Water constantly evaporates from plants through transpiration. Flowering plant leaves and petals are covered with minuscule openings called stomata through which water evaporates. The lost water needs to be replaced for the plant to stay healthy. Flower stems draw water up from the roots to the petals through a series of channels, or tubes, named the xylem. Cut flowers placed in water continue to draw water up through the xylem to keep the leaves and flower petals well hydrated. As water droplets leave the stomata, they are replenished in a way that is similar to how a paper towel becomes soaked even if only one end touches a water source. Capillary action is the force responsible for drawing water up vertically through the stems to the flower's petals.
Just as humans can become dehydrated through sweating, flower petals will dry out from transpiration if they do not have a source of fresh water. You have probably observed that you need to refill the water in a vase of cut flowers as the water level drops. Planted flowers need regular watering to keep the soil around the roots moist. Climatic conditions such as humidity, direct sunlight and wind speed affect how quickly a plant draws water to its petals. On cooler or cloudy days, plants transpire less and thus need less frequent watering. Flowers are affected by the quantity and quality of the water that is available to them.
Observing Capillary Action
Do an easy experiment at home to see plant transpiration and capillary action in flowers. Color the water in a vase blue, red or green with food coloring. Stick a white carnation in the colored water. Within a day, you will notice the petals changing color. Eventually, the whole flower completely transforms color as the plant draws water to the petals. As water passes through the tube like xylem, you will also observe color changes on leaf veins.
For an even more dramatic experiment, use a sharp knife to divide the stem into two sections up the middle. Leave about 5 inches of the stem intact between the split and the flower. Choose two different food colorings to dye the water in two glasses, such as red and blue. Place the glasses side by side, then put one of the split carnation stem halves in one glass, the other half in the second glass. Transpiration will soon draw the two colors to the petals, and you will have a multicolored carnation.