Is water going up plants cohesion or adhesion?

One of the greatest mysteries of plants is how they transport water from their roots all the way to their leaves. This process, known as the transpiration stream, is essential for plant survival, as water is needed for photosynthesis and keeping the plant hydrated. But how does water move upwards against the force of gravity? Is it due to the cohesive properties of water or the adhesive properties of the plant's vascular tissues?

The Cohesion Theory

The cohesion theory, also known as the cohesion-tension theory, proposes that water is pulled up through the plant due to the cohesive nature of water molecules. Water molecules are attracted to one another through the formation of hydrogen bonds, creating a strong cohesion between them. As water is transpired from the leaves, this cohesive force pulls up more water molecules to fill the void. This creates a continuous flow of water up the plant, known as the transpiration stream.

This theory is supported by several pieces of evidence. One is the observation that the rate of transpiration is affected by humidity levels; the drier the air, the faster water is transpired from the leaves, which pulls up more water from the roots. Additionally, water can be pulled up to great heights, exceeding the height of the tallest trees, supporting the idea that the force of cohesion is strong enough to overcome gravity.

The Adhesion Theory

The adhesion theory proposes that it is the adhesive properties of the plant's vascular tissues that allow water to move up the plant. Adhesion occurs when water molecules are attracted to the walls of the plant's xylem vessels through hydrogen bonding. This attraction creates a tension between the water molecules and the walls, which helps to pull water up the plant against gravity.

This theory is supported by several pieces of evidence as well. One is the observation that water can only move up through narrow spaces, such as the xylem vessels, which indicates that adhesion is at work. Additionally, the rate of water movement is influenced by temperature, which affects the viscosity of the water and its ability to adhere to surfaces.

The Truth

The truth is that both cohesion and adhesion are at work in the transpiration stream. Without the cohesive properties of water, there would be no force to pull the water molecules up the plant. And without the adhesive properties of the xylem walls, the water would simply wet the insides of the vessels and not move up the plant. Thus, both forces complement each other to create a cohesive and adhesive system that allows for the efficient transport of water from the roots to the leaves.

In conclusion, the movement of water up plants is not due to one single force but rather the combination of both cohesion and adhesion. Understanding the complex interplay between these forces is essential for studying the water transport mechanisms of plants, which can have important implications for agriculture and environmental conservation.