Root cells grow faster on the side where there is the smallest amounts of auxin. Because of the large amount of auxin in the roots, the roots have no place to go except down. Every 2 hours from the time you put the celery into the cups, remove 1 of the stalks and put into the correct towel. Each time you remove a stalk from the water, carefully peel the rounded part with a vegetable peeler to see how far up the stalk the purple water has traveled. For water to start at the roots and move to the leaves it relies on a characteristic common to water.
Water molecules sticking to each other is how plants get there necessary water supply.
Inflate a plastic bag to make sure it does not have any holes. Drop a pebble into the bag so that the bag hangs down. Use a twist tie to securely close the plastic bag around the twig. Examine the plastic bag after 24 hours. Carefully pour the water from the plastic bag into a measuring cup.