In a biology lecture, I was told that the xylem vessels were 'narrow so that the water column does not break easily and capillary action can be effective'.

My question is why is this?

Is the reasoning similar to an artery in the human body, whereby the narrow property can increase the pressure of the blood and force it out faster?

Or is the reasoning different?


  • $\begingroup$ While this is a slightly different question biology.stackexchange.com/questions/42223/… , the answer will help to explain how adhesion and cohesion work to maintain the water column in the xylem. This is different from the human circulatory system, $\endgroup$ – AMR Jan 12 '16 at 18:35
  • $\begingroup$ I'd suggest reading about capillary action. The article is a bit technical in places, but what part of it boils down to is the narrower the capillary (up to a certain point), the stronger the effect and so the water column can go higher. $\endgroup$ – MattDMo Jan 12 '16 at 19:24
  • $\begingroup$ Related answer to How do trees lift water higher than 10 meters? $\endgroup$ – fileunderwater Jan 13 '16 at 23:15
  • $\begingroup$ What are you referring to with "...can increase the pressure of the blood and force it out faster" (specifically the "force it out faster" part) $\endgroup$ – fileunderwater Jan 14 '16 at 9:24

Xylem's main job is to transport water, and therefore the inorganic nutrients in the water. Now, to answer your question, we need to know how water is transported.

Water is transported due to both cohesion, adhesion, and surface tension. Water is known for forming hydrogen bonds, and this is known as cohesion. When it forms hydrogen bonds with other molecules, this is called adhesion. In the xylem, some of the water molecules are evaporating, but pulls the water molecules below back up. The adhesive forces also do a similar job of pushing the water up.

Now, if the xylem was smaller in diameter, then the surface the water is exposed to increases. This means the adhesive forces are stronger. Therefore, the water is transported a higher distance. This is important for really tall plants.

Now to answer your second part, the arteries analogy is so wrong. This has nothing to do with how fast the waters goes up, but the height at which is reaches. Also, the human blood vessels depend on pressure and pumping (the heart's job).

  • $\begingroup$ Surface tension is the result of the differential between cohesion and adhesion, it isn't a property per se. "At liquid-air interfaces, surface tension results from the greater attraction of liquid molecules to each other (due to cohesion) than to the molecules in the air (due to adhesion)." $\endgroup$ – AMR Jan 14 '16 at 2:40
  • $\begingroup$ Also, read the OPs question again, they are asking about a comparison between xylem and the circulatory system of the human body. Your answer really doesn't address that. $\endgroup$ – AMR Jan 14 '16 at 2:47
  • $\begingroup$ @AMR ok. I will edit ASAP $\endgroup$ – TanMath Jan 14 '16 at 2:47

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