Plants breathe using a system called respiration. They release carbon dioxide and take in oxygen from the air around them.
Plants, unlike other living things, can produce their own oxygen in a process called photosynthesis. Photosynthesis is the opposite of respiration. In photosynthesis, plants' carbon dioxide is absorbed by the plant and from that, oxygen is produced.
Plants respire all the time, day or night. However, they only photosynthesize during the day, when the sun is out. Every part of a plant breathes. It "inhales" oxygen through pores along the entire length of the plant, from the roots to the flowers. The roots get oxygen from air pockets in the surrounding soil. The plant "exhales" carbon dioxide the same way.
Certain plants produce more oxygen than others. Plants that produced high amounts of oxygen like the elodea canadenis are often used in ponds and aquariums because they do such a good job of oxygenating water.
Photosynthesis is an entirely different process than respiration. During photosynthesis, the chlorophyll, the pigment that makes plants green, captures energy from the sun and uses it along with carbon dioxide and water to make sugars. These sugars give the plant fuel to help it stay healthy and to grow.