Diffusion plays an important role in the movement of materials, but it is a very slow process.
Consequently, most organisms have evolved some sort of specialized transport mechanism to distribute substances more rapidly between distant areas. Water and minerals enter through root epidermis, cross the cortex, pass into the stele, and flow upward in xylem. The latter is used to absorb minerals whose concentrations in soil solution are lower than those required by the plant. The opening and closing of stomata is regulated by specialized epidermal cells called guard cells.