Visualizing water-conduction pathways of living trees: selection of dyes and tissue preparation methods

To visualize water-conduction pathways in living trees, we introduced aqueous solutions of safranin and acid fuchsin into stems of Populus sieboldii Miquel. To examine the spread of each dye in the trees, we compared several techniques for preparing tissue for light microscopy. Acid fuchsin was distributed more rapidly and more widely than safranin, reflecting differences between the dye molecules in state of ionization. We prepared some sections without allowing the dye to redissolve after it had been stabilized by freeze-drying. In these sections, the dye was observed in vessels and in some of the adjacent ray parenchyma cells. Other sections were prepared without stabilizing the dye. In these sections, acid fuchsin in the sap stream left cell walls unstained, whereas safranin stained wood fibers in the vicinity of vessels, as well as the vessels themselves, provided that the sections were mounted in glycerin, which dissolves safranin. Although stained with safranin, the wood fibers contained no water. The results indicate that stabilization of the introduced dye and subsequent preparation of tissues under conditions that avoid dye resolublilization allow accurate visualization of water-conduction pathways at the cellular level.