The Path of Ascent of Sap

Because the plant's vascular tissue actually contains two separate conducting systems, xylem and phloem, a direct experiment is needed to establish which one is responsible for carrying water upward. Cutting a balsam-plant stem and standing it in a beaker of red eosin dye solution settles the question cleanly: after some time a visible red streak rises up the stem, and when a thin transverse section is cut from the coloured region and examined under a microscope, the dye is found confined strictly to the xylem elements, while the phloem in the very same section remains c …