Water evaporates from the leaves of plants by transpiration. You'll use a potometer to measure how fast the water vanishes. A capillary tube holds water that the leaves use up.
Stomata, small openings in the surface of the leaf, are where the water loss takes place. As water goes out from the leaves, sap comes up from the roots to replace it.

Without stomata, plants could not grow very tall, and we would not have tall plants and trees.