"Primitive Earth had no oxygen gas floating around, and this new chemical was a lethal, corrosive pollutant. As photosynthetic organisms evolved and oxygen began to accumulate in the atmosphere, a catastrophic extinction of early life forms followed. (Most modern anaerobes are now confined to underwater or underground habitats, where oxygen levels are low enough for them to survive.) New life forms appeared on Earth that could tolerate oxygen, and now most life on Earth is utterly dependent on a gas that was once a deadly pollutant."
There is a chemical gradient that leads to water getting absorbed into the roots. (Also there is a symbiotic relationship with stringy funguses that helps with water absorption.) Then water travels through the xylem to the leaf. It adheres to the walls of the xylem (adhesion), which chemically is a result of hydrogen bonds with whatever is in the xylem walls. It is also pulled up (cohesion) because water and its hydrogen bonds is nice and sticky. Water is pulled out of the leaves through the stomata and evaporation, and that tugs on all the water down the line.
- Chalker-Scott, L. (2015). How Plants Work: The Science Behind the Amazing Things Plants Do. United States: Timber Press.