Phloem is the plant tissue responsible for transporting sugars and other nutrients from where they’re made to where they’re needed. While xylem carries water upward from the roots, phloem moves food (primarily sucrose) in every direction, feeding roots, fruits, flowers, and growing tips that can’t produce their own energy through photosynthesis. It also serves as a long-distance communication network, carrying hormones and signaling molecules between distant parts of the plant.
Sugar Transport: The Primary Job
The core function of phloem is delivering sugars from “source” organs to “sink” organs. Sources are parts of the plant that produce more sugar than they use, mainly mature leaves running photosynthesis. Sinks are parts that consume sugar: roots, developing fruits, seeds, growing shoot tips, and storage organs like tubers. Sucrose is the dominant sugar in the phloem, though some plants also transport sugar alcohols like sorbitol and mannitol, or sugars in the raffinose family.
This source-to-sink distribution is one of the biggest factors controlling how a plant grows. A fruit tree, for instance, depends on a steady stream of sucrose flowing from its leaves into its developing fruit. If that flow is disrupted, the fruit stays small, misshapen, or low in sugar content. The same logic applies to root vegetables pulling sugars downward for storage, or to flowers fueling the energy-intensive process of producing pollen and nectar.
How the Flow Works
Phloem moves sap using a pressure-driven system first proposed by the plant physiologist Ernst Münch. The mechanism works like this: at the source end, sugars are actively loaded into the phloem tubes, raising the sugar concentration inside. This concentrated solution draws water in from surrounding tissues by osmosis, which builds up hydrostatic pressure. At the sink end, sugars are unloaded into the cells that need them, lowering the concentration and pressure there. The difference in pressure between source and sink pushes the sap through the tubes, no pump required.
This pressure flow is surprisingly effective. In cucumber plants, phloem sap moves at roughly 0.15 to 0.19 millimeters per second, which works out to about 50 to 70 centimeters per hour. That’s fast enough to supply distant organs in real time, and the speed adjusts based on how aggressively sinks are drawing sugar.
What’s Actually in the Sap
Phloem sap is more than sugar water. Analysis of sweet orange phloem sap found a total sugar concentration around 103 millimolar, with sucrose making up about two-thirds of that. The sap also carried roughly 44 millimolar of amino acids, with proline alone accounting for more than 60% of the total. Seven organic acids, including malic, citric, and succinic acid, were also present. In short, the phloem delivers a complex nutritional package: carbon for energy and growth, nitrogen in the form of amino acids, and organic acids involved in cell metabolism.
Sieve Tubes and Companion Cells
The phloem’s transport channels are called sieve tubes, built from elongated cells stacked end to end. The walls between adjacent cells are perforated with large pores, forming structures called sieve plates. These pores create a continuous pipeline running through the entire plant. To minimize resistance to flow, sieve tube cells shed most of their internal structures during development, losing their nucleus, ribosomes, and most of their cytoskeleton. What remains is a stripped-down cell containing mitochondria, some endoplasmic reticulum, and specialized proteins.
Without a nucleus, sieve tube cells can’t maintain themselves. That job falls to companion cells, smaller cells attached to the sieve tubes through numerous connections. Companion cells are metabolically hyperactive, spending energy both to keep the sieve elements alive and to power the loading of sugars into the phloem. In some plant species, companion cells use energy-dependent transporter proteins to pull sucrose from surrounding tissue into the phloem (called apoplastic loading). In others, sucrose diffuses in through cell-to-cell connections and gets chemically converted into larger sugars like raffinose and stachyose, effectively trapping it inside the phloem.
Long-Distance Signaling
Beyond nutrient delivery, phloem acts as the plant’s internal messaging system. Several hormones travel through phloem sap to coordinate responses across the whole organism. One type of cytokinin, a growth hormone produced in the shoot, moves downward through the phloem toward the roots. Jasmonic acid, a wound-response signal, travels leaf to leaf through the phloem, so when an insect chews on one leaf, distant leaves can activate their defenses before they’re attacked. A derivative of salicylic acid accumulates in the phloem and travels to distant tissues, where it’s converted back to its active form to trigger a plant-wide immune response known as systemic acquired resistance.
Small peptides, messenger RNA, and other hormone-like molecules also hitch a ride in the phloem. This makes the sieve tube system something like a combination of a circulatory system and a nervous system, delivering both food and instructions.
What Happens When Phloem Fails
The consequences of phloem breakdown illustrate just how essential it is. Huanglongbing, a devastating citrus disease, offers a clear example. The bacterial pathogen behind the disease causes phloem tissue to progressively collapse. First, the walls between sieve elements swell. Then the plant’s own defense proteins and callose (a carbohydrate sealant) plug the sieve plates, blocking flow. As sugar transport becomes obstructed, starch granules build up in the leaves because the sugars produced by photosynthesis have nowhere to go.
The visible results are striking: leaves turn yellow from nutrient imbalance, fruit becomes bitter and lopsided (because some sections of the phloem around the fruit stalk collapse while others remain functional, creating uneven sugar delivery), and each successive crop is smaller and lower in quality. Affected branches can continue producing some fruit for a time, since the phloem closest to the cambium often stays intact longer, but productivity steadily declines. Eventually the entire transport system collapses, and the tree dies.
This pattern holds across plant species. Any disease, pest, or physical damage that disrupts phloem tissue starves the parts of the plant that depend on imported sugar, while causing sugar and starch to accumulate in the leaves. Girdling a tree trunk, which strips away the bark containing the phloem, kills the roots first because they lose their food supply, even though water continues flowing upward through the xylem for a while.