It is known that illumination of the algae Chara corallina results in the formation of regions with inhomogeneous pH distribution in the proximity of the membrane. It was shown that, in a certain range of light intensities, two states with different pH distribution may correspond to the same intensity: an entirely homogeneous state and completely formed structures (pattern). The transition to the state with a high amplitude of pH peaks takes place at a higher light intensity than the transition to the homogeneous state; i.e., the system shows hysteresis. This phenomenon was studied using a mathematical model. A possible mechanism of hysteresis is discussed.