Water molecules love some other molecules and hate other molecules.

Have you ever noticed that magnets like to stick to other metals? So do water molecules. When water molecules stick to other molecules that are also little magnets, it is called "molecular adhesion." This explains why it is easy to clean up spilled water with a paper towel: the water molecule's little magnets like to stick to the cellulose molecules of the paper, which are also like little magnets. Water molecules will stick to any other molecules that are like little magnets (polar), but do not like to get involved with any molecules that hate little magnets (nonpolar), like oil. Oil and water don't mix, right? That is why you have to shake the salad dressing real hard before you pour it: the oil molecules hate the water little magnets.

Paper towels are made out of trees, and trees are made out of cellulose. The leaves make molecules of sugar out of sunlight, water, and carbon dioxide, then combine the sugars into huge sugar chains. These sugar chain molecules are called cellulose. This name comes from the fact that the plant material is made of cells, and the "-ose" word ending means "sugar". Cellulose is also a great magnet (polar molecule), so water sticks to cellulose just like a magnet to the refrigerator door!

Now that we know a bit about water molecules, let's look at how water acts in a little tube.

Water Molecules

bonded to paper towel

Water loves to rise in a tube

Meniscus shown in blue

water molecules want to stick together and stick to side

Have you ever noticed that water in a glass tends to hug the sides and even stick up above the water's surface? The edge of the water that sticks up above the water's surface is called a "meniscus."

When you put a small tube into water, the water likes to stick to each side, with a meniscus on each side. If the tube is so skinny that the meniscus on one side can touch the meniscus on the other side, the water will rise up the tube (each meniscus wants to go up the side, and they chase each other). This is called "capillary action."

The redwood tree's trunk is made up of millions of little bitty tubes (xylem), and these tubes are made of cellulose. The water molecules like to stick together and like to stick to the walls of the tubes of cellulose, so they rise up the tubes by capillary action. All the way to the top!

The water pressure decreases as it rises up the tree. This is because the capillary action is fighting the weight of the water. Although the xylem tube is very thin, and therefore the weight of the water is very low, it is not zero. Eventually, the effects of gravity on the water starts to equal the effects of capillary action. Scientists have found that the pressure inside the xylem decreases with the height of the tree, and similarly, the size of the redwood leaves decreases with the decrease in pressure. (See an excellent article in the San Francisco Chronicle, the source of the illustration to the right.)

Scientists Dr. George W. Koch of Northern Arizona University, Dr. Gregory M. Jennings of Humboldt State in Arcata, California, and Dr. Stephen D. Davis of Pepperdine University in Malibu, California, have studied the water pressure inside a coast redwood. They studied the correlations among the tree's height, its internal water pressure, leaf size, photosynthesis and other factors. Dr. Koch and his colleagues have concluded that no existing species of tree can grow higher than 130 meters (427 feet).