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Grades 7–10 Intermediate About 20 min Version 1 Awaiting review Premium

Transpiration: how plants lose water

Transpiration is the loss of water vapour from a plant, mostly through tiny pores called stomata on the underside of its leaves. As water evaporates from the leaves, more is pulled up from the roots, carrying minerals and cooling the plant. Stomata open in the light and close in the dark. Water is lost faster when it is warm (faster evaporation), windy (moist air is blown away), and dry (the air can take more water). Covering the underside of the leaves with petroleum jelly blocks the stomata.

What you will learn

  • Show that plants lose water from their leaves.
  • Investigate how light, wind, humidity and temperature affect transpiration.
  • Explain the role of stomata.

Before you start

  • Know that water evaporates faster when it is warm.

Materials

  • A leafy shoot (e.g. a balsam or bean plant), a clear plastic bag, a little cooking oil
  • Petroleum jelly

Equipment

  • Measuring cylinder, lamp, small fan

Procedure

  1. Set up the shoot in light, still and dry air at 25 °C. Record the water lost after 2 hours.
  2. Repeat in the dark, then with a fan blowing, then in humid air.
  3. Grease the undersides of the leaves and repeat.
  4. Try 35 °C.

How this simulation is simplified

  • The shoot loses 3 mL per hour in light, still, dry air at 25 °C; each condition multiplies this by a fixed factor.
  • Oil on the water stops evaporation from the cylinder, so all the water lost goes through the plant.