Functions of stomata during Day time:

Stomata (openings) in the leaf epidermis are surrounded by guard cells. During daytime guard cells are making glucose and so are hypertonic (higher concentrations of glucose) than their nearby epidermis cells. Water enters them from other cells and they swell. In this form they assume a rigid bowed shape, creating a pore between them.

The function of Stomata during Night:

At night when there are low solute concentrations in guard cells water leaves them and they become flaccid. In this form both guard, and cells rest against one another closing the opening.