Capillary Action in Plants: Definition & Examples Video

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  • 0:00 Definition and Forces
  • 1:05 The Process
  • 2:10 How Plants Use this Action
  • 3:30 Lesson Summary
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Lesson Transcript
Instructor: Kelly Robson

Kelly has taught High School Science and Applied Communications. She holds an Education Specialist Degree in Ed. Leadership.

Capillary action is the process that plants use to pull water up from the ground. This lesson explains what capillary action is, describes all the forces necessary for it to happen, and goes through examples of the process.

Definition and Forces

Capillary action is the movement of liquid along a surface of a solid caused by the attraction of molecules of the liquid to the molecules of the solid. Let's simplify.

Plants use capillary action to bring water up the roots and stems to the rest of the plant. The molecules of the water (the liquid) are attracted to the molecules of the inside of the stem (the solid). This attraction is used to help force the water up from the ground and disperse it throughout the plant.

There are three forces involved with the process of capillary action. Let's discuss them.

Adhesion is the process of attaching one thing to another. For plants, adhesion allows for the water to stick to the organic tissues of plants. Cohesion keeps molecules of the same substance together. For plants, cohesion keeps the water molecules together. Surface tension is the effect of intermolecular attraction that causes liquids to form a top or outer layer that behaves like a thin film of sorts. Surface tension is responsible for the shape of water drops and for holding the structures together as plants soak up the water.

Water molecules form hydrogen bonds with each other to give them a sticky quality allowing them to form drops. Adhesion allows them to stick to the tree shown here.
Water Drops

The Process

Although you may use water to help get sticky substances, such as syrup, off your hands, water itself is actually sticky. Water molecules not only stick to each other, but water also sticks to grass, cloth, organic tissues, soil, and paper towels. Plants take advantage of water's stickiness to organic tissues and soil.

As mentioned above, capillary action is the movement of the water through the plant. This movement occurs when the adhesion is stronger than the cohesion. Water molecules are naturally attracted to each other. This attraction comes from the temporary hydrogen bonds that they form.

So, basically, a water molecule has to choose to stay with the other water molecules or to go stick itself to the organic tissue of the plant. When the molecule is more attracted to the plant, it is pulled towards the organic tissue. Although the attraction is stronger to the tissue, the water molecule still wants to bring its buddies. Therefore, the first water molecule still attaches itself to the plant tissue and it also brings along water molecule number two.

How Plants Use Capillary Action

Plants need water to grow. Soil is on the list of things that water sticks to. Plants bury their roots deep into the soil so that it can gather the water that is sticking to the soil.

Plants dig their roots deep into the ground to find the water.
Roots and water

Let's go on a journey through the path of the water molecule. First, the water comes to the ground in the form of rain. It travels down through the soil, picking up nutrients from the soil. Eventually, the water molecule runs into a tree root. The water molecule is attracted to the organic tissue that makes up the root of the tree.

The attraction to the tree root tissue (cohesion) becomes stronger than the attraction to the other water molecules (adhesion). This sends the water drop up through the roots of the tree in narrow pipes called capillaries or xylem. The water drop does not want to travel alone, so it forms a temporary hydrogen bond with another water molecule. They each take a buddy and form a stream of water moving up the capillary.

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