Flexi Says: Animals urinate mainly to remove nitrogen-based waste, and smaller amounts of other water-soluble waste including salts and other chemicals. Plants produce less nitrogen-based waste than animals because they recycle nitrogen more efficiently. Nitrogen waste is made while breaking down proteins, and nitrogen is also needed to build new proteins. Animals usually get plenty of nitrogen from eating food, so they don't need to conserve. Since plants don't eat other living things for food, they need a source of nitrogen to grow new parts like leaves and flowers. Therefore they use up the nitrogen they get from breaking down proteins for growth. Plants do need to get rid of other waste, though, especially excess oxygen from photosynthesis, which they release through tiny openings in the leaves called stomata. Salts and other chemicals are stored in tissue like bark and leaves, which are sometimes shed seasonally. Sap or other resins can also remove waste from trees.