Plants breathe oxygen and carbon dioxide. In daylight they produce their own food at the same time by a process known as photosynthesis. During this process the plants release more oxygen into the atmosphere than they breathe in. The effect of the plant’s feeding and breathing system is that they exhale oxygen during the night. This is the reason for the old custom hospitals of removing plants and flowers from the morning.
Without enough plants, the world’s atmosphere would not be replenished with sufficient oxygen for the human race to survive. Generally plants breathe more slowly the colder it becomes. Some plants, including many trees, go into a form of hibernation during the winter, with their breathing reduced to a minimum.