Bulliform cell

'Bulliform cells or motor 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 portion of the leaf and are shown to be large, empty and colourless.

Mechanism

The activity of bulliform cells can be explained as:

During drought, the loss of water through vacuoles induces the reduced bulliform cells to allow the leaves of many grass species to close as the two edges of the grass blade fold up toward each other. Once enough water is available, these cells enlarge and the leaves are forced 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. Bulliform cells occur on the leaves of a wide variety of monocotyledon families 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.

It is unclear if this mechanism applies in all monocots, however, or whether other components such as fibers are the pieces controlling the folding and unfolding of the leaf. What is observed is that the turgidity of the bulliform cells often coincide with the folding activity, though there are cases where folding happens long after the cells have gone turgid.