Abstract. The determination of guard cell lumen volume in relation to its geometric characteristic dimensions is presented. Stomatal movements can be divided into two stages: Spannungsphase and motorphase, with a transition point between them. During the Spannungsphase movement, the lumen volume increases as a result of the change in its geometric shape. At transition, the lumen volume is approximated by a portion of a circular tube with a rounded cross-section. The volume increase during the motorphase comes from three different sources: expansion by wall stretching, increasing inner cross-section of a guard cell by wall thinning, and polar expansion. The relationship between the aperture and the lumen volume is also established. The results set forth in this geometric model are essential to studies of the pressure interaction between a guard cell and its surrounding epidermal cells.