Re: How is tensile strength of xylem related to water column maintenance?

Hi Madhu,

The book is referring to the tensile strength of the water column carried by the xylem. In shorter plants, especially in moist soils, the roots can pump water in with sufficient force to push water throughout the plant. In some cases, the water is pushed hard enough that it is forced out of the leaves as visible droplets. In taller plants, that pushing force is not enough, and an additional process is needed to get water to the top of the plant. In this case, the water is actually pulled up the plant. How do plants pull off such a feat? Water molecules are polar, with the oxygen atom surrounded by a small negative charge, and each hydrogen surrounded by a small positive charge. Because opposite charges attract, the oxygen side of a water molecule is attracted to the hydrogen side of other water molecules. In short, a characteristic of water molecules is that they are attracted to one another, a bit like magnets. This explains some of the characteristics of water, like surface tension. All the water molecules on the surface of a volume of water are weakly bound to one another like magnets.

Imagine you had a bunch of metal balls in a line on a table, each touching the ball next to it. If you pulled on the ball at the end of the line, the other balls would stay put. Now imagine that the balls were little magnets. This time, if you pulled on the ball at the end of the line, all the balls would follow, since they are magnetically attracted to one another. The same thing can happen with a chain of water molecules. Now imagine you lift the ball at the end of the line. The magnetic attraction may be enough to support a few metal balls, but once you lift high enough, and too many balls are lifted into the air, the combined weight of the balls being lifted will exceed the attraction between them (the tensile strength), and the chain will break. If the balls were not magnets, there would be no tensile strength at all, and you couldn’t pull a chain of them. Similarly, if a column of water had no tensile strength, the plant couldn’t pull water up the stem.

Within a plant, there are several xylem vessels that contain a continuous column of water that extends from the roots to the leaves. Within the leaves, the column is split to spread throughout the leaf. Water in the leaf can evaporate out of small holes called stomata, and a small meniscus is formed at each stomata. As the water evaporates, the meniscus pulls more water in from the vessels of the leaf, which pull more water in from the vessels of the stem. This is happening at many stomata in each leaf, and in all the leaves throughout the plant. The collective tension applied by all of the menisci can amount to a substantial amount of tension in the xylem vessels in the main stem of the plant, especially in very tall plants like trees. If the tensile strength of the water column is exceeded, the water column does break, a process called cavitation. The resulting gap is filled with water vapor and gases that had been dissolved in the water. Vapor is unable to transmit tension, and so the transport of water through that vessel stops until the gap is closed.

Alex Brands
Lehigh University