The Quantum Botanist

Capillary Action in Plants

June 6, 2026

As we know, plants need water to live. But how does that water, as well as other nutrients, get from the soil to the plant? Plants are able to move materials throughout their bodies by utilizing capillary action. There are also theories about how quantum tunneling may be used, and given the entire premise of this blog, you may think that would be the topic I talk about here, but we have to understand the basics before we go quantum, so we’ll be keeping this discussion classical. There are three main things we need to know about before understanding capillary action: adhesion, cohesion, and surface tension.

Adhesion

Adhesion is the force that causes particles that are not the same to stick together. After washing your hands or doing something else involving water, you may have noticed that the water sticks to your skin a bit. This is adhesion. The water is sticking to your skin, which is a different material. Water loves to stick to itself, which we’ll go over a bit later, but sometimes, it likes to stick to other things even more. Water molecules are polar, meaning that they have areas of positive charge, as well as areas of negative charge. These areas are what cause the attraction that the molecules have to other molecules. For example, glass molecules are more polar than water molecules, so the water molecules would prefer to be attracted to the glass. This attractive force can be strong enough to counteract the force of gravity, allowing water to climb up the side of the glass container.

Cohesion

Cohesion is similar to adhesion, but instead of attraction to different molecules, cohesion is the attractive force between molecules of the same type. This is how water likes to stick to itself. Once again, water is a polar molecule, so the positive parts are attracted to the negative parts. Cohesion is what gives rise to surface tension.

Surface Tension

Surface tension is a result of cohesive forces. Water wants to make hydrogen bonds to itself, but the molecules on the surface have fewer neighbors to bond with, so the bonds that they do have are stronger. This makes it so that the surface of water is harder to break.

Capillary Action

These three combine to allow water to move upwards. The adhesion will act to move the water a bit up any narrow tube, but the surface tension caused by the cohesion wants to keep the surface of the water intact, and so the cohesive forces draw the water up through the tube. The nutrients dissolved in the water are also drawn up into the tube, and so it can help spread materials throughout the plant. Capillary action cannot counteract gravity entirely, however, but the plant has different organ and mechanisms to spread them the rest of the way.

How Does It Work?

Capillary action is defined by Jurin’s law

h=\frac{2\gamma\cos\theta}{\rho gr}

where $\gamma$ is the liquid-air surface tension, $\theta$ is the contact angle, $\rho$ is the density of the liquid, $g$ is the acceleration due to gravity, and $r$ is the radius of the tube. From this equation, we can see what would cause strong or weak capillary action. For a strong force, we would want some combination of small radius, strong surface tension, low contact angle, and low density. Of note here is the radius being small, as that is the only variable we have any control over. The other variables are all fixed from the physical traits of water or gravity. Plants are filled with incredibly narrow tubes, even in their cell walls, so plants are very good at taking up water.