How Trees Move Water

 A question was put to one of my classes long, long ago, in a classroom far, far away: “How can water reach the top of a tree, 60 feet in the air without any pump?” This is where my brain started to hurt. At the time, I didn’t know anything about trees—this was why I was there.  Trees don’t have muscles to push the water through itself.  Trees know the properties of water and how to manipulate it to get what they need. 

First, water does two things we need to understand. Water clings to itself and also clings to other things. Water molecules like each other. When they touch, they hold hands. I think about watching rain hitting a window and the drops moving down get bigger as they absorb other drops. They are drawn together. However, they are also drawn to the window. They are not bouncing off the window and falling to the ground. Water hits the window and stinks to the window. These two actions of water are called adhesion and cohesion (holds to itself and holds to other materials). But water’s strongest hold is to itself more than to other materials.

Second, water wants to balance its solvents. Solvents are substances that will dissolve in water. There is an experiment that shows this. You take a plastic tube and bow it into a ‘U’ shape. Inside the tube at the lowest point, there is a membrane. A barrier that will let water in but not the solvent. Water is added to the tube at equal parts, so on each side of the membrane it is the same. Then on one side of the membrane you add the solvent. Water passes through the membrane to the solvent side making the water level higher on the solvent side. Water pushes itself high up the tube to try and balance the solvent equally, but because of the membrane, which is key, water could move higher.  Plants use membranes the same way. The membrane acts as a kind of door letting some molecules in and keeping others out. This process might sound familiar. The process is osmosis. Some water softeners use reverse osmosis to clean tap water. The solvent they use is salt.

Thirdly, we have something called evapotranspiration. The leaves at the top of the tree get a lot of sun. To keep them from wilting and dying they need a constant flow of water. Why? Because water evaporates out of the leaf all the time. Sometimes we just call it transpiring. It’s a little like sweating but for leaves. Remember, water likes itself. It has great self-esteem!  As water evaporates out of the leaf, it pulls more water up to the leaf like a chain. As long as the line is not broken, water will flow up from the ground, up the tree and out of the leaf.  All the nutrients for the whole tree flow up a super thin layer, not more than a few cells wide. This layer is just underneath the bark. It is the green you see when you cut into a branch. This layer of nutrient rich cells is called the cambium.

With that bit of information for your noggin, I leave you. If you like this, then there is more of the same on the podcast. Stay awesome to each other and Plant on Babylon!