Re: How does water get to the top of tall trees?

Water travels up the tree through long narrow tubes (sieve cells) 
lying side by side in layers all along the circumference of the tree 
trunk.  Several "forces" help move the water up the tree.  

There's a force called "osmosis" that drives the water gradient to 
be equal between cells so that water flows up into the cells with 
lower concentrations of water.  

Capillary action is also in force in the tree.  The sieve cells are 
so narrow that the water is drawn up into it.  You would see this 
happenning when you touch a paper towel to water.  The water is drawn 
up the towel.

As well, there's the force of attraction- "water tension" between 
water molecules.  If you fill a cup with water, you can add drop by 
drop more water until the top of the water is above the lip of the 
cup.  This is water tension, in which the water molecules are so 
attracted to each other that they can stay togetehr without a 
supporting wall.  This tension of the water inside the sieve cells of 
the tree keeps the flow of water continous.

Finally, there's "transpiration".  The tree is "breathing" through 
its leaves, and water is evaporating from the leaves, and as water 
evaporates from the leaves, more water is drawn into the leaves from 
the water in the tree branches.

If you look at the entire picture, the roots absorb water, then the 
sieve cells draw up the water through capillary action and osmosis.  
The water tension keeps the flow of water from breaking, with 
transpiration driving the flow in the upward direction, towards the 
leaves.

If you cut down a tree, you will see water (tree sap) flowing out of 
the sieve cells, which are located along the circumference of the 
tree.  Water won't be spurting out of the tree, and it will stop 
eventually, because there isn't any transpiration taking place any 
more.

The movement of water happens pretty much the same way in flowers 
too, and the water is easier to see.  Take a fresh flower (with 
leaves) and, holding the stem underwater, cut the stem with sharp 
scissors.  Take it out of the water and you will see a drop of water 
hanging onto the end you just cut.  That's water tension keeping the 
drop from falling!  If you watch closely, the water will slowly move 
up into the stem!! 

In fact, when you cut flowers to bring into the house, you need to 
put them in water right away.  The movement of water up the stem 
happens without stop, and if you wait to put the cut stems in water, 
the sieve cells will start to draw up air, and the air bubble will 
break the force of the capillary action, and the flower will be unable 
to draw up anymore water.  If you do need to wait before putting the 
plants into water, cut the stem a bit higher up, in water, and 
hopefully you'll have cut off the stem with the air bubble.
I hope you understand trees and plants better now  :)
Evelyn Tsang
Information taken from the Tree House in the Montreal Botanical 
Gardens.