Stomata

Stomata (singular: stoma) are small pores in the epidermis of leaves and young stems through which gases diffuse in and out. In most plants they are found mainly on the lower surface of the leaf, which is cooler and shaded, so less water is lost from them.Each stoma is surrounded by a pair of guard cells. Guard cells are unusual epidermal cells:• they are bean-shaped (kidney-shaped) in most plants
• they contain chloroplasts, unlike most other epidermal cells
• their cell walls are unevenly thickened: the inner wall next to the pore is thicker and less elastic than the outer wall
• cellulose microfibrils in the walls are arranged radially, so the cells can lengthen but not widen muchThese features allow the guard cells to change shape and control the size of the pore.Stomata are the main route for both carbon dioxide entering the leaf and water vapour leaving it. The leaf surface is covered by a waxy cuticle that reduces water loss, but the stomata must be open for carbon dioxide to reach the mesophyll. This creates a trade-off between gas exchange for photosynthesis and water loss by transpiration.Plants adapted to dry conditions (xerophytes) often have fewer stomata, stomata sunk in pits, or hairs around them. These trap a layer of humid air, reducing the water potential gradient and so the rate of water loss.