What is negative pressure in the cohesion tension mechanism?
Well water moves into the roots because there is a low water potential made by actively intaking ions. The mass amount of water taken in is mostly more than is in the xylem vessel, so it all piles in there via osmosis. Water has cohesive properties (molecules stick together) which means that water which moves up pulls up the rest with it. Water also has adhesive properties – it sticks to the sides of the xylem vessel reducing the impact of gravity upon the water molecules which are cohesively joined. Most of the movement is down to mass flow, the sheer amount of water entering the xylem vessel causes whats already there to be forced upwards. The presence of ions in the xylem vessel is also a factor to consider, ions reduce the water potential, so movement of ions causes movement of water (as water travels from a high water potential to a low water potential). – ions such as sodium (Na+ – salt) is actively pumped into the xylem vessel this is dissolved in the water and therefore moves u
