Right image from W.H. Freeman and Sinauer Associates, used by permission. The epidermal tissue functions in prevention of water loss and acts as a barrier to fungi and other invaders. Thus, epidermal cells are closely packed, with little intercellular space. To further cut down on water loss, many plants have a waxy cuticle layer deposited on top of the epidermal cells. Obviously these openings would allow gas exchange, but at a cost of water loss.
Guard cells are bean-shaped cells covering the stomata opening.
They regulate exchange of water vapor, oxygen and carbon dioxide through the stoma. Note the oval stomatal apparatuses in the center of the stem. Note the two sets of guard cells. Pea Leaf Stoma, Vicea sp. The Virtual Forest A 360 degree navigable forest.