Stomata are minute pores found on the epidermis of leaves and young shoots of plants that are used to control exchange of gases. The pore is surrounded by a pair of specialised cells called the guard cells that are responsible in regulating the size of the opening. Distribution of stomata varies between monocots and dicots, between plant species, and between the underside and top side of the leaves on a plant. Stomata are found more on plant surfaces thriving under higher light, lower atmospheric carbon dioxide concentrations and in moist environments. Usually the lower surface of a dicot leaf has a greater number of stomata while in a monocot leaf they are more or less equal on both surfaces. In most of the floating plants, stomata are found only on the upper epidermis.
This video explains how to study the stomatal distribution on the upper and lower leaf surfaces and to calculate the stomatal index.