cohesion-tension theory

Water is "pulled" up the xylem by the water lost through transpiration. The colum of water doesn't break because of the cohesive forces between the water molecules. Hydrogen bonds between indiviual water molecules is the force of attraction. The pulling action of transpiration stretches the water column in the xylem so that it is under tension.

So yeah, thats just a simpler version of what Zwitter Ion said really, but typing it helps my revision too!

The adhesive and cohesive forces also help the water move in a continuous column of water called the transpiration stream because of the difference in hydrostatic pressure.

Thanks, I'll add that sentence to my notes!

While we're on this topic could anyone please explain to me the evidence for the "Root Pressure" theory of transpiration? I only seem to have the limitations in my notes and it came up in a mock exam.

mineral ions in the soil are actively pumped into the roots, therefore decrease the water potential in the root
water will move in by osmosis, increasing hydrostatic pressure and this pressure causes water to move upwards to leaves,etc.

Surely that's the theory behind Root pressure, not the evidence supporting it?

Isn;t it something along the lines of when you cut the root, sap seeps out because of the pressure? It might not be right as I did my exams last year...

Ah that sounds more like it. Thank you

I am still a bit unsure. What evidence supports the cohesion theory?