What is apical growth

A plant grows new tissue from an apical meristem. The apical meristem is a group of cells that retain the ability to continue divisions, forming new cells continuously as the plant grows. This PRIMARY growth is responsible for growth in height.

By water evaporation from the leaves is created a negative pressure at the top of the tree that promotes the ascension  of the water through channels that are called xylem. there are also two molecule forces that are involved: the intra-water molecules cohesion (water molecules stick to each other) and the adhesion when the water molecules stick to the channels they are running in. this is what enables water to go from the roots to the top of a tall tree.