The process of photosynthesis requires water. The system of xylem vessels from the root to the leaf vein can supply the needed water. But what force does a plant use to move water molecules into the leaf parenchyma cells where they are needed ?

Explanation

As water evaporates through the stomata, since the thin film of water over the cells is continuous. It results in pulling of water, molecule by molecule into the leaf from the xylem.

Because of lower concentration of water vapour in the atmosphere as compared to the substomatal cavity and intercellular spaces, water diffuses into the surrounding air. This creates a .pull.
The forces generated by transpiration can create pressure sufficient to lift a xylem sized column of water over 130 metres high.