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. These cells are large,empty and colourless.

The mechanism of working of bulliform cells can be explained as:

When the water supply is sufficient to the plant ,the bulliform cells absorb water and due to the absorption of water ,they become turgid . When they are turgid , the leaf straightens up and it is exposed . The leaf straightening permits the loss of the water . Whereas on the other hand , at the time of insufficient water supply , these cells lose water and become flaccid due to the loss of water . When they are flaccid during the water stress , they make the leaf curled inwards so that leaf is exposed less .The curling minimises water loss . Hence, the bulliform cells minimise the water loss during the unfavourable or stress conditions.

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.