The epidermis is covered with pores called stomata. However, more stomata are found on the lower epidermis so as to prevent excessive loss of water. Stomata are pores that allow gas exchange to take place and photosynthesis to occur at top speed. Stomata are bounded by two guard cells. Guard cells are structurally specialized to control stomata and regulate the movement of gases, especially water vapour, into and out of leaf tissues by responding to changes in water pressure within the guard cells. When water pressure is high, the thin outer walls of the cells are forced into a curved shape. This pulls the think inner walls of the guard cells away from one another, opening the stoma. When water pressure decreases, the inner walls pull together and the stoma closes. Guard cells respond to conditions in the environment, such as wind and temperature, helping to maintain homeostasis within a leaf.
Leaves are covered on the top and bottom by epidermis made of a layer of tough, irregularly shaped cells. The epidermis of many leaves is also covered by the cuticle and lack chloroplast. Together, the epidermis and the cuticle form a waterproof barrier that protects tissues, limits the loss of water through evaporation and help focus the light onto the mesophyll layers, right below the upper epidermis.
The palisade layer is the top of the mesophyll layer. These closely packed cells absorb light that enters the leaf. The palisade cells are nearest the top of the leaf where most light gets through. The cells are arranges end on so that there are as few cell walls as possible between the leaf surface and the chloroplast. Beneath the palisade layer is the spongy mesophyll, a loose tissue with many air spaces between its cells. These air spaces connect with the exterior through the stomata, pore like openings in the underside of the leaf that allow carbon and oxygen to diffuse into and out of the leaf. The surface of spongy mesophyll cells are kept moist so that gases can enter and leave cells easily and rapidly. This also means that water evaporates from these surfaces and is lost to the atmosphere, a process called transpiration. The water would be replaced by water drawn into the leaf through xylem vessels in the vascular tissue.
The vascular tissue of leaves are connected directly to the vascular tissues of stems, making them part of the plant’s transport system. In leaves, xylem and phloem tissues are gathered together into bundles that run from the stem into the petiole and end among the mesophyll cells. The xylem brings water and dissolved mineral salts to the leaf from the soil via the roots and the stem. Once out of the veins, these raw materials diffuse from cell to cell right through the mesophyll of the leaf. Once the chlorophyll receives the essential raw materials, the manufactured food such as glucose made would then be transported as sucrose via the phloem to all the plant cells.
