Delphinidin

The lower epidermis of most plants generally resembles the upper epidermis, but the lower is perforated by numerous tiny pores called stomata. Some plants (e.g., alfalfa, corn) have these pores in both leaf surfaces, while others (e.g., water lilies) have them exclusively on the upper epidermis; they are absent altogether from the submerged leaves of aquatic plants.

Stomata are very numerous, ranging from about 1,000 to more than 1.2 million per square centimeter (6,300 to 8 million per square inch) of surface. An average-sized sunflower leaf has about 2 million of these pores throughout its lower epidermis. Each pore is bordered by two sausage- or dumbbell-shaped cells that usually are smaller than most of the neighboring epidermal cells. These guard cells, which originate from the same parental cell, are part of the epidermis, but they, unlike most of the other cells of either epidermis, contain chloroplasts.

The functioning of guard cells is aided by the photosynthesis that takes place within them. The primary functions include regulating gas exchange between the interior of the leaf and the atmosphere and regulation of evaporation of most of the water entering the plant at the roots. Guard cell walls are distinctly thickened but quite flexible on the side adjacent to the pore. As the guard cells inflate or deflate with changes in the amount of water within the cells, their unique construction causes the stomata to open or close. When the guard cells are inflated, the stomata are open; when the water content of the guard cells decreases, the cells deflate, and the stomata close.