Since plants don’t have mouths
Plants absorb water from the soil – through their roots – by osmosis.
When water builds up in the roots, because transpiration isn’t happening, it increases the concentration of ions and lowers the water potential in the roots. This causes a diffusion of water molecules from the area of higher concentration – in the soil – to the lower concentration – in the roots.
A Small Digression on Diffusion
Molecules jostle and bump into each other, shooting off in all directions (into other molecules) until they reach a balanced state. This occurs especially when there is an area of high concentration of ions and an area of low concentration of ions – the ions move across a ‘concentration gradient’ until the concentration is the same in both areas.
Back to root pressure…
Thus by osmosis, water moves into the plant’s roots. This in turn creates pressure in the roots and causes some water to get pushed up the stem.
(But it’s nothing like the force of transpirational pull)
Cool facts about root pressure
- You can measure root pressure by attaching a pressure gauge – or root pressure probe (RPP) – to the cut stem of a plant.
- It causes the phenomenon known as guttation – when plants exude droplets of xylem sap at night through special tissue in leaf tips or edges (called hydathodes). (example 1)