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Sep. 15, 2010

Lighting Up Celery Stalks

by Science Friday Education

Click to enlarge images
Plants and trees need water for growth and food production. The process through which water is transported throughout a plant begins at the roots. Water absorbed from the roots of a plant will travel through a series of long thin tubes, or veins, that stretch from the stem to the leaves.
 
In this activity, students will conduct a series of hands-on experiments that will demonstrate how the working of these veins, known as capillary action, enables water to travel throughout the length of a plant. Students will learn how the forces of water cohesion and adhesion contribute to the process of capillary action.
 
Grade Level: 6th – 8th grade
Subject Matter: Life Science, Chemistry
National Standards: NS.5-8.1, NS.5-8.3
Leaves have an intricate web of veins that transport nutrients and water and provide structural support. But what determines the pattern of venation? Physicists Marcelo Magnasco and Eleni Katifori, of The Rockefeller University, investigated this question using sophisticated algorithms and a little glow-in-the-dark dye.
 
Activity Materials
Wax paper, cut into small squares, one for each student
Droppers – one for each student
Clear plastic cups – one for each student
Colored water (use food dye of any color)
Coffee filters - cut into thin strips about two inches long, one per student
Magnifying lens
Tape
Celery stalks (fresh with leaves on top of stalk) – one per student
Scissors
 
Vocabulary
Capillary action: the process by which plants transport water upwards and throughout the plant.
Cohesion: the force of attraction between water molecules that causes a tendency to remain together.
Adhesion: the attraction between unlike substances that causes them to stick together.
Xylem: part of a plant that carries water and nutrients from the roots to the stem and leaves.
 
What To Do

1. Begin the lesson by viewing the Science Friday video, Lighting Up Leaves. The video examines the intricate patterns of venation that transport nutrients and water. Ask students to explain the process of how plants are able to defy gravity by transporting water from the roots at the bottom of the plant to the leaves at the top of the plant.

2. Tell students that they will conduct a series of experiments that will enable them to observe the processes that allow for water transportation in plants.

Activity 1

1. Hand out to each student 1) a small square piece of wax paper, 2) a dropper and 3) a clear plastic cup half filled with colored water.

2. Have students place five drops of the colored water on the wax paper, with each drop falling on top of each other. Observe the shape of the water. Why does the water form one big drop instead of spreading out? Will adding more drops on top make a difference? Investigate by having students place an additional five drops on top of the drop already fon the wax paper. How does the drop appear now? Why do the droplets stick together?

3. Have students place a separate, smaller drop close to the larger drop on the wax paper. Use the dropper to slowly move or drag the little drop as close as possible to the big drop without actually making contact with each other. Why does the drop follow the dropper? What happens to the small drop when it comes very close to the larger drop? Why did the little drop appear to be pulled into the big drop?

Activity 2

1. Hand out to each student a thin strip of coffee filter paper about two inches long. Have the students observe the coffee filter with a magnifying lens. Have students describe what they see. Can they tell from their observations what coffee filters are made from?

2. Have each students tape the coffee filter strip to the inside rim of the plastic cup. so that one end of the strip is dry and the bottom end of the strip is underwater. Ask students to predict what will happen to the strip.

3. After a few minutes of observation, have students discuss why the water was absorbed up the coffee filter strip.

Activity 3

1. Hand out one celery stalk to each student. Have students use scissors to cut two inches off the bottom of the celery stalk. Examine the bottom of the celery stalk. What can they see at the bottom of the celery stalk? Use the magnifying lens to observe the pattern of circles on the bottom of the stalk. What do the students think these are for?

2. Have each student place a celery stalk in the cup with colored water so that the freshly cut end is immersed in water.

3. Based on the results from the two prior activities, ask students to predict what they think will happen within the next two hours, six hours, 24 hours, etc.

4. Maintain a classroom chart that measures the movement of the water up the stalk for the next few days. In order to observe the movement of the water, have students cut the bottom of one of the celery stalks at the end of each time period that is being observed, and then use a ruler to measure the number of centimeters or inches. How long does it take for the water to reach the top of the celery stalk?

5. Have students examine thin slices of the celery stalk that has absorbed the colored water. What do the circles of colored water represent? Ask students discuss why, given their observations, the celery stalk is able to transport water up the stalk.

What's Happening?
Plants have long, thin tubes located in their xylem that stretch from the roots through the stem and leaves. Water is able to move up the xylem through a process called capillary action. Capillary action occurs when the forces of cohesion and adhesion combine in such a way that they overcome the downward force of gravity, and cause water to move upward through the thin tubes. Water cohesion is the force of attraction between water molecules so that they stick together, while adhesion is the tendency of water molecules to stick to other substances.
 
The first activity demonstrates water cohesion. The attraction between water molecules causes the water molecules to remain together, instead of spreading out all over the wax paper. This attraction is so great that when a water drop comes close to another water drop, the two will quickly merge and form one big drop.
 
The second activity demonstrates how a regularly used household item uses capillary action. Paper coffee filters are made of plant fibers and depending on the coffee filter brand, these fibers can be seen with a magnifying lens. The force of cohesion causes the water molecules to “hold” on to one another and the force of adhesion causes the water molecules to “stick” to the passageways in the coffee filter. Because the passageways the water will travel through in the coffee filter are so small, the adhesion forces are strong enough to overcome gravity and cause the liquid to travel upwards.
 
The third activity demonstrates and tracks capillary action in a plant using a celery stalk. The xylem or tubes that transport water can easily be seen by looking for the colored circles on the bottom of the stalk. The rate of capillary action can be measured by monitoring the appearance of colored circles up the length of the stalk.
 
Topics for Science Class Discussion
• What variables can account for any discrepancies in the rate of capillary action in the celery stalks?
• How does transpiration (loss of water in a plant through holes in its leaves) affect the process of capillary action?
• What human body system can be compared to the process of capillary action in plants?
• Explain how the electrical charges of a water molecule create cohesion.
• What would happen if the tubes in the xylem were wider or thinner?
 
Extended Activities and Links
Have students design experiments using celery stalks to test the following:
• Does the rate of capillary action differ with celery stalks with leaves and without leaves?
• Does water absorbed at different time intervals mix within the plant?
• Is the celery stalk able to exchange water horizontally?
• Are different types of liquids also able to be absorbed by plants?
 
Have students look around the school or at home for other objects or items that use capillary action.
 
Use the paper wrapper of a straw to observe capillary action:
http://www.physicscentral.org/experiment/physicsathome/mealtime.cfm
 
 

 

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