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The water is transported to the top most leaves (and rest of its parts) of a tall tree by three actions:
1) root pressure (osmosis)
2) capillary action (adhesion)
3) evapotranspiration (cohesion)
The trees contain a series of interconnected tissues (xylem and phloem) that act as network for transport of water and nutrients throughout the tree. The root pressure and capillary action push the water, while the transpiration from the leaves pull the water. The root pressure is generated by osmotic movement of water from soil to the root tissues. The capillary action is caused due to adhesion of water molecules. However, root pressure and capillarity is thought to support only few meters of water column. The rest of the water transport (esp. in tall trees) is attributed to negative pressure generated by evaporation of water from leaves. This negative pressure creates a vacuum and pulls the water from the roots.
Hope this helps.
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