8.1 Xylem and phloem
- Syllabus
- 0610–2026–2027
- Topic
- 8.1
- Level
- —
Learning objectives
8.1.1Functions of xylem and phloem: (a)• State the functions of xylem and phloem: (a) xylem – transport of water and mineral ions, and support (b) phloem – transport of sucrose and amino acids8.1.2Xylem and phloem positions• Identify in diagrams and images the position of xylem and phloem as seen in sections of roots, stems and leaves of non-woody dicotyledonous plants8.1.3Relate the structure of xylem vessels• Relate the structure of xylem vessels to their function, limited to: (a) thick walls with lignin (details of lignification are not required) (b) no cell contents (c) cells joined end to end with no cross walls to form a long continuous tube
Compare the functions of xylem and phloem
Xylem and phloem are the two transport tissues in a plant vascular bundle, but they carry different substances and xylem also has a supporting role.
| Tissue | Substances transported | Additional function |
|---|---|---|
| xylem | water and mineral ions | supports the plant |
| phloem | sucrose and amino acids | — |
Water and dissolved mineral ions taken up by roots travel in xylem. Sucrose made by photosynthesis and amino acids are transported in phloem to parts of the plant that use or store them.
Xylem does not transport sucrose, and phloem does not carry the plant's main water stream. Starch is insoluble and is not the transport sugar; sucrose is.
Identify xylem and phloem in roots, stems and leaves
In a non-woody dicotyledonous plant, xylem and phloem occur together as transport tissues, but their pattern changes between root, stem and leaf sections.
| Organ section | Xylem position | Phloem position |
|---|---|---|
| root | central star- or cross-shaped region | groups between the arms of the xylem |
| stem | inner side of each vascular bundle; bundles form a ring | outer side of each vascular bundle |
| leaf | upper side of a vascular bundle in a vein | lower side of the bundle |
First identify the organ from its overall tissue pattern. Then locate each vascular bundle and use the inside–outside or upper–lower relationship to distinguish xylem from phloem.
Do not use one position rule for every organ: central xylem identifies a dicot root, whereas a ring of separate bundles identifies a non-woody dicot stem.
Relate xylem vessel structure to function
A xylem vessel is a long, hollow tube specialised to carry water and mineral ions while helping support the plant.
| Structural feature | Functional consequence |
|---|---|
| thick walls containing lignin | strengthen the vessel, support the plant and resist collapse |
| no cell contents | leaves a hollow lumen with little resistance to water flow |
| cells joined end to end with no cross walls | forms one long continuous tube so water can flow without end-wall barriers |
lignified continuous hollow vessel → strong unobstructed pathway → transport of water and mineral ions plus structural support
This syllabus objective is limited to lignified thick walls, absence of cell contents and absence of cross walls. Do not confuse a xylem vessel with a living phloem transport cell.