Bulliform cell

Bulliform cells are large, bubble-shaped epidermal cells that occur in groups on the upper surface of the leaves of many monocots.These cells are present on the adaxial or the upper surface of the leaf.They are generally present near the mid vein.

Loss of turgor pressure in these cells,during water stress, causes them to become flaccid and thus leaves "roll up". During drought, the loss of water through vacuoles induces bulliform cells to cause the leaves of many grass species to close as the two edges of the grass blade fold up toward each other. Once adequate water is available, these cells enlarge and the leaves open again. 

 Folded leaves offer less exposure to sunlight, so they are heated less thus reducing evaporation and conserving the remaining water in the plant and occur on the leaves of many monocotyledons but are probably best known in grasses. They are thought to play a role in the unfolding of developing leaves and in the rolling and unrolling of mature leaves in response to alternating wet and dry periods.