The protoplasm of Nitella forms a thin layer surrounding a large central vacuole filled with sap. At the inner surface of the protoplasm is a non-aqueous layer called Y and at the outer surface is a similar layer called X. At each of these layers there is a potential due to the diffusion of KCl in contact with the layer. We thus have P = PX + PY in which P is the total potential, PX is the potential at X, and PY the potential at Y. We assume that when stimulation occurs PY disappears and PX remains unaltered. The loss of part of the potential therefore involves no violation of the all-or-none law since the Y layer loses all of its potential and the X layer loses nothing.
If the concentration of KCl in the external solution and in the sap is known the concentration in the protoplasm can be calculated at each stimulation by measuring the height of the spike.