When water in the soil or in the xylem is put under tension, due to the evaporation of water from a particular site(whic当土壤中或木质部中的水受到张力作用时，由于特定部分水的蒸发（沿着蒸发方向形成水势梯度），毛细现象可以使水克服水分子之间的内聚力并同时沿着水势梯度逆着重力作用移动。