This experiment helps explain the how plants drink water. The students have learnt that it is a process called capillary action. It is the ability for a liquid to flow in narrow spaces without the assistance of, and in opposition to, external forces like gravity. The effect can be seen in the drawing up of liquids between the veins/cells of the celery. It occurs because of intermolecular forces between the liquid and surrounding solid surfaces. If the diameter of the tube is sufficiently small, then the combination of surface tension will aid this occurrence. In short, the capillary action is due to the pressure of cohesion and adhesion which cause the liquid to work against gravity. What's more the combination of the two colours blue and red, moving up the separate stems, makes a purple combination at the top of the plant. Isn't science fun!