Sunday, October 15, 2006

How Do Leaves Get Water from Roots?

Plants need water, carbon dioxide and nutrients to live. Water and nutrients come to the plant from the ground, whereas the carbon dioxide comes from the air. The roots of the plant, which are under the ground, absorb water and nutrients for the whole plant above the ground. Have you ever wondered how water reaches the leaves of tall plants, especially the ones at the top? Water has to climb several hundred feet before it can reach the leaves of the top-most branches in some trees like oaks, pines or eucalyptus.

Capillary action and leaf pressure are two important factors that help move water in plants. Capillary action can simply be termed as the automatic movement of water through extremely narrow tubes known as capillaries (hair-thin in some cases) present inside the plant stem. In plants these tubes are called xylem. Capillary action helps plants in getting water to such heights against the downward force of gravity.

Leaf pressure is a property whereby the leaf is able to convey a message to the roots that the pressure of water is low in the leaf and hence it needs more water.

So what is it that helps the water drops stay together and climb up through the minute vessels in the plant body by capillary action? A physical property called surface tension helps water molecules stay together. Surface tension can be termed as the amount of force holding two molecules of the same kind together.

This property is quite high for water than many other liquids. Any impurities like dirt or salt in water tend to lower its surface tension. Because the surface tension for pure water is fairly high, when the water drops are absorbed by the tips of the plant vessels subsequent water drops cling on to the initial ones rising in the plant vessels and begin a journey to the top.

The high surface tension of water is also a reason why rain drops tend to be spherical. For more information on capillary action, read the article at

Little Lion Experiment 1:

Items Needed:

  • A small glass of bottled water
  • A few leaves with waxy surfaces
  • A candle
  • Some flat cardboard pieces

You can apply wax to a cardboard piece by rubbing a candle lengthwise on the cardboard for a few minutes. This experiment is to show that surface tension is lowered due to addition of impurities.

  1. Place a drop of water on the leaf or the waxes cardboard using an ink dropper.
  2. Observe what happens to the water drop.
  3. If the drop is still on the wax surface, try adding a few salt particles to the water drop.
  4. Observe what happens.
  5. If the drop of pure water had rolled off then mix one teaspoon salt to the water.
  6. Add a drop of the salted water to the leaf/waxed cardboard.

Little Lion Experiment 2:

Items Needed:

  • A tall juice glass
  • A small thin cardboard piece
  • A small needle or thumb tack to make holes
  • Cotton thread (white thread works best)
  • Scissors
  • A small bowl of sugar (crystal sugar preferred)

This experiment will aim to demonstrate the movement of nutrients through capillaries.

  1. Cut the cardboard piece to a size slightly larger than the juice glass opening.
  2. Make several small holes in the cardboard piece using the needle or thumb tack (be careful).
  3. Cut the thread into several pieces as tall as the glass.
  4. Now push one thread piece through each hole such that the thread reaches at least below the half way mark in the glass. Keep the cardboard piece with the threads aside.
  5. Now fill the glass halfway with water, add a spoon of sugar to it and mix till all the sugar dissolves.
  6. Place the cardboard lid on top so that the threads all touch the water at least a little.
  7. Leave the glass undisturbed for 2-3 hours.
  8. Now carefully lift the lid off along with the threads and pour away all the water in the glass.
  9. Let the thread dry over a few hours.
  10. Observe what has happened on the thread. What do you see?

You should see some sugar crystals or at least some powdery white substance on the dry threads.

For more information on growing sugar crystals you can see or