Topic 8.1 · Core + Supplement
Xylem and Phloem
Xylem and phloem run through a plant side by side, often in the same vascular bundle, which is exactly why questions about them are so easy to answer vaguely and so easy to get wrong. Learn each tissue by what it carries first, then by where it sits in a section — the two skills are tested separately, and neither one substitutes for the other.
What each tissue transports — Core
Xylem transports water and mineral ions, and it also helps support the plant. Phloem transports sucrose and amino acids. That’s the whole required list at this level, and it’s worth holding onto it precisely: the sugar phloem carries is specifically sucrose, not glucose and not starch, and swapping the word in an answer can cost you a mark even when the rest of the sentence is correct.
| Xylem | Phloem |
|---|---|
| Water and mineral ions | Sucrose and amino acids |
| Also supports the plant | Carries organic nutrients the plant has assimilated |
The two tissues commonly occur together as vascular tissue, bundled into the vascular bundles you’ll meet in the next section, but that physical closeness is exactly why an exam wants you to name the correct tissue rather than just say “the plant’s transport system.” Water and mineral ions belong to xylem. Sucrose and amino acids belong to phloem. Treating every substance moving through a plant as interchangeable is the single most common way marks are lost on this syllabus point.
Position of xylem and phloem in root, stem and leaf — Core
You’re expected to identify xylem and phloem from their position in diagrams and images of the roots, stems and leaves of non-woody dicotyledonous plants — and the arrangement genuinely changes depending on which organ you’re looking at, so no single shortcut covers all three.
Root
In a transverse section of a typical dicot root, xylem sits in the central region, usually forming a star- or cross-like shape. Phloem lies between the arms of that xylem.
Stem
In a non-woody dicot stem, xylem and phloem occur together in vascular bundles, and those bundles are arranged in a ring near the outside of the stem. Within each individual bundle, xylem sits on the inner side, closer to the centre of the stem, and phloem sits on the outer side, closer to the edge.
Leaf
Xylem and phloem occur together in the vascular bundles that form the veins of a leaf. In a typical leaf cross-section, xylem lies on the upper side of the vascular bundle and phloem lies on the lower side.
A compact way to hold all three organs in your head at once: root — central xylem, phloem between its arms; stem — xylem inside, phloem outside; leaf — xylem upper, phloem lower. But treat that as a memory aid rather than a rule you can apply blindly — “xylem inside, phloem outside” describes a stem bundle specifically, and shouldn’t be pasted onto a root or leaf diagram where the arrangement is different.
Structure of xylem vessels — Supplement
A xylem vessel’s structure is built almost entirely around one job: forming a long, largely unobstructed tube that water and dissolved mineral ions can move through, while also giving the plant support. Cambridge limits the required structure to three features, and each one maps onto a specific function.
Thick walls containing lignin strengthen the vessel and help it support the plant, and also help it resist collapsing while water is being pulled through it under tension. No cell contents — a mature xylem vessel is dead, and its centre is empty — leaves a clear internal pathway for water and mineral ions to move through. Cells joined end to end with no cross wallsmeans the individual cells that made up the vessel form one continuous tube rather than a series of separate compartments, so water doesn’t have to repeatedly cross cell boundaries as it travels.
Cambridge is explicit that the details of how lignin is actually laid down in the wall aren’t required, so there’s no need to go further than the structure–function link above. A vessel isn’t assembled the way a pipe is manufactured, either — it forms from living cells that die and lose their cross walls as the plant develops, and describing it as if it were built externally is a mechanism an examiner would push back on.
Cross-links
Once you know what xylem carries and where to find it, the natural next step is following what actually happens to that water on its way through the plant — see water uptake. The phloem side of this page gets its full treatment, including how a plant part can switch between releasing and using sucrose, on the translocation page.