Rex Trulove
We are taught early in life that water normally doesn’t flow uphill. We are also taught that plants produce sugar through photosynthesis, which can only occur with sunlight, carbon dioxide, and water. Many trees stand over 200 feet tall and in fact, redwoods and Douglas firs can be well over 300 feet tall. So how does the water get to the leaves at the top of the tree?
Reason for confusion
It isn’t magic that gets the water clear to the top of a tree, it is a tremendous design. However, it can sure seem like magic and it can be confusing. Here is why it is confusing.
Some time ago, it was found that if you put a glass tube in a container of water, water will move up the tube. The narrower the tube, the farther up the tube that water will move. However, there is a limit to how high the water will go. That is a bit more than 30 feet.
The reason for this is simple. The cause of the water moving up the tube is the air pressure that is pushing down on the water in the container. In fact, this is the basis of how atmospheric or barometric pressure is measured. The difference is that mercury is used instead of water, to measure the pressure. Thus, if the weatherman says that the barometric pressure is 30.1 and falling, what they are saying is that the mercury will move 30.1 inches up a tube and that level is lessening. It is simply easier to use mercury and only measure around 30 inches than it is to use water and measure around 30 feet.
The important part in regard to the original question is that if water will only go 30 feet up a tube from air pressure, it might seem impossible for the needed water to get all the way to the top of a 300-foot tree. Obviously, it isn’t impossible, but it also isn’t magic.
Adhesion of water
One of the properties of water is its adhesion. That is, it sticks together and it isn’t easy to break it apart. This is what gives water its surface tension.
You can see the surface tension in action by putting water in a clear glass, the looking at the water surface at eye level. Where the water touches the glass is higher than where the surface is in the center. This is a display of surface tension. It is also because of water cohesion that it is possible to put more than a cup of water in a glass that holds one cup. At eye level, the water would appear mounded above the rim.
Transpiring trees
The adhesion is quite important in getting the water to the top of a tall tree, but this is only part of it. The other part of the puzzle is called transpiration.
Transpiration is the process where water is evaporated into the air by the leaves of a plant. In a way, this is sort of like the way we perspire or sweat. This can amount to an enormous amount of water that is evaporated off by a tree. For instance, an oak tree can transpire many tons of water every day.
At the top of a tree, though, the transpiration causes a partial vacuum in the tiny tubes and tissues inside the tree that transport water. Because of the adhesion of the water, the fluid moves up to fill the vacuum. This is the same principle as sucking a drink through a straw. The adhesion is so strong, in fact, that it allows the tree to suck up water from over 300 feet below!
If a person thinks about it, this is astonishing. Put in another way, at the pressure of our atmosphere at sea level, water is incapable of rising above 30 feet. However, because the tiny water tubes in a tree are so small in diameter, plus the strength of the cohesion of water, plus the vacuum caused by transpiration, the tree generates a pressure of over 1,000 gravities.
The process is amazingly simple, yet it shows tremendous design. It also took mankind a long time to figure out how it was done. One of the amazing things is that the best pumps designed by man can’t push water straight up 300 feet without increasing the water pressure at the bottom. Trees have no pumps and function with water that is at the same pressure as the water that is around it.
The water isn’t pushed up. That is the secret. Instead, it is pulled up.
Now you know how a tree is able to get water to the very top leaves.
Very interesting and informative post indeed!
Thank you very much. When I was young, I was very curious about things like this. I lived in a place where there were many trees over 75 feet tall. When I first learned that water will only reach 30 feet under normal circumstances, I couldn’t help but wonder how water got to the top of those trees. That is the way my mind works when it is exposed to new facts.
This is very interesting information. If I have a student who is interested in plants, I will share this post. Please put the measurements in Metric as well. as many people from all over the globe don’t use the same system of measurement that is used in the United States.
I’ll try to remember to use metric the next time I write an article like this. Usually, I do, but I forgot this time.