In plants, this is the evaporation of water from the leaves. Water moves up the xylem tubes from the roots to the leaves, mainly by capillary action and the attraction of water molecules for each other. Water enters the roots of the plant by osmosis.

Transpiration starts in the root hairs. Water and minerals are brought into the roots by active transport, which uses ATP. The water and minerals have to reach the top of the plant, and they are brought up in the xylem, continuous tubes formed from dead cells, called vessel members.

There are numerous factors in the water uptake or water translocation. Water coheres to itself, and adheres to the cell walls, so there is capillary action on the water column. When stomata are open, water evaporates out the leaves. This creates a negative leaf pressure, and water from the roots is drawn up the xylem. When stomata are closed, root pressure builds up, and pushes water up the xylem anyway.

Stomata open and close to let carbon dioxide in. As earlier said, water also evaporates out these pores. When a stoma ‘wants’ to close, the guard cells around the stoma get a large concentration of potassium ions and cause lower water potential. Through the wonders of osmosis, the neighboring cells give up some of their water for the guard cells. This causes an expansion of the cell, until the membrane is full, and then the stoma closes.

In the leaves, photosynthesis occurs. The carbon dioxide brought in by the stoma enters the spongy and palisade mesophyll. In the mesophyll, sucrose and oxygen are created. The oxygen goes out the stoma, and the sucrose mixes with water, and creates sap. There is a higher concentration in the leaves, so osmosis takes it down to the roots. The sap travels down the vein in the phloem, which consists of sieve cells and sieve-tube members. Each sieve-tube members is adjacent to a specialized parenchyma cell called a companion cell. It carries out some of the metabolic functions needed to keep the sieve-tube member alive. From the phloem, the sap travels down the stem until it reaches the places it is needed.

Using Transpiration to Survive

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.

Tran`spi*ra"tion (?), n. [F. transpiration.]

1. Physiol.

The act or process of transpiring or excreting in the form of vapor; exhalation, as through the skin or other membranes of the body; as, pulmonary transpiration, or the excretion of aqueous vapor from the lungs. Perspiration is a form of transpiration.

Cudworth.

2. bot.

The evaporation of water, or exhalation of aqueous vapor, from cells and masses of tissue.

3. Physics

The passing of gases through fine tubes, porous substances, or the like; as, transpiration through membranes.

 

© Webster 1913.

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