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Transpiration: The Water Cycle in Plants

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Author: Janice VanCleave

Plants give off water from their surfaces in a process called transpiration. This water leaves through stomata, which are tiny openings on the surface of a plant and are especially abundant on the undersides of leaves. Stomata open and close to let gases in and out. Water vapor is one of the released gases. Some leaves have more than a million stomata. In forests, trees need a large amount of water because about 98 percent of the water they take in is lost through the stomata. The leaves of tropical and temperate plants have stomata that are open longer periods of time and have larger openings than those of desert plants. In the desert, plants can survive with less water because the stomata of the plants do not open as wide and remain closed most of the time. The time stomata remain open as well as the size of their openings are largely controlled by the availability of water and of sunlight.

As the water evaporates from leaves during transpiration, more water is pulled into the plant at the roots. The water moves from the roots to the leaves through tubelike structures called xylem tubes. The water carries nutrients through plants.

Purpose

To determine how the number of stoma affects water loss.

Materials

  • 4-by-8-inch (10-by-20-cm) piece of poster board
  • two 10-ounce (25-cm-diameter) plastic cups
  • pencil
  • scissors
  • paper hole punch
  • tap water
  • black marking pen
  • transparent tape

Procedure

  1. Fold the poster board piece in half by placing the short sides together.
  2. Stand one of the plastic cups upside down on the poster board.
  3. Use the pencil to trace around the mouth of the cup on the poster board.
  4. Cut out the circle tracing, cutting through both layers of the poster board.
  5. Use the paper hole punch to cut 2 holes in one of the paper circles, one hole across from the other.
  6. Randomly cut 20 holes in the other paper circle.
  7. Fill the cups with an equal amount of water so that they are about three-fourths full with water.
  8. Use the pen to mark a line on each cup that is even with the surface of the water.
  9. Use the tape to secure the edges of one paper circle over the opening of each cup. You want the holes in the circle to be the only openings.
  10. Set the cups near a window that gets direct sunlight.
  11. After 3 or more days, compare the level of the water in each cup to the black water mark on each cup.

Transpiration

Results

There is less water in the cup with many holes in its cover than in the cup with only two holes in its cover.

Why?

The fact that the water level goes down shows that water has left the cups. Water in the cups evaporates, forming water vapor, which escapes through the holes in the covering. The amount of vapor escaping increases with the number of holes in the covering. The holes represent stomata in the leaves of plants. The more stomata in the leaves, the greater the amount of water lost by transpiration. Desert plants have fewer stomata, which are closed most of the time, as well as stomata with small openings, so they lose a smaller amount of water than do other plants, which have leaves containing many large stomata that are open most of the time.

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