Science corner

6th June 2003 at 01:00
Sophie Duncan investigates transpiration

Transpiration is the process by which water evaporates from the leaves of plants. Water is drawn through the roots into the plant. Leaves are full of tiny holes called stomates, from which the water evaporates.

Several activities can be used to examine this process. The simplest is to take a green plant and cover it with a plastic bag. Leave the plant on a windowsill for a few hours. When you return you should observe water droplets in the inside of the bag. This is water that has evaporated from the plant and condensed on the inside of the bag.

Take a number of white carnations (or other white flowers). Place them in a vase of water with added food colouring (red or blue works well). Leave for at least 12 hours.

The petals will be coloured by the water. To investigate further, cut the stems across or length ways. You should be able to see where the water has travelled.

Try placing vases containing carnations in different temperatures. You should find the dye travels more quickly up the stem in hotter conditions, as the plants transpire more.

Take a leaf from a non-toxic green plant. Fill a one-litre plastic bottle with water until it is 5cm from the top. Make a bung of clay with two holes in it, one for a straw, and one for the leaf. Put the bung in the bottle and make the seals airtight.

The stem of the leaf and the straw should be above the water. Blow into the straw. This should be easy, as the holes through the leaf will let out excess air. Remove the leaf and seal the hole.

Blow through the straw again. This time it should be hard, as there is nowhere for the air to go and the pressure in the bottle increases, making it harder to blow air in.

Sophie Duncan is project manager for science at the BBC

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