In grasses certain adaxial epidermis contains groups of larger thin-walled protruding and turgid cells over the region of veins. They are called bulliform or motor cells. The cells are highly vacuolate and can store water, if available. However, in case of water deficiency the bulliform cells lose water and become flaccid. As a result the leaf gets rolled up to reduce the exposed surface. The bulliform cells are also useful in the unrolling of leaf during this development.