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Diagram of Stomata

Stoma (singular), usually called as Stomata (plural), is an opening found the leaf epidermis and stem epidermis used for gaseous exchange in plants . In Dicotyledons , more stomata is present in the lower epidermis of leaves than in the upper epidermis. On the other hand, Monocotyledons have same number of stomata on their upper epidermis as well as in the lower epidermis.[Note]

Air containing Carbon dioxide and Oxygen enters the plant through these openings. The ratio of carbon dioxide to oxygen consumption is more as most of it is used in for photosynthesis and some of the oxygen is consumed for respiration and a major amount of oxygen goes out as the by-product(but this happens only during the day). At night, oxygen is used in large amounts for respiration. Water vapour is also released in the process called as Transpiration .


As shown in the diagram in its most basic form, a stoma is just an opening surrounded by parenchyma cells called Guard cells. The guard cells are responsible for maintaining the size of the pore. The guard cells contain cholroplastids which contain chlorophyll(essential for photosynthesis).

Stomata in Dicotyledons[]

In Dicotyledons, the lower epidermis have more stomata than the upper epidermis of the leaf. The explanation lies in how the stomata works. They regulate any gases that pass through them. When the temperature is too hot, it will close to conserve water vapor but this will block CO2 coming in also. Therefore plants don't want to have their stomata closed all the time nor opened all the time. Sunlight hits the top of the leaves and therefore the temperature and light intensity is higher on the top surface of the leaves. If many stomata were to be located on the top, the plant would loose much of its water and when the stomata closes to conserve water, it won't get as much CO2. Therefore for most plants, the stomata stay on the bottom where the temperature is a bit cooler. Also it is protected from the wind since wind can cause evaporation of water. The explanation for the point "only Dicotyledons have the Stomatal pores in the lower epidermis" can be given by relying on the fact that Dicotyledons through Evolution came after the Monocotyledons.