Redwood forest near Santa Cruz. Photo: Uwe Hacke
Looking up the trunk of tall trees like these, one may wonder how water reaches the upper leaves. According to the cohesion theory of sap ascent, water in the xylem is pulled up by transpiration. During the growing season, water is lifted into the leaves by a gradient in negative pressure. Several factors make this possible, the tensile strength of water and the intricate anatomical structure of the xylem are two key factors. See Martin Zimmermann's book "Xylem Structure and the Ascent of Sap" - chapter 3 for more facts. Or study the 2nd edition (Tyree and Zimmermann), which is very detailed on this topic.