Key concepts
Plant biology
Capillary action

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?

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.

•     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)

•     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.

•     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.

Observations and results
When you put the flowers in the dyed water, did you see some of the flowers start to show spots of dye after two hours? Did you also see some dye in the stems? After 24 hours did the flowers overall have a colored hue to them? Did this hue become more pronounced, or darker, after 48 and 72 hours?

Water moves through the plant by means of capillary action. Specifically, the water is pulled through the stem and then makes its way up to the flower. After two hours of being in the dyed water, some flowers should have clearly showed dyed spots near the edges of their petals. The water that has been pulled up undergoes a process called transpiration, which is when the water from leaves and flower petals evaporates. However, the dye it brought along doesn't evaporate, and stays around to color the flower. The loss of water generates low water pressure in the leaves and petals, causing more colored water to be pulled through the stem. By 24 hours the flowers should have gained an overall dyed hue, which darkened a little over time. The stems should have also become slightly dyed in places, particularly where the leaves branch off.

More to explore
Plant Parts: What Do Different Plant Parts Do? from Missouri Botanical Garden
Capillary action from The U.S. Geological Survey, Water Science School
The Water Cycle: Transpiration from The USGS Water Science School
Transpiration in Plants from
Suck It Up: Capillary Action of Water in Plants from Science Buddies

This activity brought to you in partnership with Science Buddies