Water is pulled up through the xylem mainly by transpiration pull : evaporation of water from the leaves creates tension that pulls a continuous column of water upward, helped by cohesion and adhesion between water molecules and xylem walls.

In slightly more detail, as water evaporates from moist cell walls inside the leaf and diffuses out through stomata (transpiration), it lowers the water potential and creates negative pressure (tension) in the leaf xylem. Because water molecules stick to each other (cohesion) and to the xylem walls (adhesion), this tension is transmitted all the way down the continuous water column from the leaves to the roots, effectively pulling water upward against gravity.