Say, you're lost in the wilderness, with very low water resources. You can live weeks without food, but only days without water. Transpiration bags are the best way to gather water in the wild as far as I'm concerned. They use the natural forces of evaporation and condensation to get you water. Using this method means you don't have to worry about drinking out of a polluted stream. It also means you don't have to deal with making clunky water stills on the ground, and you're not wasting energy looking for water. Unless you're in the desert, then just disregard this, since I know nothing about survival in the desert!
You will need:
A large plastic bag.
Two pieces string or other tie that will fit around a branch.
A rock or large stick.
First thing, choose a tree for your use. Plants with large root systems work best as the roots are what gather water through osmosis. Put the bag around a leafy tree branch then tie it off. Clear bags work the best, as sun shines through the bag causing photosynthesis, which will in turn cause transpiration. Make sure there are no air leaks, or it will impede the process, if you have a handkerchief or something similar, use it as a gasket around the mouth of the bag. As the bag heats up, it draws water from the selected branch and the hot air evaporates it, eventually leaving condensation on the side of the bag. This is where the rock or stick comes in, afixing one to the bottom of the bag draws all the water to the bottom. Letting the bag sit in the sunlight for 12 hours is best, but you should be able to collect about 250 ml. every 4 hours. When you feel enough water has collected in the bottom of the bag, either remove the bag, or make a cut in the bag above the water line and squeeze the water into the container of your choice. Don't freak if there are leaves in the water, it will just give it flavor; it's really a rather pleasant green, growing, sappish taste. Oaks and other high tannin trees will make the water taste bitter but it's not poisonous.
It's possible to get up to 2 Liters of water from one tree branch. Sometimes though, the tree branches just wilt and cook in the sun, and if you leave the bag on for more than a day, it surely will. So switch the bags from branch to branch, tree-to-tree, assuming you have that many trees around. I have done this on Black Oaks, Live Oaks, Sugar Pine, Pecan, & Walnut, but I live in a very forested region. One last word of caution, I DO NOT recommend using any trees for this that you do not recognize. So you might want to brush up on your tree recognition in any areas you visit.
Sources:
http://wwmag.net/Pages/myths2.htm
http://www.true-patriot.com/water.htm#Transpiration_Bag
http://www.edibleplants.com/month/transpir.htm
Living in the
boonies with a
survivalist dad.