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Name: Paul K Harding
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Question:
How can trees get that tall? How can the transport of water function that good? Normally one could think that trees can get only 10 m high because then the pressure of the air would be to weak to transport the water and a vacuum would occur. Or do trees use a system of one pump-system after another to solve that problem?



Replies:
Okay, the answer is two-fold. One, the column of water travelling up the trunk of the tree is only a couple of molecules wide, moving up from cell to cell. You can think of each cell as a "valve" if you will, holding the water and keeping it from flowing down the trunk (unless you WANT it to go that way, in the case of sugar laden water moving to storage areas in the roots) until it can be moved to the next cell up, and so on and so on. The second part involves diffusion. Cells can diffuse certain chemical compounds through their cell walls while simultaneously holding onto others. By having a gradient of increasing chemical concentration as you go up the trunk, you can cause a gradual diffusion of water up the trunk (since water travels relatively freely through the cell walls) and thereby creates and exception to the rule of 10 meters being the maximum height for a column of water -- you've effectively added energy to the system to "pump" water up the trunk.

--Wordsworth



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