Because of its size, the tree has been studied for its water pull. Water from the roots can be pushed up only a few meters by osmotic pressure but can reach extreme heights by using large negative pressures[15] in the tree's xylem (the water tubules) and sub-pressure from evaporating water at the leaves.[16] Sequoias supplement water from the soil with fog, taken up through air roots, at heights to where the root water cannot be pulled.[17]