How does tree sap go up very tall trees?

I would guess capillarity probably plays an important role; I read online that the production of sap generates CO2 which helps building pressure, but I would like to find a detailed explanation.

My girlfriend is reading a book by Peter Wohlleben called The Hidden Life of Trees.
This books claims that we do not understand that mechanism, which is something I have a hard time to believe.

  • 1
    I am reading the same book. Quite often I have the same feeling as you do. I tought many natural mechanisms are well understood, but it does not seem so, according to the author. We can, let's say, observe a behavior thet seem to have a logical purpose, but do not know the cause or origin, etc.
    – VividD
    Dec 16, 2017 at 20:08

2 Answers 2


You're right in that an element of capillary action is involved in the process. The usual explanation for how fluids flow through xylem tissue is to do with the interconnected process of transpiration. Fluids are taken in by the root hairs, into the roots, into the xylem, and moved up the plant through the hollow spaces within the xylem known as lumen, rather like a straw or straws. This link gives more information in regard to this process as well as phloem and translocation, but it's not the most technical link http://www.bbc.co.uk/schools/gcsebitesize/science/add_ocr_gateway/green_world/planttransportrev1.shtml

A more scientific explanation is to do with the nature of water/fluid itself, that is transpiration, cohesion, capillary action and adhesion, and that is explained more clearly here http://www.dummies.com/education/science/biology/how-plants-pull-and-transport-water/

Essentially, transpiration from the leaves acts like you or I sucking on a straw - the higher the rate of transpiration, the more fluid gets drawn up through the lumen in the xylem tissue to the leaves.

I would just add that not everything is entirely proven or completely known when it comes to living plants, any more than it is with human bodies; when I did my training years ago, we were told it was assumed there were plant hormones that caused certain things to happen, but only some had been proven to exist and named. Over time, of course, many of these suppositions and theories have been proven, but I'm willing to bet not all will have been. But for now, the explanations given in the links above are still taught today, regardless whether anyone has or hasn't actually proven they're 100% accurate.

Note: 'sap' refers to fluids found in both xylem and phloem systems, rather than just that flowing up the plant through the xylem tissue.


Trees used to be able to grow much much taller than they've been able to for the past couple of millennia. I was finally able to locate a decent article which explains in detail the reasons for growth upwards and sideways: Hydraulic Limits to Tree Height and tree growth.

  • Interesting link, if a bit heavy for me for first thing Sunday morning, not properly awake yet! But I can't find anything in it that supports your statement 'trees used to be able to grow much taller.... millennia', am I missing something?
    – Bamboo
    Dec 17, 2017 at 10:53
  • I'll go find a decent article on past eons and past atmospheric compositions and past height of trees...like dinosaur days, right? Lots of O2 more than we have now, was the reason centipedes were the size of large dogs. The reason dinosaurs were 3 to 5 times larger than any organism on this planet today. Of course the CO2 was higher.
    – stormy
    Dec 18, 2017 at 4:30
  • Well, shoot, Bamboo...I am unable to find what I remember...partly because it was tough to tell a prehistoric tree's height! The gravity defined the height and that gravity is the same gravity we have today. Our own Redwoods (not necessarily modern...) have been measured to be 116 meters. Dunno where I read that bit on trees being higher but I am humbled that that is a property of gravity not atmosphere and that has not changed. Hey, it takes me a good 4 hours to wake up. And my hubby is one heck of a chatty cathy he is a morning person!
    – stormy
    Dec 18, 2017 at 4:56

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