Figure 5.4   The same leaf as in the frontispiece for Chapter 5, showing one large and three small longitudinal veins, with transverse veins connecting them. Water moves along the leaf only in large (supply) veins, and enters small (distribution) veins through transverse veins. In small veins it can move in either direction in response to local pressure gradients. Small veins could carry up to two-thirds ofthe evaporated water. Semi-dark-field optics, blue filter. Scale bar = 100 µm (Photograph courtesy M. McCully)