You will need:
Find the vascular bundles in celery with this simple experiment!
You should have seen that the food coloured water travelled up the stem of the celery and into the leaves.
How does food colour get up there? Gravity should be holding the water down right?
Water is found all the way through the celery: in the stems, the leaves and the roots. The water in the leaves of the celery evaporates through the surface of the leaves, and this leaves a space inside the leaves where the water was. This process is called transpiration. That new empty space inside the leaves creates low pressure, and like a drinking straw this low pressure allows water below the leaf to travel up the stem. You’ll see the little tubes the water travels up when you cut the celery stem, and you can see the colour up in the leaves. These tubes are called Xylem and are part of the plant's vascular system. This how plants transport the water and nutrients from the soil up to the very highest leaves. By the way, the tubes that transport sugars from the leaves downwards are called phloem).
Related experiment: make different coloured petals