Cut flowers usually can't carry out normal photosynthesis because they lack the parts necessary for the process. However, the stem can absorb a sugar solution, which provides it with the food it needs to survive longer.
Flowers last longer when they also have access to an acidifier and a biocide. To prepare your own flower-food solution at home, mix 2 tbsp. of lemon juice, 1 tbsp. of sugar and 1/4 tsp. of bleach in 1 qt. of warm water. Lemon juice serves as an acidifier, and bleach kills bacteria in the water.
Bubbles could form on the cut, open end of the flower stem, preventing the flower from absorbing the nutrient solution. Before placing the flower in the vase, cut 1 to 2 inches off the stem at a steep angle with a sharp knife or clippers. Remove any leaves that would fall under the water line because they could rot and introduce bacteria.