Ascent of Sap — Mechanism and Transpiration Pull

In tall trees, like the giant Redwoods (which can be over 100 meters tall), water must travel from deep underground all the way to the top leaves. The plant must defy gravity to do this, and it accomplishes this without a mechanical 'pump' like a human heart.

The most widely accepted scientific explanation for the ascent of sap is the Cohesion-Tension Theory, proposed by Dixon and Joly in 1894.

It relies on three main physical forces:

1. Transpiration Pull (Tension) As leaves lose water vapor through their stomata (Transpiration), it creates a negative pressure (a vacuum or tension) in the leaf tissues. This tension acts like a giant straw, physically pulling water up from the xylem tubes to replace the lost water.

2. Cohesion Water molecules are highly attracted to each other due to hydrogen bonding. This property is called cohesion. Because of cohesion, the water in the xylem forms a continuous, unbroken column from the roots to the leaves. When the top water molecule is pulled up, it pulls the next one below it, and so on.

3. Adhesion Water molecules are also attracted to the inner walls of the xylem vessels (which are made of cellulose and lignin). This prevents the column of water from falling back down under the weight of gravity.

While Transpiration Pull is the main driver in tall trees during the day, Root Pressure plays a minor role.

Cells in the roots actively pump mineral ions into the root xylem. Water follows via osmosis, creating a positive pressure (root pressure) that pushes water up the stem. However, root pressure is weak and can only push water up a few meters. It is mostly observable at night or in small herbaceous plants.

Soil Water → Root Hairs → Cortex → Endodermis → Pericycle → Root Xylem → Stem Xylem → Leaf Xylem → Mesophyll Cells → Stomata → Atmosphere.