MadSci Network: Botany
Query:

Re: Why does a carnation change colors when you put food coloring in the water?

Area: Botany
Posted By: Karen Culver-Rymsza, Grad student oceanography
Date: Sat May 10 13:56:25 1997
Area of science: Botany
ID: 862323530.Bt
Message:

Raquel,

This is a fun experiment which I have done with my own daughter. It demonstrates the process of ascent (or rise) of water in plants. By adding food coloring, you have made the water that is constantly moving from the roots to the shoots, visible. The change in color is because of the movement of water, which you have changed from clear to colored.

There are two things that combine to move water through plants -- TRANSPIRATION and COHESION.

Water evaporating from the leaves, buds and petals (TRANSPIRATION) pulls water up the stem of the plant. This works in the same way as sucking on a straw. Water that evaporates from the leaves "pulls" other water behind it up to fill the space left by the evaporating water, but instead of your mouth providing the suction it is due to evaporating water. This can happen because water is very sticky--to itself (called water COHESION) and because the tubes in the plant stem are very small (in a part of the plant called the XYLEM). Xylem moves water to the veins in the leaves and petals.

TRY THIS!!! TRY THIS!!! TRY THIS!!!

In fact, you can make your project even more interesting by slitting the stem with a very sharp knife (get an adult to do this) straight down the middle. Put each half into a cup of different colored water (try red and blue for 4th of July) and see what happens. Which color is soaked up?, Both? Just one? or neither? Just remember to keep the ends of the stem wet at all times and make fresh cuts on the ends.

For a cut flower, it is important for the stem tubes to be filled with water. If air gets in the tube no water can move up the stem because water doesn't stick to air too well, and the pull is lost. That is why many gardeners and florists always cut stems under water, so no air bubbles can get in to break the tube of water and make the flower wilt.

You may ask how water can pull water. Certainly your soda doesn't suck itself up the straw! Well, actually, it would if the straw was small enough! This process is called CAPILLARITY or CAPILLARY ACTION. It is also fun to demonstrate.

TRY IT!!  TRY IT!!  TRY IT!!  TRY IT!!  TRY IT!! TRY IT!!  TRY IT!!
You will need a paper towel or cloth, a shallow dish of water and a way to hold up the paper, like clothespins or tacks. Put just one edge into a dish of water. The small fibers in the paper or cloth act like tiny straws. They fill up gradually, all by themselves, without anyone sucking the water up. Before you know it, the water will have moved up the paper towel several inches. This is a little different than what is happening in your carnation because the little tubes are being filled with no pull from evaporation. They are filling because water likes to stick to other things besides itself, like paper (this is called ADHESION).

Every time a little water molecule soaks into a piece of paper, it pulls its neighbor in with it. Then the neighbor pulls it's neighbor and so on, until the water molecules have pushed and pulled themselves up the paper.

Try this experiment with different kinds of paper and compare them. Try to measure how far and how fast water soaks up into different kinds of paper. A few types to try: typing paper, construction paper, blotting paper, paper towels (those scratchy ones from school AND the nice soft ones from home). Add some food coloring to this water, too.

You can have fun and learn a lot from something as simple as water!!

K. Culver-Rymsza
University of Rhode Island
Graduate School of Oceanography


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