MOVIN' ON UP: In this cross-section, we can see capillary action tugging dye upwards through the plant's cells.
Image: George Retseck
Key concepts
Plant biology
Capillary action
Water
Dyes
Colors
Introduction
Have you ever heard someone say, "That plant is thirsty," or "Give that plant a drink of water."? We know that all plants need water to survive, even bouquets of cut flowers and plants living in deserts. But have you ever thought about how water moves within the plant? In this activity, you'll put carnations in dyed water to figure out where the water goes. Where do you think the dyed water will travel, and what will this tell you about how the water moves in the cut flowers?
Background
Plants use water to keep their roots, stems, leaves and flowers healthy as well as prevent them from drying and wilting. The water is also used to carry dissolved nutrients throughout the plant.
Most of the time, plants get their water from the ground. This means it has to transport the water from its roots up and throughout the rest of the plant. How does it do this? Water moves through the plant by means of capillary action. Capillary action occurs when the forces binding a liquid together (cohesion and surface tension) and the forces attracting that bound liquid to another surface (adhesion) are greater than the force of gravity. Through these binding and surface forces, the plant's stem basically sucks up water—almost like drinking through a straw!
A simple way of observing capillary action is to take a teaspoon of water and gently pour it in a pool on a countertop. You'll notice that the water stays together in the pool, rather than flattening out across the countertop. (This happens because of cohesion and surface tension.) Now gently dip the corner of a paper towel in the pool of water. The water adheres to the paper and "climbs" up the paper towel. This is called capillary action.
Materials
• Water
• Measuring cup
• Glass cup or vase
• Blue or red food color
• Several white carnations (at least three). Tip: Fresher flowers work better than older ones
• Knife
• Camera (optional)
Preparation
• Measure a half cup of water and pour it into the glass or vase.
• Add 20 drops of food color to the water in the glass.
• With the help of an adult, use a knife to cut the bottom stem tips of several (at least three) white carnations at a 45-degree angle. Tip: Be sure not to use scissors, they will crush the stems, reducing their ability to absorb water. Also, shorter stems work better than longer ones.
• Place the carnations in the dyed water. As you do this, use the stems of the carnations to stir the water until the dye has fully dissolved.
Procedure
• Observe the flowers immediately after you put them in the water. If you have a camera, take a picture of the flowers.
• Observe the flowers two, four, 24, 48 and 72 hours after you put them in the dyed water. Be sure to also observe their stems, especially the bumps where the leaves branch from the stem and it is lighter green (it may be easier to see the dye here). If you have a camera, take pictures of the flowers and stems at these time points.
• How did the flowers look after two hours? What about after four, 24, 48 and 72 hours? How did their appearance change over this time period?
• What does the flowers' change in appearance tell you about how water moves through them?
• Extra: In this activity, you used carnations, but do you think you'd see the same results with other flowers and plants? Try this activity with another white flower— a daisy, for instance—or a plant that is mostly stem, such as a stalk of celery.
• Extra: Try doing this activity again but use higher or lower concentrations of food color, such as one half, twice, four times or 10 times as much; be sure to mix each dye amount with the same amount of water. What happens if you increase or decrease the concentration of food color in the water?
• Extra: How would you make a multicolor carnation? Tip: You could try (1) leaving the flower for a day in one color of water and then putting it in another color of water for a second day or (2) splitting the end of the stem in two and immersing each half in a different color of water.
See what we're tweeting about
4 Comments
Add CommentDear Scientific American,
Reply | Report Abuse | Link to thisI sincerely appreciate your effort to post an inquiry-based activity using plants. However, the explanation given here is wrong, and perpetuates a common misconception. Capillary action is not the force that moves water through a plant. If you put a straw into liquid, capillary action moves liquid a tiny, almost imperceptible distance up the straw, and it has a similarly negligible effect in plants. Sucking on a straw is a good analogy for water uptake into plants, as evaporation from the leaves draws water upwards - but this is not capillary action.
Here's a link to a lesson and an animation that accurately explain this topic http://tinyurl.com/cscc8fz. Here also is a summary of common misconceptions in plant biology, including the idea that capillary action is important http://www.actionbioscience.org/education/hershey.html/.
Good illuminating instructive article .
Reply | Report Abuse | Link to thisCapillary action IS the main cause of the rise of water in plant stems. The height of rise of water in a tube is inversely proportional to the diameter of the tube. In a very thin tube the water will rise an inch or more. In a microscopic sized tube such as in plant stems, the water can rise hundreds of feet. The rise of water when sucking on a straw is due to atmospheric pressure. Decrease the pressure above the liquid and atmospheric pressure from below will push the water up the straw. However, the highest that atmospheric pressure can raise water is 32 feet. At a height of 32 feet, the weight of the column of water equals atmospheric pressure and the column can not rise any higher. Thus this explanation can not account for water rising to the tops of trees taller than 32 feet. Put a piece of a paper towel in water and see that the water will rise several feet. At that point evaporation equals the rate of water rising through the towel and so the water reaches a maximal height. If you put the paper towel in a closed container, the water will go much higher. The FLOW of water through the plant depends on TRANSPIRATION. Flow of new water with nutrients depends on evaporation from the leaves to make room for new water molecules.
Reply | Report Abuse | Link to thisMaybe this video will help explain how a negative pressure model accounts for the movement of water up a tree. Please note between time 2:10 - 2:22 - it's not capillary action.
Reply | Report Abuse | Link to thishttp://www.youtube.com/watch?feature=player_embedded&v=BickMFHAZR0#!