Figure 5.5 Transverse hand-section of a fresh wheat leaf showing a single large (supply) vein comprising three large vessels (v). The vein is surrounded by two sheaths of living cells, the inner mestome sheath (arrowheads), and outer parenchyina sheath (*). The mestome sheath of these veins is impermeable to water. There is no apoplasmic path for water through the mestome sheath of large veins, except through a connecting transverse vein. In small veins, by contrast, two or three mestome sheath cells next to the xylem permit flow of water and solutes through the cell wall apoplasm. Water enters the symplasm at the inner boundary of the parenchyma sheath (Canny 1990). Phloem (p).Toluidine blue stain, bright-field optics. Scale bar = 100 µm (see Colour Plate 17) (Photograph courtesy M. McCully and M. Canny)