Explain capillarity in very tall trees (is there a limit?) or tell me your favourite jam?

Question

I heard that this is the mechanism allowing trees to get moisture to their upper limits. Can I have some answers?

Answer 1

Trees do not get water to their leaves using capilliary action alone. Capilliary action could lift water to perhaps 9-10 metres maximum. The tallest trees such as Giant Redwoods are over 8 times that height. Capilliary action plays only a small, albeit vital, role in lifting water in trees. The wooden core of a tree trunk is largely a dense array of narrow tubes, called xylem, that carry water from the roots up to the leaves. The water moves up the xylem via an entirely passive process known as transpiration, which is driven by a combination of capillary rise and evaporation through the leaves. At the tops of the open xylem tubes, the water evaporates into spaces within the tree’s leaves, then exits to the atmosphere through pores in the leaves. As the water evaporates, capillary action—the electrostatic attraction between the water and the leaf cells and the inner surface of the xylem tubes—moves more water up the xylem and into the leaves. At the same time, the electrostatic attraction of the water molecules for one another provides enough cohesive force on the entire vertical water column to draw more water from below the ground up to the top of the tallest redwood.

The weight of the water column itself puts a good deal of tension on the internal cohesive forces at its top. Imagine a narrow tube filled with water and running to the ground from a treetop 360 feet in the air. Water is free to move in the xylem, and the walls of the xylem tube provide no direct support to the water inside. The support comes instead from the water itself. Its internal cohesiveness makes the column of water act like a long suspended string, and the tension on the molecules at any point in the column must support the weight of all the water below them. Expressed as a pressure, or force per unit area, the tension on the water in the xylem is surprisingly high: for every thirty feet of tree height, the tension increases by roughly fifteen pounds per square inch. For a xylem tube 360 feet high, the tension at the top is 180 pounds per square inch.

But water is only so cohesive; if the tension is great enough, the column will break. An air bubble at the break would obstruct the xylem. Theoretical calculations led plant physiologists in the 1990s to surmise that water transport, rather than the strength of wood or some other constraint, limits the height of a tree.

Answer 2

Capillary action:

If you have a straw full of water, and you suck it from the top, then water will go out the top into your mouth, and more will come in the bottom (provided there is some there). Capillary action is more or less the same priniciple (although instead of having one end of the 'straw' it ends in every pore in every leaf). Basically the sun evaporates water from the leaves, which acts as the suck to pull more water up.

I don't know for sure whether there is a limit, but I expect there is. The higher the water needs to be taken, the more energy is required to overcome gravity. The 'suck' caused by the evaporation must have a limit in force, and therefore there will come a height where the force of the suck will equal the force fo gravity, and water cannot get any higher. Obviously no tree will ever reach this height, as any parts which did get over this height would die instantly from lack of water.

Oh yeah, I like strawberry jam.

Answer 3

Yes sure,

tree trunks are made up of small eliptical cells that have holes either end. The water rises through a combination of vacuum, from evaporation from the leaves, and capiliary.

As the cells are in there own right an isolated "head" of water there is no realistic limit to the height ( I guess there's a thoeretical limit but this would evolve around lost of capilliary energy per cell until it couldn't maitain a head of pressure within one cell length ( miles ? )

Remember each cell is seperate so the normal 30 odd feet rule does not apply ( which wil be a relief to my oak trees )

Strawberry, so refreshing.

Answer 4

Yeah, its all to do with the tension in water caused by the forces between hydrogen molecules - don't totally quite me, cos that's just an approximation, rather than an A-level answer, and it has been 3 years (and many boozy nights in the students' union) since i sat my a=level chemistry and biology.

Ditch jam, can't bit a nice bit of marmalade...mmmmm, orangy bits.

you can come round ours for toast if you like, we have toast parties every tuesday...

Answer 5

Mixxed fruit

Answer 6

Mmmmmm seedless raspberry on toast.

Answer 7

Strawberry is good, I also like raspberry.

Answer 8

i love blacurrant jam, and damson but only home made damson.

no idea what the other thingy is.

Answer 9

lemon curd its a jam i think can never tell the difference between jam,curd and marmalade!!!

Answer 10

Mmm, raspberry.