Do Plants Suck Up Water?
The cartoon diagram shows a flower using a straw to drink water from an underground stream. I’ve never seen a flower drinking through a straw, but water underground is pulled to the surface of plant leaves and flower petals in much the same way. Instead of a straw, plants have tube-like structures from the roots to the leaves and flower petals. These tubes are called xylem.
Xylem tubes are very small. Water moves up small tubes because the water molecules are attracted to the cellulose chemical in the walls of the plant tubes. This attraction between unlike molecules is called adhesion. Water molecules also have a strong attraction for each other, which is called cohesion. In the diagram, animated diagrams of water molecules are shown. The molecules walking up the walls of the narrow tube are pulling water molecules up the center of the tube. When you look at the surface of water in a thin tube, you will notice that the water moves up the sides and sinks down in the center. This downward dip in the water level is called the meniscus. The movement of the water through tubes or spaces because adhesion and cohesion is called capillary action.
Capillary action moves water up narrow tubes, such as xylem tubes in plants.
Capillary action occurs due to the cohesive force of attraction between water molecules and the adhesive force of attraction between water molecules and the molecules in the walls of the tube.
Due to gravity, capillary action can only raise water a short distance up the xylem tubes in plants. Another process called transpiration pulls the water to the top of the xylem where it moves in to the cells of leaves, stems, flowers, and other organs.
The process of traspiration is similar to the process of drinking through a straw. Both processes raise the height of a liquid in a tube. When you suck the air out of a straw, you decrease air pressure pushing down on the liquid inside the straw.Air pressure outside the straw is still pushing down on the surface of the liquid in the glass. Thus, air pressure is pushing the liquid up the straw to your mouth.
In plants, liquid water moves from the roots to small openings in the surface of leaves and flower petals called stomata. At the surface, liquid water evaporates when a stoma is open. Evaporation of the water creates a low pressure at the top of the xylem tube. The higher pressure on the water at the bottom of the xylem pushes the water up. As long as there is available water for the roots, the xylem remains filled with water.
Nutrients in the soil that dissolve in water are carried from a plant’s roots up xylem tubes to different parts of the plant, such as leaves and flowers.
You now know why and how, but this is all secondary research. While secondary research, done by others, can be accepted as true, encourage kids to investigate in order to “see it for themselves.” In other words, experimentally prove that nutrients dissolved in water move from the roots through a plant’s stem to its leaves. This is called primary research.
Prove that nutrients dissolved in water move through xylem in a plant’s stem.
2-1 pint glass jars
red food color
2 innerstems from a stalk of celery (Pull the outter stems off and use the inner pale celery stems and leaves) lighter colored inner stems.)
* knife (adult use only)
Preparation of Materials by an Adult
Pour 1-cup (250 mL) of tap water into each glass jar.
• Add 20 drops of food color to the water in one of the glasses.
• Place the celery stems on the cutting board and with a knife cut across the bottom of each stem. Cut each stem again, cutting off a small slice.
Tip: Scissors can crush the tiny vessels in the stem, thus affecting the transport of water.
• Without delay, stand 1 celery stem in the colored water and the remaining celery stem in the glass of uncolored water.
1. After placing the celery stems in the two glasses, kids should observe and record the appearance of the surface of the celery slices. Look for the xylem tubes around the outside edge of each slice. The diagram shows the location of these tubes.
Recording observations: Colored drawings with word descriptions. In addition to student descriptions, use a camera to take a picture of the two celery slices.
2. Observe and record the color of the leaves on each celery stalk. As before, make a colored drawing.
Hypothesis-1: Ask kids to predict what they think the leaves on the celergy stalks will look like in three days. They are to record their hypothesis by drawing colored pictures.
3. After 24 hours, observe and record the appearance of the leaves. Lift each stalk and observe and record the bottom cut surface.
Note: Having kids to make colored drawings is not busy work. Instead, the time it takes to draw and color their observations helps to keep their mind focused on the topic being studied. As they make their drawings on the first day, 24 hours after the experiment started, ask them to give ideas about how the color moved from the colored water in the glass to the leaves?
Older kids can do research to discover on their own how the coloring is transferred. Provide clues by giving them this list of terms:
capillary action, transpiration, xylem
4. Conclusion: Describe the results. State your hypothesis and whether the experimental results did or didnot support your hypothesis.
Science Fair Project Ideas
Follow the previous procedure using white long stemmed carnations instead of celery stalks. Hold the flower stems under water and cut at an angle. Measure the time it takes for the color to reach the end of the flower petals. This will be the rate of petal coloration or the flow rate of water through xylem.
The results of the experiment will provide data used as a control for a science project. Your science project experiment will have only one of the variables in the experiment changed. Examples of variables that can be changed are:
- Length of stem
- Using a different white flower, such as a white daisy.
- Use different concentrations of the food coloring–higher and lower.
- Multicolored flower–block part of the stem with Vaseline to determine is only part of the flower can be colored.
- Use distilled water instead of tap water
- Method of cutting the stems–Angle of cut, cutting underwater, cutting in the air.
Examples of a Science Project Experiment Question
1. What affect does the length of the stem have on the rate of petal coloration?
2. What affect would different types of flowers have on the rate of petal coloration?