phloem, the principal food-conducting tissue of the vine and other vascular plants. Phloem is composed of a mix of cell types which lie alongside the xylem, the water-conducting tissue, and the combination makes up a system of veins or vascular bundles. Despite their proximity, phloem and xylem are entirely different: phloem has thin-walled tubes containing a strongly sugared sap under positive pressure which moves along from areas of strong to weak concentrations, while xylem consists of large, strong-walled tubes through which a dilute mineral solution moves under negative pressure (tension) by forces generated by transpiration. During the thickening of woody parts, the cambium produces cells on the outside that become the new season’s phloem; in later years these cells are added to the bark of the vine. Phloem of grapevine wood has the characteristic, unusual among deciduous trees, of reactivation after the next budbreak and can remain functional for three to four years.
