Fig. 10.5-3. Transverse section of grass leaf (Poa praetense). During drought, many grass leaves close as the two sides of the blade fold up toward each other. Once adequate water is available, the leaves open again. When folded, the leaves are less exposed to sunlight, so they are heated less. This movement is due to water being absorbed or lost by very large epidermis cells, called bulliform cells (see pages 194 and 195 in Plant Anatomy [Mauseth]). The bulliform cells (arrows) are located near the midrib. This species only has the two sets of bulliform cells, each set running the length of the midrib. In other species of grasses, there may be many sets of bulliform cells, allowing the leaf blade to curl or roll up rather than merely fold.