Figure 1.

Freeze-fracture cryo-SEM images of the intrusive veins of red oak leaves. The hydraulic index (HI) of the xylem conduits, a measure of roundness, decreases as the water potential falls (bottom left corner). At −1 MPa (A), none of the xylem conduits show any negative curvature; by −2.5 MPa (B), two of the four conduits show signs of buckling (HI = 0.47 and 0.62); at −3 MPa (C), all conduits have buckled, but there is a broad range in the degree of deformation; by −3.5 MPa (D), most conduits are highly deformed (HI = 0.01, 0.03, 0.03, 0.2), while one (HI = 0.85) persists in an undeformed state. HI scores for each vein in total were as follows: 0.79 (A), 0.6 (B), 0.2 (C), and 0.05 (D).