Xylem and phloem are the two transport tissues that move essential substances throughout a plant. Xylem carries water and dissolved minerals upward from the roots. Phloem carries sugars and other organic nutrients from wherever they’re made (usually the leaves) to wherever they’re needed. Together, they function like a plant’s circulatory system, forming connected networks of tubes that run through roots, stems, and leaves.
What Xylem Does
Xylem has two jobs: transport and support. It moves water and water-soluble minerals absorbed by the roots up through the stem and into the leaves, where the water is used for photosynthesis. It also provides structural rigidity. In fact, wood is mostly made of xylem tissue. The walls of xylem cells are reinforced with lignin, a tough compound that keeps the cells rigid even under pressure, which is why wood is hard.
Mature xylem cells are dead. They lose their internal contents as they develop, leaving behind hollow tubes stacked end to end. This is actually what makes them effective: the empty tubes create continuous pipelines for water to flow through without anything blocking the way. There are two main types of water-conducting cells. Tracheids are narrower cells where water passes through small pits in the side walls. Vessel elements are wider, with perforated end walls that overlap and connect into longer open channels. Most flowering plants have both types, while conifers rely mainly on tracheids.
How Water Moves Up Against Gravity
Getting water from roots to the top of a tall tree is a remarkable feat. Three forces work together to make it happen, but one dominates.
The primary driver is transpiration pull, explained by what’s called the cohesion-tension model. When water evaporates from tiny pores (stomata) on the leaf surface, it creates negative pressure, essentially suction, inside the xylem. That suction pulls water upward in much the same way you pull liquid up through a straw. Water molecules naturally stick to each other through hydrogen bonding, so when the top molecules get pulled upward, they drag the ones below along with them. This chain of water molecules stretches all the way down to the roots. The lignin-reinforced walls of xylem cells keep the tubes from collapsing under all that negative pressure.
Two other forces contribute on a smaller scale. Root pressure pushes water upward from below as roots absorb water from the soil by osmosis, but this force alone can only move water a few meters. Capillary action, the tendency of water to climb the walls of narrow tubes, works up to roughly one meter. Neither is strong enough to explain how water reaches the crown of a 100-meter redwood. Only the transpiration-driven pulling mechanism can do that.
What Phloem Does
Phloem transports sugars, proteins, and other organic molecules to wherever the plant needs them. Leaves produce sugar through photosynthesis, and phloem moves that sugar down to roots for storage, up to growing tips, or out to developing fruits and seeds. Unlike xylem, which flows in only one direction (up), phloem can move its contents both up and down depending on where the supply and demand are.
Scientists describe this in terms of “sources” and “sinks.” A source is any part of the plant that produces or releases sugar, like a mature leaf photosynthesizing in sunlight. A sink is any part that consumes or stores sugar, like a growing root tip, a flower bud, or a ripening fruit. Phloem always flows from source to sink, and what counts as a source or sink can change with the seasons. A root storing starch over winter, for example, becomes a source in spring when it releases sugar to fuel new growth.
How Phloem Moves Sugar
Sugar transport in phloem works through pressure differences. At the source, sugar is actively pumped into the phloem tubes. This high concentration of sugar draws water in from nearby xylem by osmosis, which builds up pressure inside the tube. At the sink end, sugar is unloaded from the phloem, water follows it out, and pressure drops. The result is a continuous flow of sugar-rich sap from high-pressure source areas to low-pressure sink areas, a process called bulk flow.
This is fundamentally different from how xylem works. Xylem relies on suction from above (transpiration pulling water up). Phloem relies on pressure from within (sugar concentration creating osmotic pressure that pushes sap along).
Living Cells vs. Dead Cells
One of the key differences between xylem and phloem is whether their transport cells are alive at maturity. Xylem cells die as they mature, becoming hollow tubes reinforced with lignin. Phloem cells stay alive, though barely. The main conducting cells in phloem, called sieve elements, shed most of their internal machinery as they develop. They lose their nucleus, their functional chloroplasts, and their vacuoles. Their mitochondria become rudimentary. They keep just enough internal structure to stay alive as living pipes.
To compensate for everything they’ve lost, sieve elements depend heavily on neighboring companion cells. Companion cells are fully functional living cells connected to the sieve elements through direct cytoplasmic links, allowing molecules to flow freely between them. Companion cells produce the messenger RNA and small proteins that sieve elements need but can no longer make for themselves. They essentially act as the life support system for the transport pipeline. The sieve elements themselves are connected end to end through sieve plates, perforated walls that allow sugar-rich sap and molecules to pass through.
Where They Sit Inside the Plant
Xylem and phloem are bundled together in structures called vascular bundles that run through the plant’s roots, stems, and leaves. In flowering plants with two seed leaves (dicots, like sunflowers and oak trees), these bundles are arranged in a ring around the stem. Xylem sits toward the inside of each bundle, phloem toward the outside. Between them is a thin layer of dividing cells called the cambium, which produces new xylem and phloem as the plant grows wider.
In plants with one seed leaf (monocots, like grasses and palms), vascular bundles are scattered throughout the stem rather than arranged in a ring, and most monocots lack a cambium. This is why grasses don’t develop thick woody trunks the way oaks do.
How Xylem and Phloem Become Wood and Bark
In trees and shrubs, the cambium continuously adds new layers of xylem inward and new layers of phloem outward. The xylem produced by this process is called secondary xylem, and it’s what we know as wood. Each year’s growth adds a new ring, which is why you can count tree rings to estimate age. The phloem produced outward is called secondary phloem, or bast, and it forms part of the inner bark.
The outer bark involves a second layer of dividing cells called the cork cambium, which produces the tough protective cork layer on the outside. Over time, older phloem gets pushed further out as new phloem forms. The sieve elements in these older layers eventually collapse and stop functioning. So while a tree trunk might contain decades of accumulated xylem (wood), only the most recent layers of phloem near the cambium are actively transporting sugar. This is why girdling a tree, cutting a ring through the bark deep enough to sever the phloem, kills it: the roots are cut off from their sugar supply even though water can still flow up through the intact xylem.
Xylem vs. Phloem at a Glance
- What they carry: Xylem transports water and minerals. Phloem transports sugars, proteins, and other organic molecules.
- Direction of flow: Xylem flows upward from roots to leaves. Phloem flows in any direction, from source to sink.
- Driving force: Xylem is pulled by transpiration (evaporation from leaves). Phloem is pushed by osmotic pressure created by sugar loading.
- Cell status: Xylem conducting cells are dead at maturity. Phloem conducting cells are alive but rely on companion cells.
- Cell wall: Xylem walls are thickened with lignin for strength. Phloem walls are thinner and flexible.
- In trees: Xylem accumulates as wood. Phloem forms part of the inner bark.