I was doing a Cambridge iGCSE past paper when I came across the question:

Describe the mechanism by which water flows through the xylem

I thought the correct answer would revolve around the xylem being composed of dead cells, thus allowing water to use capillary action to flow upwards, but it wasn't.

Instead, the mark scheme referred only to transpiration causing tension and so forth, not even mentioning capillary action.

Is capillary action not a valid answer? And if not, why not?



1 Answer 1


- NO,for capillary action 2 things are of importance 1) the radius of capillary tube[R] ,2)the tension force between molecules at surface of liquid [T] (for water=72.8 dynes/cm= 728 X 10^-6 N)

- height to which the fluid will rise due to cap.effect, H=(2T cosα)÷(ρgR)

- [ρ=density of liq,here water =1000kg/m³]

- numer.=constant(for given liq) denomin.=variable R

- so if capillary was in effect for trees 100s of meters tall the deno. wud have to be small while the ρg is a fixed value here for water = 10000 units the R wud have to be impossibly small

  • WHAT HAPPENS IS the transpiration of water droplets from the stomata causes a suction as no air can enter to fill in for the water lost the water as to rise up taking more of it from the soil
  • 1
    $\begingroup$ Could you add sources to your used formulae and reasoning? $\endgroup$
    – AliceD
    Jun 27, 2016 at 22:42

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