Sciences

Xylem and Phloem: What Each Carries, and Which Way

Xylem is dead, carries water up only, and does transpiration. Phloem is alive, carries sugar both ways, and does translocation. Four clear differences.

Xylem and phloem

Xylem and phloem are a plant's two transport tissues: dead xylem vessels carry water upwards, and living phloem tubes carry dissolved sugars in either direction.

Aussi appelé: Vascular tissue, Vascular bundles
Où les élèves le rencontrent: Grade 7 science, in the unit on transport in plants, and again in Grade 9 biology alongside transpiration, stomata and the effect of wind and humidity on water loss.

La réponse en bref
Xylem carries water and dissolved mineral ions in one direction only, upwards from the roots, through dead hollow cells strengthened with lignin. Phloem carries dissolved sugars both up and down, through living cells joined end to end by perforated sieve plates. The processes are transpiration in the xylem and translocation in the phloem.

Un exemple

Xylem: roots → leaves, water and minerals. Phloem: source ⇄ sink, sucrose.

Strip a ring of bark from around a tree trunk and you remove the phloem, which sits just under the bark, while leaving the xylem in the wood intact. Water keeps rising, so the leaves stay alive for a while, but sugar can no longer travel down past the cut. A swelling forms on the upper edge of the ring where the sugar collects. That single observation separates the two tissues by both position and cargo.

The four differences worth learning

Almost every exam question on this topic tests one of four things: direction, contents, cell type, and whether the cells are alive. Learn them as a set and the answers stop depending on which way the question is asked.

The pairing is not accidental. Xylem is dead because it needs to be nothing more than an open pipe, and phloem is alive because loading sugar into it costs energy. Once that reason is understood, the other three differences follow from it rather than needing separate memorising.

Direction — xylem carries one way, upwards only. Phloem carries both up and down, from wherever sugar is made to wherever it is needed
Contents — xylem carries water and dissolved mineral ions. Phloem carries dissolved sugars, mainly sucrose, and amino acids
Cell type — xylem vessels are hollow tubes with no end walls and walls thickened with lignin. Phloem sieve tubes have end walls perforated with pores, and each is attached to a companion cell
Living status — mature xylem cells are dead and empty. Phloem cells are alive, though the sieve tube itself has lost most of its contents to make room for the flow

Transpiration and translocation

Transpiration is the process the xylem carries out. Water evaporates from the surfaces inside the leaf and escapes through the stomata, and because water molecules cling to one another the whole column is pulled up behind it. Nothing pushes from below and no energy is spent by the plant, which is why dead cells are enough for the job.

Translocation is the phloem's process, and it works differently. Sugar is loaded into the sieve tubes at a source — usually a photosynthesising leaf — and unloaded at a sink, such as a root, a growing shoot tip or a developing fruit. Loading costs energy, which is supplied by the mitochondria packed into the companion cells. That is the reason phloem has to be alive.

Where to find them on a slide

In a stem the two tissues are grouped into vascular bundles arranged in a ring, with the xylem on the inside of each bundle and the phloem on the outside. In a root the xylem sits in a star shape at the very centre with the phloem between its arms. In a leaf they run together through the midrib and veins.

Under a microscope the xylem is the easier one to spot: thick-walled, empty, often stained a strong colour by the lignin, and roughly circular in cross section. Phloem cells look thinner-walled and messier, with the small companion cells crowded against the sieve tubes.

Questions fréquentes

Which one carries water, xylem or phloem?

Xylem. It carries water and the mineral ions dissolved in it, upwards from the roots to the leaves. Phloem carries dissolved food. A useful hook is that phloem and food both start with an f sound, and that xylem cells are dead like the xylophone's wooden bars.

Why are xylem cells dead?

Because a dead cell is a better pipe. Emptying the cell of its contents and dissolving the end walls leaves a continuous hollow tube with nothing in the way, and the lignin left behind stops it collapsing under the tension of the water column. Transpiration needs no energy from the plant, so nothing living is required.

Does phloem carry sugar up or down?

Both, and often at the same time in different tubes. Sugar moves from source to sink. In summer that usually means down from the leaves to the roots, but in spring a tree moves stored sugar upwards from the roots to feed new buds before there are any leaves to make it.

What is the difference between transpiration and translocation?

Transpiration is the loss of water vapour from the leaves and the upward flow of water through the xylem that replaces it. Translocation is the movement of dissolved sugars through the phloem to wherever they are needed. Different tissue, different substance, different direction, and only translocation costs the plant energy.

Dernière mise à jour 21 août 2026