Just like animals, plants need ways to rid themselves of metabolic wastes. The dilemma for a plant, however, is a little different from that for other multicellular organisms. Plants, unlike animals, produce their own food through a process called photosynthesis.
A flower is the specialized reproductive structure of a flowering plant. Like all other cells in a plant, the cells in a flower extract energy from sugar molecules through cellular respiration, which consumes oxygen and releases carbon dioxide. Plants produce the sugar they need for cellular respiration through photosynthesis, which consumes carbon dioxide and releases oxygen. During the daytime, the volume of oxygen from photosynthesis greatly exceeds the volume of carbon dioxide produced by cellular respiration.
Carbon dioxide and oxygen gas can diffuse across cell membranes and cell walls, so plant cells that are close to the air can rapidly rid themselves of excess oxygen or carbon dioxide. Since proteins and nucleic acids contain nitrogen, the breakdown of these molecules releases nitrogenous waste. Animals expel nitrogenous wastes through urination, but plants have developed other strategies to deal with this problem.
Nitrogenous wastes are stored in the vacuole, which is a large membrane-enclosed sac inside the cell, and can be used to make other compounds like plant alkaloids, many of which are used for self-defense. Plants can also store these waste products in structures they will shed, like leaves that die and fall off.