9
$\begingroup$

Which force actually drives water so high up, since pure atmospheric pressure will only get you up to about 10 meters if you're using suction and a long straw and yet tallest trees are over 100 meters ( trees do basically have long continuous 'straws' all the way up.)

$\endgroup$
3
  • 1
    $\begingroup$ Veritasium's video on this topic. $\endgroup$
    – Lith
    Jun 29, 2019 at 15:15
  • 1
    $\begingroup$ I don't know, but I note that osmotic pressure can be surprisingly large: it is quite easy to reach several atmospheres of osmotic pressure, so I guess this may have something to do with it. $\endgroup$ Jun 29, 2019 at 17:41
  • 2
    $\begingroup$ This question has already been asked and answered in detail on the Biology Stackexchange: biology.stackexchange.com/questions/11044/… $\endgroup$
    – Mike Scott
    Jun 30, 2019 at 5:33

2 Answers 2

13
$\begingroup$

It's not only atmospheric pressure which is involved in water delivery to a tree, but mainly the capillary action and osmosis.

Osmosis and Hydrostatic Pressure

Roots take advantage of "pressures" when water and its solutions are unequal. The key to remember about osmosis is that water flows from the solution with the lower solute concentration (the soil) into the solution with higher solute concentration (the root).

Water tends to move to regions of negative hydrostatic pressure gradients. Water uptake by plant root osmosis creates a more negative hydrostatic pressure potential near the root surface. Tree roots sense water (less negative water potential) and growth is directed towards water (hydrotropism).

(From “Process of How Trees Absorb and Evaporate Water via Roots and Leaves”, ThoughCo.)

Only for the sake of your interest:

Current hypotheses of height limitation focus on increasing water transport constraints in taller trees and the resulting reductions in leaf photosynthesis. We studied redwoods (Sequoia sempervirens), including the tallest known tree on Earth (112.7 m), in wet temperate forests of northern California.

Our regression analyses of height gradients in leaf functional characteristics estimate a maximum tree height of 122–130 m barring mechanical damage, similar to the tallest recorded trees of the past. As trees grow taller, increasing leaf water stress due to gravity and path length resistance may ultimately limit leaf expansion and photosynthesis for further height growth, even with ample soil moisture.

(From Koch, George W.; Sillett, Stephen C.; Jennings, Gregory M.; Davis, Stephen D.: “The limits to tree height”. Nature. 428 (6985): 851–854.)

$\endgroup$
0
$\begingroup$

It due to transpiration pull. Due to which is water molecules experience a force that pulls it upwards.

$\endgroup$
3
  • 4
    $\begingroup$ Please could you say what "transpiration pull" is. The name alone is of rather limited help. $\endgroup$ Jun 29, 2019 at 17:39
  • $\begingroup$ @safesphere That link is mostly woo-woo. It says "The transpiration pull is similar to the suction force when drinking some fluid from a bottle or glass with a straw," but suction can't raise water more than 10m, as anybody who has any experience of designing water pumps knows very well. $\endgroup$
    – alephzero
    Jun 30, 2019 at 1:07
  • $\begingroup$ @safesphere odd web page; it seems to be largely pseudo-science. If you agree then maybe adjust the comment? It might be useful to rule out this idea if in fact it doesn't hold water! $\endgroup$ Jun 30, 2019 at 19:09

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.