Figure 3.61. Guard cells are structured so that high turgor pressure opens the stomatal pore, and low turgor pressure closes the pore. Left (Lower image), Partially open Tradescantia virginiana stoma with 0.2 MPa guard cell turgor, as measured directly with a guard cell pressure probe (shown). Right (upper image) same stoma showing the aperture almost fully open, with 3.6 MPa guard cell turgor. (Images and data courtesy P.  Franks).