Our objective is to prepare a temporary mount of a leaf peel to show stomata.
Stoma (plural stomata), a word derived from Greek which means ‘mouth’, is a pore found in the epidermis of leaves, stems and all other plant parts found above the ground. Stomata are thus named because they permit the exchange of gases between the atmosphere and the inside of the leaf.
The size and shape of stomata vary in different plants. The structure of the stomata consists of a kidney shaped epidermal cell with an opening in the centre known as the pore. The stomata are bordered by a pair of specialised parenchyma cells known as the guard cells that are responsible for regulating the size of its opening, thus saving the plant from water loss. When fully stretched, the guard cells expand and thereby open the stomata. When the guard cells lose water, they shrink and the stomata will close. Guard cells also have large vacuoles. The cells that surround the guard cells are known as subsidiary or accessory cells. The guard cells in dicot plants are kidney shaped and dumbbell shaped in monocots.
Although stomata are found on the epidermis of leaves, the exact positioning is different from plant to plant. Stomata are found on either both sides or on just one side of the leaf. When the stomata are present on both sides of the leaf, they are called amphistomatic; epistomatic, if present on the upper side; and hypostomatic, if present on the lower side.
The distribution of stomata varies in different plants. There are more stomata on plant surfaces in conditions like lower atmospheric carbon dioxide concentrations, moist environments and higher light. In dicot plants the number of stomata is greater on the lower surface than on the upper surface of the leaf. On the other hand, monocot plants have the same number of stomata on both surfaces of the leaf. In the case of floating plants, stomata is found only on the upper epidermis.
The major function of stomata is the exchange of gases by taking carbon dioxide from the atmosphere and giving out oxygen that is used by human beings and animals. They help in photosynthesis and transpiration.
Photosynthesis is a process used by plants to manufacture food with the help of sunlight, carbon dioxide and water. It is through the stomata that the plant takes carbon dioxide from the atmosphere. The water molecule is broken down into hydrogen and oxygen, and it is through the stomata that the oxygen is then released as a by-product in the atmosphere. Stomata are the medium of gaseous exchange and cellular respiration in plants.
Transpiration is a process of evaporation of water from the surface of the plant. This keeps the plant cool and transfers minerals and other materials to different parts of the plant. As the plant takes water from the soil, the openings absorb other minerals. For a plant to take water from the soil, water needs to evaporate from the surface of the plant. Once this happens, pressure is developed that forces the roots to absorb water from the soil and transfer it to the tips of the plant. It is through the stomata that the major work of evaporation of water is done.
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