Climbers: How Does Water Move Through a Leaf?
How does water move through a leaf?
- Juice glass
- Tap water
- Red food coloring
- Large tree leaf, such as an oak leaf
- Crayons or colored markers
- 3 sheets of typing paper
- Fill the glass about one-fourth full with water.
- Add enough food coloring to make the water a deep red color.
- Cut across the end of the leaf's stem.
- Stand the leaf in the glass of colored water.
- Observe the leaf and make a colored drawing of it. Label the drawing Day 1.
- Repeat step 5 at about the same time each day for the next 2 days. Label the drawings Day 2 and Day 3.
The red color moves slowly through the leaf, first following the pattern made by the leaf's veins (conducting structures in leaves) and then throughout the rest of the leaf.
The leaf is part of a vascular plant. Like all vascular plants, the leaf has two main vascular tubes, xylem tubes and phloem tubes. Xylem tubes transport sap containing water and minerals upward from the roots through the plant. The xylem tubes also provide support for the plant because their walls are thick.
Phloem tubes transport sap containing water and food manufactured in the plant's leaves throughout the plant. In this activity, you saw the results of colored water moving through xylem tubes.
Scientists believe that transpiration (a process by which water vapor is lost through leaves) is responsible for the upward movement of water through xylem tubes against the pull of gravity (the force that pulls things toward the center of the earth). Xylem tubes from the roots to the leaves are believed to be filled with sap, which is mostly water. Some of the water in xylem tubes evaporates (changes from a liquid to a gas due to an addition of heat energy) during transpiration. As water is lost from the xylem tubes, the column of sap in the tube is pulled upward. This is because water molecules (the smallest particles of a substance that retain the properties of the substance) hold tightly to each other. As the water molecules in the xylem tubes move upward, water from the soil is pulled into the roots.
- Will water move the same way through a vascular plant that has a longer stem? Repeat the experiment, using a pale stalk of celery with pale leaves. (These can be found in the center of a celery bunch.)
- How do changes in the rate at which water evaporates from leaves affect the speed at which water moves through xylem tubes? Repeat the previous experiment, preparing 3 stalks of celery in 3 glasses. Ask an adult to cut the bottom from a 2-liter soda bottle. Cover one of the glasses with the bottle, as shown, and set the second glass next to the bottle. Set the third glass at a distance from the other glasses and in front of a blowing fan. NOTE: A dry, windy environment increases evaporation. Observe the celery stalks and leaves in each glass every 15 minutes for 1 hour and then as often as possible for 8 to 10 hours. Science Fair Hint: Display drawings of the results.
- Demonstrate transpiration by placing a clear plastic bag over a group of leaves at the end of a stem of a tree or bush. (Do not cut or break the stem off the plant.) Secure the bag to the stem by wrapping tape around the open end of the bag. Observe the contents of the bag as often as possible for 2 to 3 days.
- As water moves into a cell, pressure builds up inside the cell. This internal pressure is called turgor pressure. The turgidity of a cell is its firmness due to turgor pressure. A decrease in turgidity causes plant stems to wilt (become limp). Demonstrate wilting by placing a fresh stalk of celery with high turgidity in an empty glass. Evaluate the turgidity of the celery by trying to bend the stalk. Test the turgidity again after 24 hours.
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