Most plants “drink” water from the ground through their roots. The water travels up the stem of the plant into the leaves and glowers where it makes food and helps keep the plant rigid. When a flower is cut off the plant, it no loner has its roots but the stem of the flower still “drinks” up the water and provides it to the leaves and flowers. How does this happen?
There are two things that combine to move water through plants—transpiration and cohesion. Water evaporating from the leaves, buds, and petals (transpiration) pulls water up the stem of the plant. This works sort of like you sucking on a straw. Water that evaporates from the leaves “pulls up” other water molecules behind it to fill the space it left. Instead of a mouth providing the suction, it is due to the evaporating water. This can happen because water sticks to itself (cohesion) and because the tubes in the plant stem are very tiny. This water movement process through tiny tubes is called capillary action. Coloring the water with food coloring does not harm the plant but it allows you to see the movement of water into the flower. Splitting the stem simply proves that the tiny tubes in the stem run all the way through the stem from the water to the petals of the flowers. Our unofficial tests indicated that the blue food color went up the carnations the fastest, followed by the red and then the green food colors.