The root, the underground part of a plant or tree, absorbs water and dissolved nutrients from the soil. In trees, the roots become thick and woody. The tops of the roots have growing-points, growing usually downwards under the influence of gravity. This is called positive geotropy. As the main root branches, lateral roots are formed, and further branching creates a root-system. On the root are radical hairs that take care of the actual absorption of water and mineral nutrients. In the immediate vicinity of the root hairs (not further away than 1.5 mm) an increased biological activity takes place. This zone is called the rhizosphere. The root hairs are constantly replaced, they only have a limited life span and usually live no longer than3 days.