Evidence of the mechanical advantage of subsidiary cells was obtained by simultaneous measurements of turgor pressure potentials in adjacent subsidiary and guard cells using injection circuits with two separate needles. In Tradescantia virginiana the mechanical advantage approaches two. Using the same technique evidence was obtained that the Spannungsphase is, in the first place, a turgor relations phenomenon due to the mechanical advantage of epidermal or subsidiary cells. In addition, the evidence indicated that the elastic properties of guard cell walls may undergo changes during the Spannungsphase when potassium ion transport commences. During these measurements it was confirmed that the optimum leaf water deficit for maximum stomatal opening occurs when the epidermal turgor is near zero. Under these conditions the width of the stomatal pore is a function of the turgor pressure of the guard cells, since at zero turgor of the subsidiary cells their mechanical advantage has disappeared.