The ultimate cause of water movement in a plant stem against gravity is—

The ultimate cause of water movement in a plant stem against gravity is transpiration. This process involves the evaporation of water molecules from the leaf's aerial parts, which creates a suction or negative pressure known as 'transpiration pull' [2]. According to the cohesion-tension theory, this pull is transmitted through a continuous column of water in the xylem vessels, drawing water upward from the roots to the leaves [1]. While osmosis and root pressure contribute to water entry into the roots, they are insufficient to move water to the heights of tall trees; root pressure is primarily significant at night when stomata are closed [2]. During the day, transpiration becomes the dominant driving force, enabling plants to overcome gravitational and frictional resistance to transport water and dissolved minerals throughout the plant body [1].

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 5: Life Processes > Activity 5.8 > p. 95
2. [2] https://pmc.ncbi.nlm.nih.gov/articles/PMC5591614/