Bulliform cells are large, bubble-shaped epidermal cells that occur in groups on the upper surface of the leaves of many
grasses. Loss of turgor pressurein these cells causes leaves to "roll up" during water stress. During drought, the loss of moisture through vacuolesinduces bulliform cells to cause the leaves of many grass species to close as the two sides 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. [Moore, R. et al. (1998) "Botany." 2nd Ed. WCB/McGraw Hill. ISBN 0-697-28623]
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bulliform cell — noun : one of the large thin walled apparently empty cells that occur in the epidermis of many grass leaves and that by their turgor changes cause rolling and unrolling of the leaves thus regulating water loss called also hygroscopic cell, motor… … Useful english dictionary