Figure 5.7   Transverse section of a leaf of soybean showing two of the smallest veins surrounded by bundle sheath cells. Veins of this size are the end of the branching network shown in Figure 5.6, and supply most water that is evaporated. Before processing, this leaf had been transpiring in a solution of fluorescent dye for 40 min. The small vesels in each vien contain dye solution which has become concentrated by water loss to the symplasm and out through the bundle sheath. Dye has started to diffuse away from small vessels in the cell wall apoplasm of bundle sheath cells. Anhydrous freeze-substitution and sectioning, fluorescence optics. Scale bar = 50 µm (Photograph courtesy M. McCully and M. Canny)