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Transpiration

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Author: Janice VanCleave

So You Want to Do a Project about Transpiration!

Let's Explore

Purpose

To demonstrate transpiration, the loss of water from leaves.

Materials

  • two H)-ounce (300-mL) plastic cups
  • tap water
  • pen
  • 5-inch (12.5-cm) -square piece of poster board
  • sprig with 1 leaf and at least a 4-inch (IO-cm)
  • stem
  • scissors
  • 1 teaspoon (5 mL) cooking oil
  • transparent tape
  • desk lamp
  • ruler
  • ice cube
  • resealable plastic bag
  • adult helper

Procedure

  1. Fill one of the cups about three-fourths full with water.
  2. Ask an adult to use the pen to make a hole in the center of the poster board.
  3. Push the stem of the sprig through the hole until the bottom of the leaf rests on the poster board.
  4. Place the poster board, leaf side up, on the cup of water. The stem should be near but not touching the bottom of the cup. Trim the stem with scissors if it is too long.
  5. Tilt one side of the poster board just enough to allow you to pour the oil into the cup, but do not let the stem come out of the water. You do not want the oil to get on the cut end of the stem because it can close off the openings in the stem.
  6. Lower the poster board. Put the remaining cup upside down over the poster board to cover the leaf. The entire rim of this cup should rest on the poster board. Secure the rim to the poster board with tape.
  7. Move the cups so that the top cup is about 12 inches (30 cm) from the bulb of the desk lamp.
  8. Put the ice cube in the resealable plastic bag and place it on top of the top cup.
  9. Lift the plastic bag every half hour for 3 or more hours and observe any water collected on the inside surface of the top cup. Observe the lamp side, the side opposite the lamp, and the top. Record your observations in a Transpiration Data table like the one shown.

Water Loss

Results

Tiny droplets of water appear on the inside of the cup. More droplets appear on the top near the ice cube than on other parts of the glass.

Why?

Plants absorb (soak up) water from the soil through their roots. This water moves up the stem to the leaves, where about 90 percent is lost by evaporation (the process by which a liquid changes to a gas). The water vapor (water in gas form) leaves the leaf through stomata, which are tiny openings in leaves through which gases can exit or enter. A single corn plant may lose more than 2 quarts (2 L) of water per day, and 1 acre (2.47 ha) of corn can lose more than 1 million quarts (liters) during a growing season. This process by which plants lose water from their leaves by evaporation is called transpiration.

In this investigation, the water vapor lost by the leaves through transpiration is collected in the plastic cup. The ice as well as the air in the room cools the surface of the cup. When the water vapor touches the cool surface of the cup, condensation (the process by which a gas changes to a liquid) occurs and water droplets form on the inside of the cup. The side of the cup next to the ice is cooler than the side facing the lamp, so more water droplets form on the cooler side.

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