Plants require efficient systems to move essential substances throughout their structure. They transport water, nutrients, and the products of photosynthesis to where they are needed. Specialized tissues manage this internal movement, forming a transport network that distributes resources across the plant.
The Xylem’s Role in Plants
The xylem is one of the two primary transport tissues in vascular plants. Its function is to transport water and dissolved mineral nutrients from the roots upwards to the leaves and other aerial parts. This upward flow, known as xylem sap, consists mainly of water and inorganic ions.
The xylem tissue is composed of several cell types, including tracheids and vessel elements. These cells are dead at maturity, forming continuous hollow conduits that facilitate fluid movement. The walls of these vessels are strengthened by lignin, a tough substance that provides structural support and prevents collapse. This structural rigidity allows plants to achieve considerable heights.
Transporting water and minerals is important for plant survival and growth. Water is a reactant for photosynthesis, the process by which plants convert light energy into food. Additionally, water helps maintain turgor pressure within plant cells, providing structural support and firmness. The continuous flow of water through the xylem also aids in cooling the plant through evaporation.
The Mechanism of Transport
The driving force behind water movement through the xylem is transpiration. Transpiration involves the evaporation of water vapor from the leaves via stomata. Evaporation from the leaf surface creates negative pressure, or tension, within the xylem. This tension pulls water up the plant in a continuous column, much like sipping through a straw.
This upward pull is explained by the cohesion-tension theory. Water molecules exhibit cohesive forces, meaning they are attracted to each other due to hydrogen bonds, allowing them to form an unbroken column within the xylem vessels. Water molecules also adhere to the hydrophilic (water-attracting) walls of the xylem vessels, helping counteract gravity and prevent the water column from breaking. The continuous loss of water from the leaves maintains this tension, drawing more water from the roots.
While transpiration pull is the main mechanism, root pressure also contributes to water transport, especially when transpiration rates are low. Root pressure is generated by the active absorption of mineral ions into the root xylem, lowering the water potential inside the roots. Water then moves into the roots by osmosis, creating positive pressure that pushes water upwards. However, root pressure alone is generally insufficient to account for water movement to the tops of very tall trees.