When the water a plant needs is taken up by the roots it travels along tiny tubes (like water pipes) inside the stem of the plant. Here are the ends of the tubes as you look at a cut celery stalk:
These tubes transport the water to the leaves. One really good way of seeing this in action is to take some stalks of celery (a plant you can eat as a vegetable) and place them in some coloured water:
Leave them for a while and you can see the leaves turn to the colour of the water - red in this case, but blue works just as well! If you cut the celery you can see the ends of the tubes the water is travelling along as they change to the colour of the water.
Imagine that you have placed five celery stalks in some red-coloured water. Every 15 minutes you cut one of the celery stalks in half. This is what you see when you look down at the cut end of the stalks after each cut:
Looking at the evidence, how long did it take the water to reach halfway up the celery stalk?