Some of the simplest questions, when posed, may seem too trivial to warrant any interest and more importantly, necessitate critical thinking. Among such questions is how can trees transport water from the roots to the top.
At the surface of the question, this question might not seem to be mind-boggling. However, when you start to account for all the possible variables, this question starts to be mind-bending.
Facts About Water Moving In Columns
Water has a height limit to which it can be sucked up a column – 10 meters. Beyond the 10 meters, a perfect vacuum is created, and water starts to evaporate spontaneously in the tube.
For trees that are 100 meters, the trees have to create a pressure difference of 10 atmospheres to be able to transport the water that high.
So, how do trees transport water up to the highest point of the tree?
What People Think
Hypothesis #1: For most people, this feat is achieved by transpiration. This is where water molecules evaporate from leaves, exacting a pulling force to molecules that are behind the evaporated molecules.
In some regard, most people would be correct as it provides some suction mechanism to pull water molecules in trees. However, this water transportation mechanism would have its limits. The suction force is not sufficient enough to overcome the 10-meter limit.
Hypothesis #2: The second hypothesis is that trees do not have continuous columns. In this regard, the ‘tubes’ have valves regulating the height of the column of water in the form of xylem cells. The pressure to transport water up the tree is thus provided by lateral ‘squeezing’ pressure from the cells.
However, this hypothesis is discounted by the account of the fact that scientists agree that the xylem tubes is made up of a continuous tube of water. Additionally, the provision of lateral ‘squeezing’ pressure would be energy costly for the trees.
Hypothesis #3: the third common hypothesis is the utilization of osmotic pressure. The osmotic pressure created by the soil having less solute than the roots would push water inside the roots and provide the water with pressure to drive it up the tree. However, this hypothesis is also discounted by the fact that some trees thrive in salty soils such as mangrove trees, where the osmotic pressure acts in the opposite direction.
Hypothesis #4: Capillary Action where the thinner the tube the higher the water can climb. However, this is still wrong as the tubes are too wide – 2 to 5 micrometers.
How Water Is Transported Up High Trees
One fact that most people disregard is that you can have negative pressures in water unlike in air, which has the least pressure as zero pressure in a perfect vacuum. Consider the negative pressure as tension in solid.
When water molecules evaporate, they create an enormous pressure of – 15 atm. The tiny (2 to 5 nanometres) stomata prevent the water meniscus from breaking.
As you move from top to bottom, the pressure increases from the maximum negative pressure, depending on the height of the tree up to the atmosphere, creating a high-pressure difference between the top of the tree to the bottom.
The water does not boil in the tubes as it does not have activation energy that initiates boiling, usually in the form of water bubbles. Moreover, since the xylem tubes have been filled, and bubble-less from the start, water can remain a meter-stable liquid state in the tubes.