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X is for Xylem

PURPOSE:

To investigate how capillary action pulls water up plant xylem cells.

NATIONAL SCIENCE EDUCATION STANDARD:

Life Science, Grades 58, Structure and Function in Living Systems: Specialized cells perform specialized functions in multicellular organisms. Groups of specialized cells cooperate to form a tissue....

TIME:

Setup time: 15 minutes Observation and discussion: 2030 minutes a few hours later

MATERIALS:

  • plastic knives

  • celery stalks with leaves

  • red food coloring

  • clear plastic tumblers

  • paper and pencils

  • white carnations (optional)

EXTENSION:

Have students research the difference between primary and secondary xylem. Which one is found primarily in woody plants? (Secondary xylem.) Which primitive member of the grass family is widely used in Asia as a source of wood? (Bamboo.) How does this woody plant differ from a tree? (It is hollow in the center, and does not taper toward the top.) Students may want to research the unusual reproductive pattern of bamboo. (It grows from underground stems called rhizomes, but also reproduces sexually at regular but well-spaced intervalsusually 30, 60, or 120 years.)

PROCEDURE:

  • Warn students to use caution when handling the plastic knives. Have students use a plastic knife to cut the bottom 1/2 inch from a stalk of celery and place the stalk in a tumbler of colored water. Ask them to record their predictions of what will happen to the celery-water system in a few hours.

  • After a few hours, have students observe the celery stalks again. (They will see that the red color has moved up the stalk and into the leaves.) Students should record their observations. Older students might want to record the rate at which the red liquid moves through the stalk, using the equation rate distance/time.

  • Have students discuss what conclusions they can draw from their observations. (Water moves from the bottom of the celery, through long tubes, to the top of the celery.)

  • Review with students what they learned about the function of xylem in X is for Xylem.(It is responsible for moving water and dis solved minerals from the roots to the upper regions of the plant.) Have students describe what happened in the celery stalk using the term xylem.

  • Now ask students how they think the water was able to move against the pull of gravity. Record their ideas on the board.

  • Define the movement of water upward through small tubes as capillary action. Explain that Leonardo da Vinci was the first to describe capillary action back in 1490. Capillary action is based on the fact that water molecules are attracted to one another (cohesion) and to other materials (adhesion). In plants, as water evaporates from the leaves, water molecules are pulled up the xylem tubes, and these in turn pull the molecules behind them.

  • Students might enjoy using their understanding of capillary action in xylem tubes to color white carnation petals using food coloring and water. (Flowers placed in colored water will eventually draw the colored water up to the petals, tinting them the color of the dye.)

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