TREES ARE GREAT SUCKERS !
Torricelli [in 1643]... was.trying to find out whether there was a limit to how high suction pumps could draw up a column of water—a serious problem in irrigation. He poured mercury to the top of a glass tube about 1 meter long, closed at the bottom. He then sealed the opening at the rim with his thumb and turned it upside down, into a bowl of mercury, taking his thumb away. When he did this, some of the mercury ran out of the tube back into the bowl, but the remaining column was about 76 centimeters high. The empty space at the top of the tube, he argued, was a vacuum, one of the very first vacuums produced in a laboratory. He knew that mercury was about 13.6 times denser than water, so he could calculate that the maximum length of a water column—which was what he really wanted to know—would be about 34 feet [10 meters]. ... [pp 66-67]
Consider trees. Calm, silent, immobile, slow, uncomplaining—they employ dozens of biological strategies to combat the force of gravity as well as hydrostatic pressure. What an achievement to sprout new branches every year, to continue putting new rings on its trunk, making the tree stronger even as the gravitational attraction between the tree and the earth grows more powerful. And still a tree pushes sap up into its very highest branches. Isn’t it amazing that trees can be taller than about 10 meters [34 feet]? After all, water can only rise 10 meters in my straw, never higher; why (and how) would water be able to rise much higher in trees? The tallest redwoods are more than 300 feet tall, and somehow they pull water all the way up to their topmost leaves. [pp 72-73]
From: 'For the love of physics' by Walter Lewin
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