Xylem is the specialized vascular tissue in plants responsible for transporting water and dissolved nutrients throughout the organism. It functions as the plant’s internal plumbing system, ensuring that water absorbed by the roots reaches all aerial parts, such as the stems and leaves. The ability of the xylem to conduct this upward flow against gravity is fundamental for the survival and growth of almost all vascular plants.
Structure and Composition
Xylem is composed of two types of elongated, tube-like cells known as tracheary elements: tracheids and vessel elements. These cells lose their protoplasts and die once they reach functional maturity, forming hollow, continuous pipes. Tracheids are long, narrow cells with tapered ends, and they are the only type of water-conducting cell found in most gymnosperms and seedless vascular plants. Vessel elements are shorter and wider, connecting end-to-end through perforated end walls to form long, continuous vessels, which are the main water-conducting structures in flowering plants (angiosperms).
A defining feature of all tracheary elements is the presence of lignin, a complex organic polymer deposited in their secondary cell walls. Lignin provides structural support and rigidity to the xylem tissue. This polymer makes the cell walls strong enough to withstand the negative pressure, or tension, that develops inside the water column during transport. The lignified walls are also hydrophobic, meaning they repel water, which helps increase the efficiency of the water-conducting pathway.
Primary Function: Water and Mineral Distribution
The primary function of the xylem is the unidirectional movement of xylem sap. This sap consists mainly of water absorbed from the soil, carrying dissolved inorganic mineral nutrients essential for plant metabolism. Water enters the roots, travels into the xylem, and moves upward through the stem to supply the leaves, where it is used in photosynthesis and to maintain cell turgidity. The transport of these dissolved minerals is critical for synthesizing proteins, nucleic acids, and other organic molecules throughout the plant.
This upward movement is a continuous stream that compensates for the constant loss of water vapor from the leaves. Supplying water is necessary for photosynthesis, as the chemical reaction requires water as a raw material. Beyond transport, the xylem’s heavily lignified structure provides mechanical strength that allows plants, particularly large trees, to stand upright and resist bending forces. The movement of water and minerals through the xylem is a passive process, meaning the plant does not expend metabolic energy to directly pump the fluid.
The Mechanism of Ascent
The mechanism that drives water and dissolved minerals upward through the xylem is best explained by the Cohesion-Tension Theory. This theory centers on the physical properties of water and transpiration, the evaporation of water vapor from the leaves through small pores called stomata. As water evaporates from the moist surfaces of the leaf cells, it creates a powerful negative pressure, or tension.
This tension acts as a pulling force and is the primary driver of water movement. The pull is transmitted down the water column due to cohesion, a property of water. Water molecules are polar and form hydrogen bonds, causing them to stick together strongly and maintain a continuous, unbroken column from the leaves down to the roots. This cohesive force gives the water column a high tensile strength, preventing it from breaking under the intense tension created by transpiration.
The second physical property supporting the mechanism is adhesion, the attraction between water molecules and the hydrophilic, lignified walls of the xylem vessels. Adhesion helps to counteract gravity and prevent the water column from pulling away from the walls. This system is entirely passive, powered by the sun’s energy driving evaporation, which generates the tension that pulls the cohesive water column upward.