The tree is a complex "machine" capable of extracting, transporting and chemically transforming material to nourish itself and to promote its own development.
The tree draws water and mineral salts present in the soil by its rootlets (absorbant hairs of roots). This solution constitutes the crude sap which is transported from the roots to the leaves through the xylem.
The trunk and the branches transport the raw sap from the roots towards the leaves and the refined (nutritive) sap from the leaves to the roots. The minerals are depositied along the way and intervene in the cellular reproduction of the tree.
The leaves are the main place for chemical reactions where the raw sap is enriched by the sugars generated by photosynthesis.