Cohesion tension theory is the primary explanation for how water moves upward through the xylem in plants. This theory posits that water molecules exhibit strong cohesive properties, allowing them to stick together and form a continuous column within the xylem vessels. As water evaporates from the leaves during transpiration, it creates a negative pressure that pulls more water upward from the roots through the xylem. This process is essential for maintaining the flow of water and nutrients throughout the plant.

While cohesion tension theory is widely accepted, other theories, such as root pressure theory, also contribute to our understanding of water movement. Root pressure theory suggests that positive pressure builds up in the root xylem due to the absorption of water, which occurs at a rate greater than the rate of transpiration. This pressure is generated when ions are actively transported into the root xylem, creating a lower water potential compared to the surrounding soil. Consequently, water enters the roots via osmosis, increasing the volume of water in the xylem and generating positive pressure. This phenomenon can lead to guttation, where excess water is expelled from the leaves, particularly in the morning when root pressure is at its peak.

To fully grasp the movement of water through xylem, it is crucial to understand capillary action, which is the ability of water to move through narrow spaces. Capillary action is influenced by three main factors: adhesion, cohesion, and surface tension. Adhesion refers to the attraction between water molecules and the walls of the xylem vessels, while cohesion describes the attraction between water molecules themselves. This cohesive force allows water to form a continuous column. Surface tension, which occurs at the air-water interface, creates a 'skin' on the water's surface, enabling small objects, like a paper clip, to float without breaking the surface tension.

In summary, the interplay of adhesion, cohesion, and surface tension facilitates capillary action, allowing water to rise against gravity in the xylem. The cohesive forces transmit the pull created by surface tension and adhesion, ensuring that water can effectively travel from the roots to the leaves. Understanding these principles is essential for comprehending how plants transport water and nutrients, ultimately supporting their growth and survival.