Water transport in xylem refers to the upward movement of water and dissolved minerals from the roots to the leaves and other aerial parts of a plant. The xylem is a complex vascular tissue consisting mainly of tracheids and vessel elements, forming a continuous network throughout the plant. This movement is unidirectional and crucial for photosynthesis, maintaining turgor, and nutrient distribution.

The primary mechanism explaining this long-distance transport is the Cohesion-Tension theory (or Cohesion-Tension-Transpiration Pull theory):

1. Transpiration Pull: Water evaporates from the leaf surface through stomata (transpiration). This creates a negative pressure, or "pull," on the water column in the xylem vessels, much like sucking through a straw. This transpiration pull is the main driving force.

2. Cohesion: Water molecules are polar and highly attracted to each other through hydrogen bonds. This strong cohesion creates a continuous, unbroken column of water extending from the roots to the leaves. As one water molecule evaporates from the leaf, it pulls the next molecule up.

3. Adhesion: Water molecules are also attracted to the hydrophilic walls of the xylem vessels (a property called adhesion). This adhesion helps to counteract the downward pull of gravity and prevents the water column from breaking, especially in narrow xylem conduits.

4. Tension: The transpiration pull creates a continuous tension (negative pressure) throughout the entire water column, extending down to the roots. This tension draws water from the root xylem, then from the root cortical cells, and finally from the soil.

Essentially, the sun's energy driving transpiration at the leaves creates a tension that, due to the cohesive and adhesive properties of water, pulls a continuous column of water upwards through the xylem, overcoming gravity and resistance. Minor contributions from root pressure can also push water up in the absence of transpiration, but transpiration pull is the dominant force.