Transpiration, also referred to as sweating, is the process by which plants lose water through their leaves especially, but also stems, flowers and fruits. This water is converted from liquid form to vapor and released to the atmosphere.
Water is absorbed through the roots of plants and moves up through hollow tubes in the leaves. Plants lose water through tiny pores (spaces called stomata) mainly on the underside of their leaves. Depending on how much water is in the soil and how hot or humid the air is and how much wind there is the plant will transpire more or less water into the atmosphere. An increase in temperature will cause an increase in transpiration rate because an increase in temperature will cause more water to evaporate from the cell walls.
Desert plants and conifers have specially adapted structures such as thick cuticles, reduced leaf areas, sunken stomata (pore spaces) and hairs to reduce transpiration and conserve water. The stomata are closed during the day and open at night when transpiration will be lower.
Clear plastic bags work best for transpiration, as sun shines through the bag causing photo syntheses, which will in turn cause transpiration.
Carefully slide the bag over a leafy branch containing at least 3 or 4 healthy green leaves that are dry on the surface. Tie the top of the bag off with whatever is available (handkerchief, hair band, twist tie, shoelace, etc.) Any of these will act as a gasket to make sure there are no air leaks. Do not tie off around more than one branch as this would permit air leaks and that will hurt the process efficiency considerably.
As the bag heats up, it draws water from the selected branch and the hot air evaporates it, eventually leaving condensation on the inside of the bag.
You can affix a rock or stick to the bottom of the bag to draw the water to the bottom. You can also tie the branch to the trunk of the tree to hold it in a downward position.
Let the bag sit in the sunlight until you feel enough water has collected in the feel enough water has collected in the bottom of the bag. You can either remove the bag or cut a small slit in the bag above the water line and squeeze the water into your mouth or a container.
Any leaves in the water will color and flavor the water. Oaks and other high tannin plants will make the water taste bitter but it's not poisonous.
Sometimes, the tree branches that you may choose just wilt and cook in the sun (especially if you leave the bag on for more than a day for example). So if you have many trees around, switch the bag from branch to branch.