If you carefully observe a cross section of a leaf under the microscope or in any picture you will notice that some cells contain green dots these Green Dot are cell organelles called chloroplast which contain chlorophyll. Stomata which are tiny pores present on the surface of the leaves. Massive amount of gaseous exchange takes place in the leaves through these pores for the purpose of photosynthesis. But it is important to note here that exchange of gases occurs across the surface of stems, roots and leaves as well. Since large amount of water can also be lost through the stomata, the plant closes the pores when it does not need carbon dioxide for photosynthesis. The opening and closing of the pores is a function of the guard cells. The guard cells swell when water flows into them, causing the stomatal pores to open similarly, the pores close is in the guard cell shrink.