In darkness, leaves takes in oxygen and release carbon dioxide owing to the the continuous process of respiration. As dawn breaks, photosynthesis begins in the mesophyll cells and, since this is the reverse of respiration, the following sequence of events takes place. The release of carbon dioxide from the leave slows down and eventually stops because this gas is absorbed in increasing amount by mesophyll as a raw material of photosynthesis.
During daylight hours, sieve tubes of the phloem continually transport sugar made in the leaves to growth and food storage regions in the plant.
