The vascular tissues, xylem and phloem, form the plant’s internal plumbing system, a complex network of conduits extending from the deepest roots to the farthest leaf tips. This system distributes resources efficiently across the entire structure.
Xylem: The Water Delivery Network
The primary function of the xylem is to transport water and dissolved mineral nutrients upward from the roots to the rest of the plant body. This passive process occurs through specialized cells—tracheids and vessel elements—which are dead at maturity, forming hollow, continuous tubes. Their rigid cell walls are reinforced with lignin, preventing the tubes from collapsing under pressure.
Water is pulled up through the xylem using the cohesion-tension theory, driven by transpiration. Transpiration is the evaporation of water vapor from the leaves through stomata, which creates a powerful tension or negative pressure.
This tension is transmitted down the water column because water molecules exhibit strong cohesion, sticking tightly to one another due to hydrogen bonding. This cohesive property maintains an unbroken column from the roots to the leaves, allowing the transpirational pull to draw water upward against gravity. Water also adheres to the lignified walls of the vessels, which helps prevent the column from breaking.
Phloem: The Food Distribution System
The phloem is responsible for translocation, transporting sugars, primarily sucrose, produced during photosynthesis to all other parts of the plant. Sugars move from a “source” (e.g., a mature leaf) to a “sink” (any area requiring energy for growth or storage, like roots or growing tips).
The conducting cells, called sieve tube elements, are living at maturity but lack a nucleus and most organelles to maximize sap flow. Each sieve tube element is associated with a companion cell, which carries out the necessary metabolic functions.
Sugar transport is explained by the pressure flow hypothesis, driven by the active transport of sucrose into the phloem at the source. This loading significantly increases the solute concentration within the sieve tube elements. The increased concentration causes water to move rapidly from the adjacent xylem into the phloem by osmosis.
This influx of water generates a high turgor pressure at the source end of the sieve tube. This pressure gradient pushes the sugar-rich phloem sap toward the sink, where the sugar is actively unloaded. Once the sugar is removed, water moves back into the xylem, completing the cycle and lowering the pressure.
Vascular Tissues: Interdependence and Structure
The xylem and phloem are found together in distinct strands called vascular bundles. These bundles are arranged differently depending on the plant organ: in dicot stems, they are typically organized in a ring, while in monocot stems, they are scattered.
The two transport systems are functionally interdependent. The xylem provides the water necessary for phloem transport to operate. Conversely, the phloem transports the sugars that fuel the energy needs of all living plant cells, including the root cells responsible for water and mineral uptake into the xylem.
The lignified xylem tissue forms the wood of trees, allowing them to grow vertically. The distribution of resources by both tissues allows plants to maintain leaves for photosynthesis, develop reproductive structures, and store energy.