Phloem is the specialized vascular tissue within a plant responsible for transporting food throughout its structure. This tissue forms a complex network, functioning much like a circulatory system, ensuring that the energy produced during photosynthesis reaches all living cells. Without this internal delivery system, plants could not grow, flower, or store the energy reserves necessary for survival. Understanding phloem means recognizing its fundamental importance to the entire plant organism, from the tallest tree to the smallest seedling.
Phloem’s Role in the Plant
The job of phloem is translocation, which involves moving organic compounds, primarily the sugar sucrose, to areas where they are needed. These sugars are produced in a “source,” typically mature leaves, which are actively converting sunlight into chemical energy. The sucrose is then transported to “sinks,” which are parts of the plant requiring energy for growth or storage.
Sinks include developing fruits, growing root tips, or storage organs like tubers and bulbs. The direction of flow is dynamic and determined by the plant’s current metabolic needs, meaning the transport can move both upward and downward within the stem. Sucrose must first be actively loaded into the phloem tissue at the source, which is an energy-intensive process requiring metabolic energy in the form of ATP.
This loading increases the solute concentration inside the phloem elements, causing water to move in from the adjacent water-conducting tissue through osmosis. The influx of water creates a high internal pressure, which drives the sugary solution, known as phloem sap, through the tubes toward the lower-pressure sinks. This pressure-driven mass flow allows for rapid distribution of energy over long distances.
Where Phloem is Located and How it Appears to the Naked Eye
Phloem is always situated toward the exterior part of the plant’s vascular system. In non-woody or herbaceous plants, phloem is found alongside water-conducting tissues in discrete structures called vascular bundles. These bundles are often arranged in a ring just beneath the surface in dicot stems, or scattered throughout the stem in monocots.
In woody plants, the phloem is a distinct and continuous layer that makes up the innermost part of the bark. It lies just outside the vascular cambium, the layer responsible for producing new vascular tissue. This location is why phloem is sometimes referred to by its older name, “bast.”
Macroscopically, phloem tissue appears quite different from the hard, woody material it surrounds. If you were to peel back the outer bark of a tree, the phloem is the soft, often moist or slimy layer immediately visible. It typically has a lighter color, sometimes a pale green or off-white, and lacks the structural rigidity of the wood.
This soft, external location explains why removing a complete ring of bark from a tree trunk, a process known as girdling, is lethal to the plant. When the phloem ring is severed, the downward flow of sugars to the roots is interrupted, effectively starving the roots even though the leaves and upper trunk remain healthy for a period.
The Cellular Components of Phloem
The function of phloem is carried out by four primary types of specialized cells adapted for transport and support.
Sieve Tube Elements
The main conducting channel is the sieve tube element, consisting of elongated, living cells joined end-to-end to form a continuous tube. These cells are unique because they are functionally alive at maturity but have lost their nucleus and most other organelles, creating an open pathway for the phloem sap to flow. The ends are marked by sieve plates, which look like perforated colanders under a microscope. These pores allow the cytoplasm and phloem sap to pass freely, minimizing resistance to flow. The interior of these tubes appears largely clear due to the absence of a nucleus or large vacuole.
Companion Cells
Closely associated with every sieve tube element is a companion cell, which provides the metabolic support the sieve tube element cannot provide for itself. Companion cells are recognizable by their dense cytoplasm and prominent nucleus, indicating high metabolic activity. They are linked to the sieve tube elements by numerous small connections called plasmodesmata, which facilitate the loading and unloading of sugars.
The phloem tissue also contains supportive cells:
- Phloem parenchyma cells, which are generally unspecialized cells scattered throughout the phloem. Their primary role is to store starches, fats, and other organic compounds, and they appear as typical plant storage cells.
- Phloem fibers and sclereids, which provide mechanical strength and protection. These cells have thick, lignified secondary cell walls and are typically dead at maturity, appearing as bundles of strong, supportive tissue, often at the outer edge of the phloem layer.
Phloem vs. Xylem: A Visual Comparison
Understanding phloem requires contrasting its structure with xylem, the plant’s other major vascular tissue, which transports water. The most apparent visual difference in a cross-section is their relative position: phloem is always located toward the outside of the plant body, while xylem forms the hard, inner core or is situated toward the center of the vascular bundle.
The structural composition is vastly different at the cellular level. Xylem is primarily composed of tracheids and vessel elements that are dead at maturity, creating hollow tubes with thick, intensely lignified cell walls that provide immense structural rigidity. This is why xylem is the material we call “wood.”
Conversely, the conducting sieve tube elements of the phloem are living cells with relatively thin cell walls. When viewed in a mature stem cross-section, the phloem tissue often appears less organized, softer, and can look somewhat crushed due to the pressure exerted by the surrounding, expanding xylem.
The thick walls of the xylem vessels make them appear rigid and clearly defined under a microscope, often forming distinct, large openings that facilitate water flow. Phloem, however, presents a collection of thinner-walled sieve tube elements and their associated companion cells, resulting in a softer, less geometrically precise appearance in the overall tissue structure.