour at a time: watch the guard cells swell or go flaccid, the leaves wilt or recover, and plot the water lost against the pore width you measure.
Transpiration and wilting: stomata open and close as guard cells become turgid or flaccid, water loss through stomata and the cuticle, and wilting when a plant loses water faster than its roots take it up · covers the O-Level, IGCSE, GCSE topic
What the sim shows
Stomata, turgor and wilting. A young bean plant in a small pot stands on a top-pan balance. Cling film over the pot stops water evaporating from the compost, so the mass lost is the water the plant loses by transpiration: water evaporates from the wet cells inside the leaves and the vapour diffuses out through the stomata. Each stoma is a pore between two guard cells. In the light the guard cells take in water by osmosis and become turgid; their inner walls are thicker, so they curve apart and the pore opens, letting in carbon dioxide for photosynthesis. In the dark, or when the plant is short of water, they lose water, become flaccid and the pore closes. When the plant loses water faster than its roots take it up, its cells lose water and turgor: flaccid cells no longer press against each other, so the leaves droop and the plant wilts. Watering it, or less light (so less water is lost), lets the cells become turgid again. Simplified on purpose: every number is illustrative (≈), in the range of a small pot plant in a school lab: up to about 5.2 g of water an hour with the stomata fully open and 0.2 g an hour through the waxy cuticle when they are closed; pores up to 10 µm wide; the lamp gives up to about 12000 lux. Each hour is long enough for the stomata and the leaves to settle a step toward the new conditions; real stomata take minutes to an hour to respond. The temperature, humidity and air movement stay the same, and the LED lamp barely warms the leaf. The water loss is shared evenly over the stomata, and one stoma is shown; a real leaf has thousands on its lower surface. The close-up's cells are not to scale with the plant. Wilting here is temporary: the plant recovers when it is watered. A fresh 100 cm³ beaker of water is used each time, and only as much is poured as the compost can hold.
- Lit knob: the lamp's dimmer
- Green dots in the close-up: chloroplasts
- Pale cone: light from the lamp
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