How does capillarity contribute to water transport in xylem?

Capillarity aids in water transport in xylem by creating an upward force that moves water against gravity.

Capillarity, also known as capillary action, is a physical phenomenon that allows liquid to flow in narrow spaces without the assistance of external forces like gravity. In the context of plant biology, this process plays a crucial role in the transportation of water from the roots to the upper parts of the plant through the xylem.

The xylem is a type of tissue in vascular plants that primarily functions for the long-distance transport of water and nutrients. It is composed of hollow, tube-like structures that are dead at maturity, providing an uninterrupted pathway for water movement. The walls of these tubes are made up of a substance called lignin, which is hydrophilic (water-attracting) in nature. This property is essential for capillary action to occur.

Capillarity in xylem works on the principle of cohesion-tension theory. Water molecules are polar, meaning they have a positive and a negative end. This polarity allows them to form hydrogen bonds with each other (cohesion) and with the walls of the xylem vessels (adhesion). When water is lost from the leaves during transpiration, it creates a tension or suction force which pulls up more water from below.

The narrow diameter of the xylem vessels enhances the capillary action. The smaller the diameter of the tube, the higher the water can rise against gravity. This is due to the increased interaction between the water molecules and the tube walls, which creates a stronger adhesive force.

In summary, capillarity is a vital mechanism in the xylem's function of water transport. The hydrophilic nature of the xylem walls and the cohesive and adhesive properties of water molecules work together to facilitate this upward movement of water, enabling plants to receive the necessary hydration and nutrients for their survival and growth.