Many leaves that are capable of rolling up in dry, unfavourable conditions, and reopening again under conditions when there is no water stress, have special, thin-walled water-containing cells that enable them to make these movements. These are the bulliform or motor cells. Examples may be found in grasses, for example marram grass, Ammophila arenaria, and many mem-bers of the bamboosoid grasses. The shape, size and disposition of such cells can be used as an aid to classification and identification. (Fig. 6.18).
Cells with similar properties are present at the pulvinus and at the attach-ment regions of the leaflets to the rachis in many plants whose leaves fold at night.